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Home > Archive > Electrical Engineering > April 2008 > What happens when you blow a fuse really hard?
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| Author |
What happens when you blow a fuse really hard?
|
|
|
| Hi.
What happens if one were to overload a fuse so strongly the fuse's
maximum safe interrupting rating was exceeded? My guess is that it
would start arcing enough to allow high current through the device it
is protecting for a (relatively) protracted period of time, therefore
potentially causing damage to it (and hence defeating the purpose of
the fuse, which is to _protect_ the device from damage). Not to
mention the arc itself might be damaging as well (due to the high
temperature.). Is this right?
Is there a video or something showing what happens?
| |
| gfretwell@aol.com 2008-03-20, 5:25 pm |
| On Thu, 20 Mar 2008 13:51:50 -0700 (PDT), mike3 <mike4ty4@yahoo.com>
wrote:
>Hi.
>
>What happens if one were to overload a fuse so strongly the fuse's
>maximum safe interrupting rating was exceeded? My guess is that it
>would start arcing enough to allow high current through the device it
>is protecting for a (relatively) protracted period of time, therefore
>potentially causing damage to it (and hence defeating the purpose of
>the fuse, which is to _protect_ the device from damage). Not to
>mention the arc itself might be damaging as well (due to the high
>temperature.). Is this right?
>
>Is there a video or something showing what happens?
This is probably discussed at length on the Bussman site
| |
|
| On Mar 20, 5:07=A0pm, gfretw...@aol.com wrote:
> On Thu, 20 Mar 2008 13:51:50 -0700 (PDT), mike3 <mike4...@yahoo.com>
> wrote:
>
>
>
>
> This is probably discussed at length on the Bussman site
Can you provide a link to the discussions?
| |
| no_one 2008-03-21, 3:25 am |
|
"mike3" <mike4ty4@yahoo.com> wrote in message
news:f2ecd5e7-0825-47c7-a814-c8c56a4564ea@s19g2000prg.googlegroups.com...
On Mar 20, 5:07 pm, gfretw...@aol.com wrote:
> On Thu, 20 Mar 2008 13:51:50 -0700 (PDT), mike3 <mike4...@yahoo.com>
> wrote:
>
>
>
>
> This is probably discussed at length on the Bussman site
Can you provide a link to the discussions?
If you think about it the interrupting rating is likey due to the voltage of
the fused circuit rather than the current rating; A highly inductive
circuit will look like a transient high voltage source and possibly arc over
the fuse and sustain a plasma arc.
BTW, a fuse is there to prevent the source wiring from burning up, not keep
the load device from getting damaged; the load is already damaged if it is
going to pull enough current if one assumes the fuse was sized correctly in
the first place.
| |
| Tim Perry 2008-03-21, 3:25 am |
|
"mike3" <mike4ty4@yahoo.com> wrote in message
news:f2ecd5e7-0825-47c7-a814-c8c56a4564ea@s19g2000prg.googlegroups.com...
On Mar 20, 5:07 pm, gfretw...@aol.com wrote:
> On Thu, 20 Mar 2008 13:51:50 -0700 (PDT), mike3 <mike4...@yahoo.com>
> wrote:
>
>
Your question can be interpreted in multiple ways. My assumption is that you
are talking about something like getting hit by lightning as opposed to an
instantaneous zero ohm load.
[color=darkred]
> My guess is that it
Until the assembly melts.
[color=darkred]
> therefore
Wrong, fuses are to prevent fires, mostly. Protecting equipment is a
secondary effect (mostly).
[color=darkred]
>Not to
>
>
> This is probably discussed at length on the Bussman site
Can you provide a link to the discussions?
| |
| John Rye 2008-03-21, 1:25 pm |
| Hello Mike
In article
<d4c1e90d-595f-439e-b250-f8d9069944b1@e23g2000prf.googlegroups.com>,
mike3 <mike4ty4@yahoo.com> wrote:
> Hi.
> What happens if one were to overload a fuse so strongly the fuse's
> maximum safe interrupting rating was exceeded? My guess is that it
> would start arcing enough to allow high current through the device it
> is protecting for a (relatively) protracted period of time, therefore
> potentially causing damage to it (and hence defeating the purpose of
> the fuse, which is to _protect_ the device from damage). Not to
> mention the arc itself might be damaging as well (due to the high
> temperature.). Is this right?
> Is there a video or something showing what happens?
There are substantial textbooks written about fuses and how they work. (For
example "Electric Fuses" by A Wright & P G Newbury ISBN 0 906048 78 8)
Operation is different on alternating current and direct current.
To try and keep it simple on alternating current in correct operation :-
(1) If the current is high enough for long enough it melts a short bit of
metal which is thinner than the rest of the conductor and leaves a gap.
(2) An arc forms across the gap.
(3) The material surrounding the arc cools the arc, and when the current
drops to zero the arc goes out.
(4) Providing the resistance across the gap is big enough the arc does not
restrike and the circuit is broken.
If the fault current is higher than the fuse is designed to handle too much
of the element will be melted before a current zero occurs and the arc will
restrike.
If the fuse is used on a higher voltage system than it is designed for the
thinner part of the element that is intended to melt will not produce a big
enough gap and the arc will restrike.
If the arc restrikes in the fuse and keeps burning the heat will destroy the
body of the fuse, and the arc will then continue burning outside of the fuse.
In either case the result is usually considerable damage at the site of the
fuse, and also at the site of the problem which caused the fuse to operate in
the first place.
John Rye
--
John Rye
Hadleigh IPSWICH England
<http://web.ukonline.co.uk/jrye/index.html>
---< On Line using an Acorn StrongArm RiscPC >---
| |
| phil-news-nospam@ipal.net 2008-03-21, 1:25 pm |
| On Fri, 21 Mar 2008 15:17:59 +0000 (GMT) John Rye <jrye@ukonline.co.uk> wrote:
| Hello Mike
|
| In article
| <d4c1e90d-595f-439e-b250-f8d9069944b1@e23g2000prf.googlegroups.com>,
| mike3 <mike4ty4@yahoo.com> wrote:
|> Hi.
|
|> What happens if one were to overload a fuse so strongly the fuse's
|> maximum safe interrupting rating was exceeded? My guess is that it
|> would start arcing enough to allow high current through the device it
|> is protecting for a (relatively) protracted period of time, therefore
|> potentially causing damage to it (and hence defeating the purpose of
|> the fuse, which is to _protect_ the device from damage). Not to
|> mention the arc itself might be damaging as well (due to the high
|> temperature.). Is this right?
|
|> Is there a video or something showing what happens?
|
| There are substantial textbooks written about fuses and how they work. (For
| example "Electric Fuses" by A Wright & P G Newbury ISBN 0 906048 78 8)
|
| Operation is different on alternating current and direct current.
|
| To try and keep it simple on alternating current in correct operation :-
|
| (1) If the current is high enough for long enough it melts a short bit of
| metal which is thinner than the rest of the conductor and leaves a gap.
|
| (2) An arc forms across the gap.
|
| (3) The material surrounding the arc cools the arc, and when the current
| drops to zero the arc goes out.
|
| (4) Providing the resistance across the gap is big enough the arc does not
| restrike and the circuit is broken.
|
| If the fault current is higher than the fuse is designed to handle too much
| of the element will be melted before a current zero occurs and the arc will
| restrike.
|
| If the fuse is used on a higher voltage system than it is designed for the
| thinner part of the element that is intended to melt will not produce a big
| enough gap and the arc will restrike.
|
| If the arc restrikes in the fuse and keeps burning the heat will destroy the
| body of the fuse, and the arc will then continue burning outside of the fuse.
|
| In either case the result is usually considerable damage at the site of the
| fuse, and also at the site of the problem which caused the fuse to operate in
| the first place.
If the current of the arc is sufficiently high, the damage can also occur
in the wiring supplying the fuse (for a main fuse or single circuit) and
in the wiring between the fuse and the faulty load. That damage could be
as extensive as complete building destruction due to fire and the death of
a great many people inside depending on the lack of escape opportunities
(e.g. a fire at night in a home full of small children).
IMHO, fuse and breaker interruption ratings are one thing that should never
be grandfatherable. In cases where the utility increased the fault current
availability, they should pay the upgrade costs for all customers involved.
Or they should split up LV circuits when they upgrade to transformers to
deal with increased power demands.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2008-03-21-1054@ipal.net |
|------------------------------------/-------------------------------------|
| |
|
| no_one wrote:
> "mike3" <mike4ty4@yahoo.com> wrote in message
> news:f2ecd5e7-0825-47c7-a814-c8c56a4564ea@s19g2000prg.googlegroups.com...
> On Mar 20, 5:07 pm, gfretw...@aol.com wrote:
>
> If you think about it the interrupting rating is likey due to the voltage of
> the fused circuit rather than the current rating; A highly inductive
> circuit will look like a transient high voltage source and possibly arc over
> the fuse and sustain a plasma arc.
..
John Rye has a nice answer. A high current can produce an arc larger
that the fuse was designed to extinguish destroying the fuse (and a lot
more).
One of the ways to make a fuse that is safe with very high available
fault currents is for the fuse to open before the first current peak so
the fuse does not have to interrupt the full available fault current.
..
> BTW, a fuse is there to prevent the source wiring from burning up, not keep
> the load device from getting damaged; the load is already damaged if it is
> going to pull enough current if one assumes the fuse was sized correctly in
> the first place.
..
Fuses primarily protect load side wiring and distribution equipment.
They will in many cases protect load side equipment - like a stalled
motor (usually protected by means other that a fuse).
--
bud--
| |
|
| On Mar 21, 9:17 am, John Rye <j...@ukonline.co.uk> wrote:
> Hello Mike
>
> In article
> <d4c1e90d-595f-439e-b250-f8d906994...@e23g2000prf.googlegroups.com>,
> mike3 <mike4...@yahoo.com> wrote:
>
>
> There are substantial textbooks written about fuses and how they work. (For
> example "Electric Fuses" by A Wright & P G Newbury ISBN 0 906048 78 8)
>
> Operation is different on alternating current and direct current.
>
> To try and keep it simple on alternating current in correct operation :-
>
> (1) If the current is high enough for long enough it melts a short bit of
> metal which is thinner than the rest of the conductor and leaves a gap.
>
> (2) An arc forms across the gap.
>
> (3) The material surrounding the arc cools the arc, and when the current
> drops to zero the arc goes out.
>
> (4) Providing the resistance across the gap is big enough the arc does not
> restrike and the circuit is broken.
>
> If the fault current is higher than the fuse is designed to handle too much
> of the element will be melted before a current zero occurs and the arc will
> restrike.
>
> If the fuse is used on a higher voltage system than it is designed for the
> thinner part of the element that is intended to melt will not produce a big
> enough gap and the arc will restrike.
>
> If the arc restrikes in the fuse and keeps burning the heat will destroy the
> body of the fuse, and the arc will then continue burning outside of the fuse.
>
> In either case the result is usually considerable damage at the site of the
> fuse, and also at the site of the problem which caused the fuse to operate in
> the first place.
>
So then my guess was right, it fails to stop the current (therefore
defeating
the purpose of the fuse), not to mention causing direct damage due to
the
arc itself.
| |
|
| On Mar 21, 12:05 pm, bud-- <remove.budn...@isp.com> wrote:
> no_one wrote:
>
>
>
> .
> John Rye has a nice answer. A high current can produce an arc larger
> that the fuse was designed to extinguish destroying the fuse (and a lot
> more).
>
But destroying the fuse does not mean the circuit was broken, at least
not quickly enough to protect the equipment that the fuse was there
to protect, not to mention it may cause direct damage itself.
Is this right?
> One of the ways to make a fuse that is safe with very high available
> fault currents is for the fuse to open before the first current peak so
> the fuse does not have to interrupt the full available fault current.
> .> BTW, a fuse is there to prevent the source wiring from burning up, not keep
>
> .
> Fuses primarily protect load side wiring and distribution equipment.
> They will in many cases protect load side equipment - like a stalled
> motor (usually protected by means other that a fuse).
>
So what happens if the current got so high the fuse was totally
destroyed?
Would the motor still have been protected or not?
| |
| phil-news-nospam@ipal.net 2008-03-21, 5:25 pm |
| On Fri, 21 Mar 2008 11:53:40 -0700 (PDT) mike3 <mike4ty4@yahoo.com> wrote:
| So what happens if the current got so high the fuse was totally
| destroyed?
| Would the motor still have been protected or not?
The arc is still a high current channel. It is dissipating some of that
power in the form of a voltage drop over the arc channel times the current
flowing through the arc.
If there is a fault in the motor that results in the high current flow,
the exact scenario depends on the nature of that fault. If there is an
arc in the motor itself, such as in the windings, that could get very
destructive. If there is a bolted fault that is not producing an arc,
then the damage would be from the dissipation of the fault current in
the wiring. That will be less until it reaches the point where wiring
fails and an arc likely begins at that point. Or maybe that arc will
cool sufficiently and extinguish at the next zero cross.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2008-03-21-1454@ipal.net |
|------------------------------------/-------------------------------------|
| |
| phil-news-nospam@ipal.net 2008-03-21, 5:25 pm |
| On Fri, 21 Mar 2008 11:49:08 -0700 (PDT) mike3 <mike4ty4@yahoo.com> wrote:
| So then my guess was right, it fails to stop the current (therefore
| defeating
| the purpose of the fuse), not to mention causing direct damage due to
| the
| arc itself.
It would be a failure of the fuse. It would be a failure of the process
to select the proper fuse and supporting equipment to ensure that the
existing potential conditions (voltage and available fault current) can
be dealt with. But this by no means justifies omitting any kind of
current interruption capability.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2008-03-21-1502@ipal.net |
|------------------------------------/-------------------------------------|
| |
| daestrom 2008-03-21, 8:25 pm |
|
"John Rye" <jrye@ukonline.co.uk> wrote in message
news:4f83974f66jrye@ukonline.co.uk...
> Hello Mike
>
> In article
> <d4c1e90d-595f-439e-b250-f8d9069944b1@e23g2000prf.googlegroups.com>,
> mike3 <mike4ty4@yahoo.com> wrote:
>
>
>
> There are substantial textbooks written about fuses and how they work.
> (For
> example "Electric Fuses" by A Wright & P G Newbury ISBN 0 906048 78 8)
>
> Operation is different on alternating current and direct current.
>
> To try and keep it simple on alternating current in correct operation :-
>
> (1) If the current is high enough for long enough it melts a short bit of
> metal which is thinner than the rest of the conductor and leaves a gap.
>
> (2) An arc forms across the gap.
>
> (3) The material surrounding the arc cools the arc, and when the current
> drops to zero the arc goes out.
>
> (4) Providing the resistance across the gap is big enough the arc does not
> restrike and the circuit is broken.
>
> If the fault current is higher than the fuse is designed to handle too
> much
> of the element will be melted before a current zero occurs and the arc
> will
> restrike.
>
> If the fuse is used on a higher voltage system than it is designed for the
> thinner part of the element that is intended to melt will not produce a
> big
> enough gap and the arc will restrike.
>
> If the arc restrikes in the fuse and keeps burning the heat will destroy
> the
> body of the fuse, and the arc will then continue burning outside of the
> fuse.
>
> In either case the result is usually considerable damage at the site of
> the
> fuse, and also at the site of the problem which caused the fuse to operate
> in
> the first place.
>
I would add a couple of more points.
The interrupting rating needed is *not* a function of the load connected,
but of the *source*. If the source is a hi power supply that can
theoretically supply several thousand amps (i.e. it has a low internal
impedance), then protective devices such as fuses and breakers must have a
higher interrupting rating. Good design assumes that a bolted, zero-ohm
fault occurs right at the wiring leaving the fuse box and calculates the
maximum current that could occur (based on the source impedance and
voltage). Then size protective devices to interrupt that current.
Some fuses are 'slo-blow' meaning they have a larger thermal mass so it
takes more heating to melt them. These are for service where the normal
load can occasionally draw higher than fuse rating current. Such as motor
starting.
Some fuses I've used were filled with sand to increase the interrupting
rating. The idea is that when the fuse melts, bits of sand would 'fall' in
between the melted ends and help interrupt the arc. If the arc got hot
enough, the grains of sand would melt together into a crude form of glass.
A small, 10A fuse of this type that measures just 2 1/2 inches long is rated
for interrupting 10 kA.
daestrom
| |
|
| On Mar 21, 2:02=A0pm, phil-news-nos...@ipal.net wrote:
> On Fri, 21 Mar 2008 11:53:40 -0700 (PDT) mike3 <mike4...@yahoo.com> wrote:=
>
> | So what happens if the current got so high the fuse was totally
> | destroyed?
> | Would the motor still have been protected or not?
>
> The arc is still a high current channel. =A0It is dissipating some of that=
> power in the form of a voltage drop over the arc channel times the current=
> flowing through the arc.
>
So then I was right, the fuse fails to do it's job of stopping the
current.
> If there is a fault in the motor that results in the high current flow,
> the exact scenario depends on the nature of that fault. =A0If there is an
> arc in the motor itself, such as in the windings, that could get very
> destructive. =A0If there is a bolted fault that is not producing an arc,
> then the damage would be from the dissipation of the fault current in
> the wiring. =A0That will be less until it reaches the point where wiring
> fails and an arc likely begins at that point. =A0Or maybe that arc will
> cool sufficiently and extinguish at the next zero cross.
>
> --
> |---------------------------------------/---------------------------------=
-=AD|
> | Phil Howard KA9WGN (ka9wgn.ham.org) =A0/ =A0Do not send to the address b=
elow |
> | first name lower case at ipal.net =A0 / =A0spamtrap-2008-03-21-1...@ipal=
..net |
> |------------------------------------/------------------------------------=
-=AD|
| |
|
| On Mar 20, 9:24=A0pm, "Tim Perry" <timpe...@nospammeadelphia.net> wrote:
> "mike3" <mike4...@yahoo.com> wrote in message
>
> news:f2ecd5e7-0825-47c7-a814-c8c56a4564ea@s19g2000prg.googlegroups.com...
> On Mar 20, 5:07 pm, gfretw...@aol.com wrote:
>
>
>
>
> Your question can be interpreted in multiple ways. My assumption is that y=
ou
> are talking about something like getting hit by lightning as opposed to an=
> instantaneous zero ohm load.
>
Anything that would cause a sudden and enormous surge of current.
>
> Until the assembly melts.
>
But that would take longer than just the fuse wire itself melting.
>
> Wrong, fuses are to prevent fires, mostly. =A0Protecting equipment is a
> secondary effect (mostly).
>
And because of the extreme heat of the arc, it would fail to do that
too,
and perhaps make even more fires.
> =A0>Not to
>
>
>
>
> Can you provide a link to the discussions?
| |
|
| On Mar 21, 2:05=A0pm, phil-news-nos...@ipal.net wrote:
> On Fri, 21 Mar 2008 11:49:08 -0700 (PDT) mike3 <mike4...@yahoo.com> wrote:=
>
> | So then my guess was right, it fails to stop the current (therefore
> | defeating
> | the purpose of the fuse), not to mention causing direct damage due to
> | the
> | arc itself.
>
> It would be a failure of the fuse. =A0It would be a failure of the process=
> to select the proper fuse and supporting equipment to ensure that the
> existing potential conditions (voltage and available fault current) can
> be dealt with. =A0But this by no means justifies omitting any kind of
> current interruption capability.
>
Nope, I guess not, as even a chance of it working is better than none
at all.
| |
| gearhead 2008-03-21, 9:25 pm |
| On Mar 21, 4:32=A0pm, mike3 <mike4...@yahoo.com> wrote:
> On Mar 21, 2:02=A0pm, phil-news-nos...@ipal.net wrote:
>
e:[color=darkred]
>
>
at[color=darkred]
nt[color=darkred]
>
> So then I was right, the fuse fails to do it's job of stopping the
> current.
>
>
>
n[color=darkred]
[color=darkred]
[color=darkred]
>
---=AD=AD|[color=darkred]
below |[color=darkred]
al.net |[color=darkred]
---=AD=AD|- Hide quoted text -[color=darkred]
>
> - Show quoted text -
All this talky-talk has convinced you, somehow, that fuses don't
work... because the mains have low impedance, or something. I just
gotta say, what a crock.
The fuse melts when the current through it exceeds the fuse's current
rating. Doesn't take long. Now you have a gap, which interrupts the
current.
Here we get to the misleading part.
Somebody comes along and tells you'll have an unstoppable arc, because
the mains can deliver hundreds of amps, or thousands, or whatever.
Or you came up with the idea on your own, I don't know. Whatever.
It ain't gonna happen unless you have the VOLTAGE to make an arc jump
that gap. And if you have the right fuse installed it has a current
rating which -- wait for it -- exceeds the voltage available from the
mains! What a concept!
Install a fuse with the proper ratings, and it will work.
| |
| Beachcomber 2008-03-21, 9:25 pm |
|
>Some fuses I've used were filled with sand to increase the interrupting
>rating. The idea is that when the fuse melts, bits of sand would 'fall' in
>between the melted ends and help interrupt the arc. If the arc got hot
>enough, the grains of sand would melt together into a crude form of glass.
>A small, 10A fuse of this type that measures just 2 1/2 inches long is rated
>for interrupting 10 kA.
>
Years ago, organizations such as UL and the National Fire Protection
Association did studies on thousands of homes with fuses vs. circuits
breakers. The bottom line was that the houses with fuse boxes were
more likely to burn down in an electrical fire vs. the homes with
circuit breaker panels.
Now that could be because the wiring was older in the fusebox homes,
or the owners were so stupid that they were more likely to insert
objects such as coins when they ran short of replacement fuses, etc.
But, for the most part, if the circumstances are not right, it is
possible for even the standard Edison-base safety fuses to explode
during a severe short or overload and melt their holders. If the arc
cannot be contained, wires can melt and damage can extend beyond the
fuse box.
Fortunately, this is rare... but it is also among the reasons that
circuit breakers are considered the more modern and safe choice.
| |
| Martin Crossley 2008-03-22, 3:25 am |
| http://www.era.co.uk:80/services/devices.asp
Half way down the page is a video clip of a fuse failing to meet BS1362 for
fuses in BS1363 / IS401 (British, Irish, etc.) plugs.
(Warning: There are now lots of (mainly Chinese-made) travel
adaptors/adapters on sale worldwide to fit these sockets that don't even
have a fuse!)
Martin.
| |
| phil-news-nospam@ipal.net 2008-03-22, 3:25 am |
| On Fri, 21 Mar 2008 16:32:25 -0700 (PDT) mike3 <mike4ty4@yahoo.com> wrote:
| On Mar 21, 2:02?pm, phil-news-nos...@ipal.net wrote:
|> On Fri, 21 Mar 2008 11:53:40 -0700 (PDT) mike3 <mike4...@yahoo.com> wrote:
|>
|> | So what happens if the current got so high the fuse was totally
|> | destroyed?
|> | Would the motor still have been protected or not?
|>
|> The arc is still a high current channel. ?It is dissipating some of that
|> power in the form of a voltage drop over the arc channel times the current
|> flowing through the arc.
|>
|
| So then I was right, the fuse fails to do it's job of stopping the
| current.
Indeed. It can happen. FYI, I have personally seen a circuit breaker
fail to trip on solid short circuit. After the short itself opened a
couple seconds later, the lights came back up (so I know it was not a
case of tripping and arcing).
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2008-03-21-2309@ipal.net |
|------------------------------------/-------------------------------------|
| |
| phil-news-nospam@ipal.net 2008-03-22, 3:25 am |
| On Fri, 21 Mar 2008 16:38:07 -0700 (PDT) mike3 <mike4ty4@yahoo.com> wrote:
| On Mar 21, 2:05?pm, phil-news-nos...@ipal.net wrote:
|> On Fri, 21 Mar 2008 11:49:08 -0700 (PDT) mike3 <mike4...@yahoo.com> wrote:
|>
|> | So then my guess was right, it fails to stop the current (therefore
|> | defeating
|> | the purpose of the fuse), not to mention causing direct damage due to
|> | the
|> | arc itself.
|>
|> It would be a failure of the fuse. ?It would be a failure of the process
|> to select the proper fuse and supporting equipment to ensure that the
|> existing potential conditions (voltage and available fault current) can
|> be dealt with. ?But this by no means justifies omitting any kind of
|> current interruption capability.
|>
|
| Nope, I guess not, as even a chance of it working is better than none
| at all.
In cases where the wiring is done correctly, and the proper fuse socket is
installed that only admits the proper fuse time, and that proper fuse type
is installed, the only failure mode I can imagine is a manufacturing defect
in the fuse. And I have never heard of that happening in fuses. Cases I
have heard of fuses failing are when some rating of the fuse (the voltage
or the fault current) is exceeded (and then it's a failure of who selected
the wrong fuse). OTOH, I have personally experienced a circuit breaker
failure.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2008-03-21-2312@ipal.net |
|------------------------------------/-------------------------------------|
| |
| phil-news-nospam@ipal.net 2008-03-22, 3:25 am |
| On Sat, 22 Mar 2008 02:10:53 GMT Beachcomber <invalid@notreal.none> wrote:
|
|>Some fuses I've used were filled with sand to increase the interrupting
|>rating. The idea is that when the fuse melts, bits of sand would 'fall' in
|>between the melted ends and help interrupt the arc. If the arc got hot
|>enough, the grains of sand would melt together into a crude form of glass.
|>A small, 10A fuse of this type that measures just 2 1/2 inches long is rated
|>for interrupting 10 kA.
|>
|
| Years ago, organizations such as UL and the National Fire Protection
| Association did studies on thousands of homes with fuses vs. circuits
| breakers. The bottom line was that the houses with fuse boxes were
| more likely to burn down in an electrical fire vs. the homes with
| circuit breaker panels.
I'd still feel a bit safer if I had protection on the main entrance by
both (with appropriate or excessive interruption rating, of course).
The question I've pondered is, if it is better to have the main breaker
first followed by the fuses, or have the fuses first followed by the
breaker. I suspect the former might be a bit safer than the latter as
it gives you an extra disconnect to completely de-energize the box when
replacing a fuse.
| Now that could be because the wiring was older in the fusebox homes,
| or the owners were so stupid that they were more likely to insert
| objects such as coins when they ran short of replacement fuses, etc.
Don't people get shock out of that?
I do know my dad has inserted the wrong size fuse, before.
| But, for the most part, if the circumstances are not right, it is
| possible for even the standard Edison-base safety fuses to explode
| during a severe short or overload and melt their holders. If the arc
| cannot be contained, wires can melt and damage can extend beyond the
| fuse box.
|
| Fortunately, this is rare... but it is also among the reasons that
| circuit breakers are considered the more modern and safe choice.
the only failure I've ever experienced of an OCPD is a breaker, not a
fuse. But my experiences are not statistically significant.
FYI, I do make sure all the breakers where I live right now (my dad's
house) get flipped off and back on at least once a year. The power goes
out often enough for that to be no additional inconvenience.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2008-03-21-2319@ipal.net |
|------------------------------------/-------------------------------------|
| |
| Beachcomber 2008-03-22, 3:25 am |
|
>| Fortunately, this is rare... but it is also among the reasons that
>| circuit breakers are considered the more modern and safe choice.
>
>the only failure I've ever experienced of an OCPD is a breaker, not a
>fuse. But my experiences are not statistically significant.
>
Breakers are not necessarily intrinsically safer than fuses... This
brings to mind a somewhat infamous example of slipshod manufacturing
and safety testing.
There was a whole series of one brand of breakers (Federal Pacific, I
think - Google to check it out) that, more often than not, simply
failed to open during an overload...
Here is one link detailing a similar problem:
http://www.inspect-ny.com/fpe/fpepanel.htm
| |
| Long Ranger 2008-03-22, 9:25 am |
| If you blow a fuse really hard, your cheeks will puff out, and you will
become dizzy if you do it for very long. Try using compressed air.
| |
|
| On Mar 21, 1:49 pm, mike3 <mike4...@yahoo.com> wrote:
> So then my guess was right, it fails to stop the current (therefore
> defeating the purpose of the fuse), not to mention causing direct
> damage due to the arc itself.
Ambiguous is the definition of 'hard'. What is 'hard' on a fuse is
voltage that a fuse must interrupt. All fuses have a maximum
voltage. If that voltage is too high, then arcing problems, discussed
by John Rye, means that fuse may not protect from fire.
For example two cylinder type fuses appear same. The 3AG fuse once
used in autos is only rated to interrupt 32 volts. A standard line
fuse for appliance operation (same physical dimensions) must be rated
to interrupt 250 volts. Using that old style 32 volt automobile fuse
in an appliance may not properly interrupt AC mains voltage for
reasons cited by John Rye. IOW interrupting 120 VAC with a 32 volt
automotive fuse could be 'too hard' on that fuse.
| |
|
| daestrom wrote:
>
> "John Rye" <jrye@ukonline.co.uk> wrote in message
> news:4f83974f66jrye@ukonline.co.uk...
>
> I would add a couple of more points.
>
> The interrupting rating needed is *not* a function of the load
> connected, but of the *source*. If the source is a hi power supply that
> can theoretically supply several thousand amps (i.e. it has a low
> internal impedance), then protective devices such as fuses and breakers
> must have a higher interrupting rating. Good design assumes that a
> bolted, zero-ohm fault occurs right at the wiring leaving the fuse box
> and calculates the maximum current that could occur (based on the source
> impedance and voltage). Then size protective devices to interrupt that
> current.
>
> Some fuses are 'slo-blow' meaning they have a larger thermal mass so it
> takes more heating to melt them. These are for service where the normal
> load can occasionally draw higher than fuse rating current. Such as
> motor starting.
>
> Some fuses I've used were filled with sand to increase the interrupting
> rating. The idea is that when the fuse melts, bits of sand would 'fall'
> in between the melted ends and help interrupt the arc. If the arc got
> hot enough, the grains of sand would melt together into a crude form of
> glass. A small, 10A fuse of this type that measures just 2 1/2 inches
> long is rated for interrupting 10 kA.
>
> daestrom
>
The issue, for the questions of the OP, is not the current or voltage
rating of the fuse. It is the fuse rating for the available fault
current of the supply, as daestrom wrote.
Fuses (and circuit breakers) meant for use in power circuits [US] will
have a rating for available fault current. Fuses are readily available
[US] that can be used on circuits with an available fault current of
200,000A. FRN fuses from Buss are one type (time delay, relatively
inexpensive, widely available). As I wrote previously, these fuses will
open far before the first current peak approaches 200,000A. Fuses that
open that fast are called "current limiting". Circuits are designed so
the the current peak that can get through the fuse is safe for what is
connected downstream.
If you have a point in the power system that has an available fault
current of 50,000A and install a fuse rated for 5,000A, with a heavy
fault the fuse and a considerable amount of surrounding equipment may
disappear. That is not a failure of the fuse. It is a failure of the
design or maintenance that allowed a fuse with inappropriate ratings to
be used.
--
bud--
| |
|
| Beachcomber wrote:
> Breakers are not necessarily intrinsically safer than fuses... This
> brings to mind a somewhat infamous example of slipshod manufacturing
> and safety testing.
>
> There was a whole series of one brand of breakers (Federal Pacific, I
> think - Google to check it out) that, more often than not, simply
> failed to open during an overload...
>
> Here is one link detailing a similar problem:
> http://www.inspect-ny.com/fpe/fpepanel.htm
>
Not just slipshod testing. FPE sent fraudulent test information to UL.
This was discovered after FPE was sold to Reliance. Reliance informed UL
and most of the FPE line was delisted. Reliance sued the company that
sold FPE and got a lot of money to cover liability.
--
bud--
| |
| Tim Perry 2008-03-23, 1:25 pm |
| > > On Thu, 20 Mar 2008 13:51:50 -0700 (PDT), mike3 <mike4...@yahoo.com>
>
>
>
you[color=darkred]
> are talking about something like getting hit by lightning as opposed to an
> instantaneous zero ohm load.
>
>Anything that would cause a sudden and enormous surge of current.
>
>
>But that would take longer than just the fuse wire itself melting.
>
> secondary effect (mostly).
>
>And because of the extreme heat of the arc, it would fail to do that
too,
and perhaps make even more fires.
You are making an assumption based on broadly defined criteria.
No meaningful answer can be given except of the most general nature.
Either the fuse will function or it wont. If the fuse does not function it
was the wrong type/size for the application.
| |
| Paul Hovnanian P.E. 2008-03-24, 3:25 am |
| daestrom wrote:
>
[snip]
>
> I would add a couple of more points.
>
> The interrupting rating needed is *not* a function of the load connected,
> but of the *source*. If the source is a hi power supply that can
> theoretically supply several thousand amps (i.e. it has a low internal
> impedance), then protective devices such as fuses and breakers must have a
> higher interrupting rating. Good design assumes that a bolted, zero-ohm
> fault occurs right at the wiring leaving the fuse box and calculates the
> maximum current that could occur (based on the source impedance and
> voltage). Then size protective devices to interrupt that current.
Now, try to teach this to a utility that serves its customers from a
networked secondary system or with primary service. Back when a customer
originally requested service, the fault current available on a 480V
service might have been around 50 kA. Their service equipment was sized
to interrupt this level of fault current.
As the years go by and more capacity is added, the fault current
available can go up to 100 or 150 kA. Does anyone bother to notify the
customers? Not as far as I've seen.
The problem is either caught when someone does a major remodel,
requiring the service equipment design to be revisited, or when the
service equipment (and sometimes the building) fails catastrophically.
--
Paul Hovnanian mailto:Paul@Hovnanian.com
------------------------------------------------------------------
I could get a new lease on life but I need the first and last month
in advance.
| |
| grinder@oh-yoohoo.edu 2008-03-24, 5:25 pm |
| It will buy you dinner.
| |
| phil-news-nospam@ipal.net 2008-03-24, 9:25 pm |
| On Sun, 23 Mar 2008 19:32:15 -0800 Paul Hovnanian P.E. <paul@hovnanian.com> wrote:
| The problem is either caught when someone does a major remodel,
| requiring the service equipment design to be revisited, or when the
| service equipment (and sometimes the building) fails catastrophically.
And the lawyers get rich.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2008-03-24-2106@ipal.net |
|------------------------------------/-------------------------------------|
| |
| phil-news-nospam@ipal.net 2008-03-24, 9:25 pm |
| On Thu, 20 Mar 2008 13:51:50 -0700 (PDT) mike3 <mike4ty4@yahoo.com> wrote:
| What happens if one were to overload a fuse so strongly the fuse's
| maximum safe interrupting rating was exceeded? My guess is that it
| would start arcing enough to allow high current through the device it
| is protecting for a (relatively) protracted period of time, therefore
| potentially causing damage to it (and hence defeating the purpose of
| the fuse, which is to _protect_ the device from damage). Not to
| mention the arc itself might be damaging as well (due to the high
| temperature.). Is this right?
|
| Is there a video or something showing what happens?
You've gotten a lot of answer so far. Obviously some good and some bad.
Now it's back in your court. Let me give this one suggestion: make the
video in MPEG or Theora format.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2008-03-24-2108@ipal.net |
|------------------------------------/-------------------------------------|
| |
| Paul Hovnanian P.E. 2008-03-26, 8:25 pm |
| Long Ranger wrote:
>
> If you blow a fuse really hard, your cheeks will puff out, and you will
> become dizzy if you do it for very long. Try using compressed air.
The most dangerous aspect of this is when you inhale before blowing. If
you aren't careful, you could swallow the fuse.
--
Paul Hovnanian mailto:Paul@Hovnanian.com
------------------------------------------------------------------
It's a damn poor mind that can only think of one way to spell a word.
-- Andrew Jackson
| |
| Ben Miller 2008-03-27, 3:25 am |
| Paul Hovnanian P.E. wrote:
>
> Now, try to teach this to a utility that serves its customers from a
> networked secondary system or with primary service. Back when a
> customer originally requested service, the fault current available on
> a 480V service might have been around 50 kA. Their service equipment
> was sized to interrupt this level of fault current.
>
> As the years go by and more capacity is added, the fault current
> available can go up to 100 or 150 kA. Does anyone bother to notify the
> customers? Not as far as I've seen.
>
> The problem is either caught when someone does a major remodel,
> requiring the service equipment design to be revisited, or when the
> service equipment (and sometimes the building) fails catastrophically.
I suspect this is a much bigger problem than people realize. Even some
residential areas have increased as more & bigger homes are added. Your
house might be on the same pole pig, but the primary side could have much
more available SCC than it used to. Even if the utility informed customers,
how many companies or homeowners are going to replace entire service
panels/switchboards? As you said, it will only get done when it is upgraded
or redesigned for some other reason or when it fails.
--
Benjamin D Miller, PE
www.bmillerengineering.com
| |
| John Rye 2008-03-27, 9:25 am |
| Hello Paul
In article <47E720BF.B1E5FC19@hovnanian.com>,
Paul Hovnanian P.E. <paul@hovnanian.com> wrote:
> daestrom wrote:
> [snip]
[color=darkred]
> Now, try to teach this to a utility that serves its customers from a
> networked secondary system or with primary service. Back when a customer
> originally requested service, the fault current available on a 480V
> service might have been around 50 kA. Their service equipment was sized
> to interrupt this level of fault current.
> As the years go by and more capacity is added, the fault current
> available can go up to 100 or 150 kA. Does anyone bother to notify the
> customers? Not as far as I've seen.
> The problem is either caught when someone does a major remodel,
> requiring the service equipment design to be revisited, or when the
> service equipment (and sometimes the building) fails catastrophically.
>
If these figures are correct there appears to be a big difference between Uk
& US practice. The vast majority of domestic/small commercial supplies in the
UK will be 230V phase to neutral from a 3 phase transformer with a maximum
size of 1 MVA, and an impedance of about 4.5%. This gives a maximum potential
short-circuit current of 33 kA, which will be reduced by HV supply impedance,
and the cable between the transformer and the customer.
At the end of the utility cable supplying the customer will be a fuse box
containing current limiting fuses rated for this short-circuit current. This
does not rule out the possibility that the customers switchgear is
inadequate, but it does provide a back stop in that if the customers's
switchgear fails the utilities fuses will usually clear the fault.
John
--
John Rye
Hadleigh IPSWICH England
<http://web.ukonline.co.uk/jrye/index.html>
---< On Line using an Acorn StrongArm RiscPC >---
| |
| phil-news-nospam@ipal.net 2008-03-27, 9:25 am |
| On Wed, 26 Mar 2008 22:27:46 -0500 Ben Miller <Ben@somewhere> wrote:
| Paul Hovnanian P.E. wrote:
|
|>
|> Now, try to teach this to a utility that serves its customers from a
|> networked secondary system or with primary service. Back when a
|> customer originally requested service, the fault current available on
|> a 480V service might have been around 50 kA. Their service equipment
|> was sized to interrupt this level of fault current.
|>
|> As the years go by and more capacity is added, the fault current
|> available can go up to 100 or 150 kA. Does anyone bother to notify the
|> customers? Not as far as I've seen.
|>
|> The problem is either caught when someone does a major remodel,
|> requiring the service equipment design to be revisited, or when the
|> service equipment (and sometimes the building) fails catastrophically.
|
| I suspect this is a much bigger problem than people realize. Even some
| residential areas have increased as more & bigger homes are added. Your
| house might be on the same pole pig, but the primary side could have much
| more available SCC than it used to. Even if the utility informed customers,
| how many companies or homeowners are going to replace entire service
| panels/switchboards? As you said, it will only get done when it is upgraded
| or redesigned for some other reason or when it fails.
And whose responsibility is it to pay for that upgrade "out of the blue"
when the utility decides to save their own money by increasing the size
of the transformer somewhere along the line, rather than splitting up the
services among smaller transformers? Or, another way, when it turns out
a home burns down because the OCPD failed due to the now higher available
fault current, but was correctly sized when installed, and this is due to
the fact the owner could not afford the upgrade right away even though he
was notified by the utility. What if someone dies in that event?
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2008-03-27-0825@ipal.net |
|------------------------------------/-------------------------------------|
| |
| phil-news-nospam@ipal.net 2008-03-27, 9:25 am |
| On Wed, 26 Mar 2008 16:16:50 -0800 Paul Hovnanian P.E. <paul@hovnanian.com> wrote:
| Long Ranger wrote:
|>
|> If you blow a fuse really hard, your cheeks will puff out, and you will
|> become dizzy if you do it for very long. Try using compressed air.
|
| The most dangerous aspect of this is when you inhale before blowing. If
| you aren't careful, you could swallow the fuse.
Just turn your head sideways before inhaling. I found it works quite well.
I'd be more worried about what diseases the fuse might have.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2008-03-27-0834@ipal.net |
|------------------------------------/-------------------------------------|
| |
|
| John Rye wrote:
> Hello Paul
>
> In article <47E720BF.B1E5FC19@hovnanian.com>,
> Paul Hovnanian P.E. <paul@hovnanian.com> wrote:
>
>
>
> If these figures are correct there appears to be a big difference between Uk
> & US practice. The vast majority of domestic/small commercial supplies in the
> UK will be 230V phase to neutral from a 3 phase transformer with a maximum
> size of 1 MVA, and an impedance of about 4.5%. This gives a maximum potential
> short-circuit current of 33 kA, which will be reduced by HV supply impedance,
> and the cable between the transformer and the customer.
>
> At the end of the utility cable supplying the customer will be a fuse box
> containing current limiting fuses rated for this short-circuit current. This
> does not rule out the possibility that the customers switchgear is
> inadequate, but it does provide a back stop in that if the customers's
> switchgear fails the utilities fuses will usually clear the fault.
>
Is it normal UK practice to install fuses at the source end of the
utility cable?
"Current limiting" fuses should also reduce the available fault current
downstream.
A replacement service downtown [US]was a mere 800A. Right the other side
of the basement wall was a utility transformer vault for part of a
downtown block. The utility said the available fault current was 200,000A.
All the service cable connections in the vault were "cable limiters" - a
fuse crimped to the wire on one end and a lug bolted to the vault
bussbars on the other end. I have never seen information on the cable
limiters - probably "current limiting".
--
bud--
| |
| Ben Miller 2008-03-28, 3:25 am |
| bud-- wrote:
> A replacement service downtown [US]was a mere 800A. Right the other
> side of the basement wall was a utility transformer vault for part of
> a downtown block. The utility said the available fault current was
> 200,000A. All the service cable connections in the vault were "cable
> limiters" - a fuse crimped to the wire on one end and a lug bolted to
> the vault bussbars on the other end. I have never seen information on
> the cable limiters - probably "current limiting".
Just think of them as big fuses designed for short-circuit protection only.
They are generally used on parallel sets of lines, so a fault on one line
will open its limiter, removing that line from the system. This prevents
more catastrophic damage. The service remains on, but that phase is then
running with one less conductor.
--
Benjamin D Miller, PE
www.bmillerengineering.com
| |
| Paul Hovnanian P.E. 2008-03-28, 1:25 pm |
| Ben Miller wrote:
>
> Paul Hovnanian P.E. wrote:
>
>
> I suspect this is a much bigger problem than people realize. Even some
> residential areas have increased as more & bigger homes are added. Your
> house might be on the same pole pig, but the primary side could have much
> more available SCC than it used to. Even if the utility informed customers,
> how many companies or homeowners are going to replace entire service
> panels/switchboards? As you said, it will only get done when it is upgraded
> or redesigned for some other reason or when it fails.
Back when I was a distribution engineer, my standard practice was to
calculate the secondary fault current available based on a distribution
transformer located just outside our largest standard substation. Even
if the customer was 20 miles down a farm road. One never knows if the
next big substation will go in just across the street from him.
But this wasn't standard practice across the company. And for larger
commercial/industrial customers, they need more information than just a
maximum fault current level. Any significant change to the system needs
to be looked at. But when I'd call a customer about a system upgrade (a
12 kV to 34.5 kV cutover for example) and suggest they forward it on to
an engineer, I was usually met with a blank stare. Even some primary
service customers, who are supposed to have "qualified" personnel
operate and maintain their systems had no one on call. Some of our
linemen made good money on the side, as they were the only people in
town with the tools and know-how to do H.V. maintenance.
> --
> Benjamin D Miller, PE
> www.bmillerengineering.com
--
Paul Hovnanian paul@hovnanian.com
-----------------------------------------------------------------------
Procrastinators: The leaders for tomorrow.
| |
| Paul Hovnanian P.E. 2008-03-28, 1:25 pm |
| phil-news-nospam@ipal.net wrote:
>
> On Wed, 26 Mar 2008 16:16:50 -0800 Paul Hovnanian P.E. <paul@hovnanian.com> wrote:
> | Long Ranger wrote:
> |>
> |> If you blow a fuse really hard, your cheeks will puff out, and you will
> |> become dizzy if you do it for very long. Try using compressed air.
> |
> | The most dangerous aspect of this is when you inhale before blowing. If
> | you aren't careful, you could swallow the fuse.
>
> Just turn your head sideways before inhaling. I found it works quite well.
> I'd be more worried about what diseases the fuse might have.
Always use protection. I guess this is what they mean by protection
coordination.
--
Paul Hovnanian paul@hovnanian.com
-----------------------------------------------------------------------
Procrastinators: The leaders for tomorrow.
| |
|
| Ben Miller wrote:
> bud-- wrote:
>
> Just think of them as big fuses designed for short-circuit protection only.
> They are generally used on parallel sets of lines, so a fault on one line
> will open its limiter, removing that line from the system. This prevents
> more catastrophic damage. The service remains on, but that phase is then
> running with one less conductor.
>
But for a parallel run the wire is still backfed from the load end
(unless there are cable limiters on both ends). The utility is protected
but I would think the customer would still have a failure.
And opening one wire puts the load on the other parallel runs (but what
better option).
Not obvious to me why there is more advantage on parallel runs than
single unless because the wires are usually larger.
-----------------
The building where the service was replaced had burned down so only the
basement and 1st floor slab remained. That was used for parking. The
original 208/120 service in the back corner remained - 6 parallel runs.
Someone had the bright idea of storing snow melting salt on top of the
switchgear. In the resulting burndown some of the wires burned back into
the service conduits. At least a couple of the wires welded to the
conduit - the utility broke a come-along trying to pull one. Some of the
limiters didn't blow - must have burned free. But the utility was protected.
--
bud--
| |
| Paul Hovnanian P.E. 2008-03-28, 8:25 pm |
| bud-- wrote:
>
> John Rye wrote:
> Is it normal UK practice to install fuses at the source end of the
> utility cable?
> "Current limiting" fuses should also reduce the available fault current
> downstream.
>
> A replacement service downtown [US]was a mere 800A. Right the other side
> of the basement wall was a utility transformer vault for part of a
> downtown block. The utility said the available fault current was 200,000A.
> All the service cable connections in the vault were "cable limiters" - a
> fuse crimped to the wire on one end and a lug bolted to the vault
> bussbars on the other end. I have never seen information on the cable
> limiters - probably "current limiting".
Its possible to install current limiting fuses ahead of service
equipment that would not, by itself, withstand an increase in available
fault duty. But, from what I've seen, utilities rarely know when their
upgrades will push customer equipment beyond its rating.
Utilities I've worked with do not keep track of customer facility
specifications. If a knowledgeable customer gets wind of an upgrade,
they may do a coordination study and install CL fuses (rather than
upgrading the service equipment). But very few customers are
knowledgeable and most, if warned, wouldn't understand the consequences
of ignoring the heads up.
--
Paul Hovnanian mailto:Paul@Hovnanian.com
------------------------------------------------------------------
Rube Goldberg is alive and working for Microsoft.
| |
|
| From: (mike3)
Hi.
What happens if one were to overload a fuse so strongly the fuse's
maximum safe interrupting rating was exceeded? My guess is that it would
start arcing enough to allow high current through the device it is
protecting for a (relatively) protracted period of time, therefore
potentially causing damage to it (and hence defeating the purpose of the
fuse, which is to _protect_ the device from damage). Not to mention the
arc itself might be damaging as well (due to the high temperature.). Is
this right?
Is there a video or something showing what happens?
--------------------------------
{ All the on going discussions aside & there valid points }
If you were able to get a micro camera inside device boxes
& conduit we would see the wires tense and pull about when ever there is
a trip/short or fuse blow out - to what extent varies, but it can
actually create a pull strong enough to dislodge wire connections
causing further trouble in the circuit., in time enough of those Hard
Blows and it's bound to happen .....
I once found a loose wire connection in a perfectly
closed heating circuit that had tript several times from an overloaded
condition., they corrected the overload, but were stumped as to why it
still flickerd and tripped when the circuit was re-established within
tolerance - When I traced & checked the adjacent gang box I found
several loose wires proceedng from the wire caps & the one flickering
was almost fully dislodged.....so probe & troubleshoot around some
more.. or call me }
Roy Q.T. ~ US/NCU ~ E.E. Technician
[have tools, will travel]
| |
| phil-news-nospam@ipal.net 2008-03-29, 1:25 pm |
| On Fri, 28 Mar 2008 11:04:35 -0700 Paul Hovnanian P.E. <paulh@seanet.com> wrote:
| Ben Miller wrote:
|>
|> Paul Hovnanian P.E. wrote:
|>
|> >
|> > Now, try to teach this to a utility that serves its customers from a
|> > networked secondary system or with primary service. Back when a
|> > customer originally requested service, the fault current available on
|> > a 480V service might have been around 50 kA. Their service equipment
|> > was sized to interrupt this level of fault current.
|> >
|> > As the years go by and more capacity is added, the fault current
|> > available can go up to 100 or 150 kA. Does anyone bother to notify the
|> > customers? Not as far as I've seen.
|> >
|> > The problem is either caught when someone does a major remodel,
|> > requiring the service equipment design to be revisited, or when the
|> > service equipment (and sometimes the building) fails catastrophically.
|>
|> I suspect this is a much bigger problem than people realize. Even some
|> residential areas have increased as more & bigger homes are added. Your
|> house might be on the same pole pig, but the primary side could have much
|> more available SCC than it used to. Even if the utility informed customers,
|> how many companies or homeowners are going to replace entire service
|> panels/switchboards? As you said, it will only get done when it is upgraded
|> or redesigned for some other reason or when it fails.
|
| Back when I was a distribution engineer, my standard practice was to
| calculate the secondary fault current available based on a distribution
| transformer located just outside our largest standard substation. Even
| if the customer was 20 miles down a farm road. One never knows if the
| next big substation will go in just across the street from him.
|
| But this wasn't standard practice across the company. And for larger
| commercial/industrial customers, they need more information than just a
| maximum fault current level. Any significant change to the system needs
| to be looked at. But when I'd call a customer about a system upgrade (a
| 12 kV to 34.5 kV cutover for example) and suggest they forward it on to
| an engineer, I was usually met with a blank stare. Even some primary
| service customers, who are supposed to have "qualified" personnel
| operate and maintain their systems had no one on call. Some of our
| linemen made good money on the side, as they were the only people in
| town with the tools and know-how to do H.V. maintenance.
One of those MBA type people, had they known what you did, might have been
able to accuse you of not saving the company as much money as you could.
Never mind the liabilities you avoided ... or even the higher future costs.
As for H.V. maintenance ... I watched a substation near here being upgraded
for a couple months period along my daily drive route to work. It appears
all the work was contracted out. Only a couple times did I see AEP trucks
there. The rest of the time was one or more of about 3 different private
contracting companies. I suppose the industrial customers could have used
them as well.
I sure don't want the hassles of having 12 kV to 34.5 kV coming in to my
house :-) I'll stick with the "under 600 volt" service as long as the
available fault current stays under 22kA (the rating on my panel main).
I'm on a 100 kVA pad with 2 or 3 other houses (it's how to tell how the
two pads in the neighborhood are exactly devided up without calling in
the guys that come spray paint red lines all over the yards). It would
have to be under 2% to exceed my ratings, assuming infinite distribution
fault current and superconductor underground service. I do get noticeable
light dimming all over the house when my A/C compressor starts, lately, so
I've probably got plenty of impedance.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2008-03-29-0910@ipal.net |
|------------------------------------/-------------------------------------|
| |
| Ben Miller 2008-03-31, 3:25 am |
| bud-- wrote:
> But for a parallel run the wire is still backfed from the load end
> (unless there are cable limiters on both ends). The utility is
> protected but I would think the customer would still have a failure.
>
Yes, you need them at both ends in order to isolate the cable.
> And opening one wire puts the load on the other parallel runs (but
> what better option).
Exactly. Of course, without some type of fault detector, nobody is aware
that a fault occured or that the service is not running on the full
conductor ampacity, so if it is heavily loaded it is likely to fail over the
long term anyhow.
> Not obvious to me why there is more advantage on parallel runs than
> single unless because the wires are usually larger.
They protect the cable in either case. However, with parallel conductors
they add the function of isolating the one faulted conductor, and allowing
continued operation. I have never seen them on a single service conductor.
Here is the Bussman info: http://www.bussmann.com/pdf/1042.pdf
--
Benjamin D Miller, PE
www.bmillerengineering.com
| |
| Paul Hovnanian P.E. 2008-03-31, 5:25 pm |
| bud-- wrote:
>
> Ben Miller wrote:
> But for a parallel run the wire is still backfed from the load end
> (unless there are cable limiters on both ends). The utility is protected
> but I would think the customer would still have a failure.
>
> And opening one wire puts the load on the other parallel runs (but what
> better option).
>
> Not obvious to me why there is more advantage on parallel runs than
> single unless because the wires are usually larger.
Larger? The individual conductors of a parallel run would be smaller
than a single conductor.
That is one reason to use cable limiters. For a single conductor (per
phase), one can protect them with a transformer primary side fuse. On
the other hand, each conductor of a paralleled group is significantly
smaller than the total service rating.
In the event than one of the group opens, the remaining conductors will
carry more load current, even though the upstream protection will 'see'
nothing unusual. Once the load current exceeds the cable limiters
rating, it will open. As the load current shifts to the remaining
cables, their limiters will open as well.
--
Paul Hovnanian paul@hovnanian.com
-----------------------------------------------------------------------
Procrastinators: The leaders for tomorrow.
| |
|
| Paul Hovnanian P.E. wrote:
> bud-- wrote:
>
> Larger? The individual conductors of a parallel run would be smaller
> than a single conductor.
I did not clearly state what I mean. The wire for parallel runs is
likely to be larger than a different run that was just single. The idea
was parallel runs are near the max size wire that is economical, but a
single run is not because the loads vary from small to large.
>
> That is one reason to use cable limiters. For a single conductor (per
> phase), one can protect them with a transformer primary side fuse. On
> the other hand, each conductor of a paralleled group is significantly
> smaller than the total service rating.
If a transformer feeds only one set of wires, the transformer primary
fuse may provide short circuit protection. Utility transformers often
feed multiple services.
The one place I saw cable limiters was in a downtown utility transformer
vault. The 800A service I was working on was almost trivial compared to
the transformer rating. But if there was a fault on my wires why
couldn't it propagate into the vault and cause some real excitement? Why
wouldn't cable limiters (utility end only) be useful on my single set of
conductors?
>
> In the event than one of the group opens, the remaining conductors will
> carry more load current, even though the upstream protection will 'see'
> nothing unusual. Once the load current exceeds the cable limiters
> rating, it will open. As the load current shifts to the remaining
> cables, their limiters will open as well.
>
Ben, and the Buss data sheets he provided [thanks], say cable limiters
are for short circuit protection. The curves on the data sheets do not
go out to a long enough time to determine what kind of overload
protection the cable limiters provide.
(The data sheets show those limiters are current limiting.)
--
bud--
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