Electrical code Compliance

Author

Fuse ratings

gab

2005-06-17, 11:31 pm

Hy guys,
the question is: why fuses are rated with voltage values too?
Isn't it enough to indicate rated current?
In fact I can use a 250V fuse also on a 400V supplied circuit.
Any suggestion?
Thanx
BR

Dale Farmer

2005-06-17, 11:31 pm

gab wrote:

quote:

> Hy guys,
> the question is: why fuses are rated with voltage values too?
> Isn't it enough to indicate rated current?
> In fact I can use a 250V fuse also on a 400V supplied circuit.
> Any suggestion?
> Thanx
> BR

The voltage ratings also specify AC or DC. It has to do with
the interrupting capacity of the fuse. When the fuse blows under
a dead short, can the fuse actually stop the current flow? Of does
the arc formed at the moment it blows just keep the current flowing?
That's why a fuse ( to pick some numbers out of my XXX) that is rated
to stop 600V AC is only rated to stop 200V DC. DC arcs, are for
some reason I never understood, harder to stop. The higher the
voltage, the longer the arc.

--Dale

phil-news-nospam@ipal.net

2005-06-17, 11:31 pm

On 16 May 2005 06:49:02 -0700 gab <gabrio.spinelli@gmail.com> wrote:

| the question is: why fuses are rated with voltage values too?
| Isn't it enough to indicate rated current?
| In fact I can use a 250V fuse also on a 400V supplied circuit.

There is also another rating to be concerned with: interruption current

A voltage higher than the fuse is rated for can result in arcing,
possibly sustained, across between the otherwise broken elements.
It can also result in explosive destruction of the fuse and damage
to the fuse holding equipment.

Generally, a lot of margin exists in these ratings, so a 250 volt fuse
is probably tested at 500 volts or more. Reasons behind this include
being sure the 250 volt level is always met even with some variations
in manufacturing, as well as handling unexpected (but often seen) rises
in voltage (surges) that can cause overcurrent situations (e.g. the
fuse is more likely called upon to interrupt during these times). So
you could very well routinely see a 250 volt fuse work fine at 400 volts.

But are you just seeing it work with the circuit working fine, or are
you actually testing the fuse on a 400 volt circuit under situations
where the fuse is expected to blow? What will a 50 amp 250 volt fuse
rated for 25000 amps interruption current do when subjected to 22000
amps on a 400 volt circuit?

Do you know what your available fault current is? Do you know what
you system supply impedance is?

--
-----------------------------------------------------------------------------
| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
| (first name) at ipal.net | http://phil.ipal.org/ http://ka9wgn.ham.org/ |
-----------------------------------------------------------------------------

Spehro Pefhany

2005-06-17, 11:31 pm

On 16 May 2005 06:49:02 -0700, the renowned "gab"
<gabrio.spinelli@gmail.com> wrote:

quote:

>Hy guys,
>the question is: why fuses are rated with voltage values too?
>Isn't it enough to indicate rated current?
>In fact I can use a 250V fuse also on a 400V supplied circuit.
>Any suggestion?
>Thanx
>BR

Because a fuse that is rated for 250V typically won't safely interrupt
a 400V circuit under fault conditions. For example, an arc may form
from one end of the fuse to the other, allowing hundreds of amperes to
flow through the ionized gas and other material.

You can use a fuse at LESS than its rated voltage safely. So a 32V
automotive fuse is okay at 12V or 6V, but must NOT be used on mains
voltage.

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

Gary Schafer

2005-06-17, 11:31 pm

On Mon, 16 May 2005 15:17:42 GMT, Dale Farmer <dale@cybercom.net>
wrote:

quote:

>
>
>gab wrote:
>
>
> The voltage ratings also specify AC or DC. It has to do with
>the interrupting capacity of the fuse. When the fuse blows under
>a dead short, can the fuse actually stop the current flow? Of does
>the arc formed at the moment it blows just keep the current flowing?
>That's why a fuse ( to pick some numbers out of my XXX) that is rated
>to stop 600V AC is only rated to stop 200V DC. DC arcs, are for
>some reason I never understood, harder to stop. The higher the
>voltage, the longer the arc.
>
> --Dale
>

An AC arc is easier to extinguish because the voltage swings from
maximum to zero during the AC cycle. When it is at zero the arc has a
chance to stop. With DC there is no change in voltage so once an arc
starts there is nothing to stop it. With AC the arc has to restart
each half cycle if it is going to arc.

An arc has a very low resistance. Once it starts heavy current will
flow through it.

Regards
Gary

operator jay

2005-06-17, 11:31 pm

"Gary Schafer" <gaschafer@comcast.net> wrote in message
news:oe4i8119b03cl9ddmo4l3n5h0svfet53f8@4ax.com...

quote:

> On Mon, 16 May 2005 15:17:42 GMT, Dale Farmer <dale@cybercom.net>
> wrote:
>
>
> An AC arc is easier to extinguish because the voltage swings from
> maximum to zero during the AC cycle. When it is at zero the arc has a
> chance to stop. With DC there is no change in voltage so once an arc
> starts there is nothing to stop it. With AC the arc has to restart
> each half cycle if it is going to arc.
>
> An arc has a very low resistance. Once it starts heavy current will
> flow through it.
>
> Regards
> Gary

I always hear people refer to the zero crossing of the current as the point
where the arc extinguishes. Sometimes this can be when voltage as it or
near maximum.

j

Tomi Holger Engdahl

2005-06-17, 11:31 pm

"gab" <gabrio.spinelli@gmail.com> writes:

quote:

> Hy guys,
> the question is: why fuses are rated with voltage values too?
> Isn't it enough to indicate rated current?

Just the current is not enough for fuse rating.

The votlage rating the the fuse tells at what voltage
that fuse can stop the current flow when the current
rating exceeds the current limit.
If you use a fuse at higher voltage than for what it
is rated for, then when the fuse blows it coudl happen
that instead of current stops flowing, there will be
a a strong arching inside the fuze that keeps the current
flowing even through the fuse wire inside it has burned.
Then the voltage on the circuit is withing fuse voltage
rating, it can stop the current flowing.

Voltage rating is not the only thig that needs to be
considered. There is also a thign called
"current breaking capacity" that tells what is the maximum
current that the fuse can safely stop.
If you have a short circuit and power supply with higher
current output capacity than the fuse current breaking
capacity rating, what can happen is that the high current
keeps flowing even the fuse is burned, because the high
current can keep up the arch... This will definately
do all kinds of harm, including excessive current still
getting to your circuit and fuse heagin very radipdly
(even causes some fuses to "explode".

So when selecting fuses you need to look first the
current rating you need for your application.
Then you need also look at the fuse has high enough
voltage rating and enough "current breaking capacity"
for the application. The needed "current breaking capacity"
depends on the application heavily, for example on normal
mains powered equipment that is plugged to wall outlet
you don't need very high current breaking capacity,
because the normal sort circuit currents are not
very high on normal outlets, and there is a second
fuse on the mains panel bereaking current and catastrophic
short circuit situatuions (the fuses on the mains panel
hav much higher "current breaking capacity", they are
designed to allow to stop the full short circuit current
available from the power company to your house in away
or another).

quote:

> In fact I can use a 250V fuse also on a 400V supplied circuit.

250V fuse protecting a circuit that operates at 400V is
not a good idea...

Can you describe more your situation.

quote:

> Any suggestion?
> Thanx
> BR
>

--
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at
http://www.epanorama.net/

Dale Farmer

2005-06-17, 11:31 pm

Gary Schafer wrote:

quote:

> On Mon, 16 May 2005 15:17:42 GMT, Dale Farmer <dale@cybercom.net>
> wrote:
>
>
> An AC arc is easier to extinguish because the voltage swings from
> maximum to zero during the AC cycle. When it is at zero the arc has a
> chance to stop. With DC there is no change in voltage so once an arc
> starts there is nothing to stop it. With AC the arc has to restart
> each half cycle if it is going to arc.

Ah. So simple. Learn something new every day.

--Dale

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