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Home > Archive > Electrical Engineering > January 2008 > Explain This!
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| I recently posted this to an electricians' forum, it currently has 158
views: but no takers
A problem easily repaired today is much more difficult to explain what
occurred because of it. We recently relocated a machine that included a 70
KVA Autotransformer & a primary disconnect.
The call-back was not due to problems running: But because when they
switched off the X-former's disconnect, it tripped out the overhead
bus-duct, (ground-fault!, at the switchgear).
I found that the X-former's secondary neutral was landed on machine ground,
(a ground wire from the X-former had fallen out of sight in the conduit to
the machine).
Though the white tape should have caused further investigation, the
electrician's oversight was still understandable since:..
1) the neutral lug on the machine was obviously not used,..
2) both would have a common point in a normal X-former.
After connecting properly & allowing production to resume...
My challenge is why did the bus fault out while disconnecting?
I would love to hear other theories before I divulge mine.
Hint: my explanation involves the electrics of a classic car!
| |
| Rheilly Phoull 2007-12-09, 9:25 am |
|
"evo" <evo@sc.rr.com> wrote in message
news:475baa5e$0$9830$4c368faf@roadrunner.com...
>I recently posted this to an electricians' forum, it currently has 158
> views: but no takers
>
> A problem easily repaired today is much more difficult to explain what
> occurred because of it. We recently relocated a machine that included a 70
> KVA Autotransformer & a primary disconnect.
> The call-back was not due to problems running: But because when they
> switched off the X-former's disconnect, it tripped out the overhead
> bus-duct, (ground-fault!, at the switchgear).
> I found that the X-former's secondary neutral was landed on machine
> ground,
> (a ground wire from the X-former had fallen out of sight in the conduit to
> the machine).
> Though the white tape should have caused further investigation, the
> electrician's oversight was still understandable since:..
> 1) the neutral lug on the machine was obviously not used,..
> 2) both would have a common point in a normal X-former.
> After connecting properly & allowing production to resume...
> My challenge is why did the bus fault out while disconnecting?
>
> I would love to hear other theories before I divulge mine.
>
> Hint: my explanation involves the electrics of a classic car!
>
>
Ohh !! those under-educated electricians. They will kill us someday. How
lucky we are to have experts on the internet to tell us when they go wrong!
--
Cheers .......... Rheilly P
| |
| BasicGuy 2007-12-09, 1:25 pm |
| On Sun, 9 Dec 2007 01:42:11 -0700, evo wrote
(in article <475baa5e$0$9830$4c368faf@roadrunner.com> ):
> I recently posted this to an electricians' forum, it currently has 158
> views: but no takers
What forum was that?
| |
| phil-news-nospam@ipal.net 2007-12-09, 1:25 pm |
| On Sun, 9 Dec 2007 03:42:11 -0500 evo <evo@sc.rr.com> wrote:
| I recently posted this to an electricians' forum, it currently has 158
| views: but no takers
|
| A problem easily repaired today is much more difficult to explain what
| occurred because of it. We recently relocated a machine that included a 70
| KVA Autotransformer & a primary disconnect.
| The call-back was not due to problems running: But because when they
| switched off the X-former's disconnect, it tripped out the overhead
| bus-duct, (ground-fault!, at the switchgear).
| I found that the X-former's secondary neutral was landed on machine ground,
| (a ground wire from the X-former had fallen out of sight in the conduit to
| the machine).
| Though the white tape should have caused further investigation, the
| electrician's oversight was still understandable since:..
| 1) the neutral lug on the machine was obviously not used,..
| 2) both would have a common point in a normal X-former.
| After connecting properly & allowing production to resume...
| My challenge is why did the bus fault out while disconnecting?
|
| I would love to hear other theories before I divulge mine.
Was this machine wired with 1 pole, 2 poles, or 3 poles of phase power?
Was the neutral terminal on the machine supposed to be connected? Or was
the machine intended to be run only on 2 or 3 poles of power?
What kind of autotransformer is this?
My first thought without knowing what the actual setup is, is that some
other circuit is at fault and a low level ground current, which in this
case is backflowing into the neutral which is measured in the switchgear
for groundfault, possibly combined with some surge, caused the trip.
I'd rather not try to think through other scenarios without narrowing
down how this was actually wired (including the machine itself, which
can probably be understood by knowing whether it needs the neutral to
work correctly, or not).
Wiring diagrams, one showing the way it should have been, and one showing
how it actually was done, might clarify.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2007-12-09-1106@ipal.net |
|------------------------------------/-------------------------------------|
| |
| de777lik3@shaw7777.ca 2007-12-10, 5:25 pm |
| which would clarify why nobody has taken a stab at explaining it yet
>
>Wiring diagrams, one showing the way it should have been, and one showing
>how it actually was done, might clarify.
| |
|
| > > My earlier message contained..................
[color=darkred]
> "Rheilly Phoull" spat back;
> Ohh !! those under-educated electricians. They will kill us someday. How
> lucky we are to have experts on the internet to tell us when they go
wrong!
> --
> Cheers .......... Rheilly P
I see you are apparently holding back on your answer untill ............?.
At least I have an explaination that I believe others may confirm...
OK, so that may have been expected in a forum for engineers..(humor?),
But understand that I have become discouraged that for two months now,
no one has confirmed my explaination off this switchgear tripping,
nor has anyone yet offered one of thier own!
Granted, a mistake was made in this installation, but as a 30 year
electrician myself; I find more fault in the pressure to meet production
than I blame my electrician's carlessness or ignorance .
| |
| Paul Hovnanian P.E. 2007-12-16, 5:25 pm |
| evo wrote:
>
> I recently posted this to an electricians' forum, it currently has 158
> views: but no takers
>
> A problem easily repaired today is much more difficult to explain what
> occurred because of it. We recently relocated a machine that included a 70
> KVA Autotransformer & a primary disconnect.
> The call-back was not due to problems running: But because when they
> switched off the X-former's disconnect, it tripped out the overhead
> bus-duct, (ground-fault!, at the switchgear).
> I found that the X-former's secondary neutral was landed on machine ground,
> (a ground wire from the X-former had fallen out of sight in the conduit to
> the machine).
> Though the white tape should have caused further investigation, the
> electrician's oversight was still understandable since:..
> 1) the neutral lug on the machine was obviously not used,..
> 2) both would have a common point in a normal X-former.
> After connecting properly & allowing production to resume...
> My challenge is why did the bus fault out while disconnecting?
Lots of things need clarifying before anyone can make an educated guess
at this. However, here's a shot in the dark:
Supposing that the secondary of this (auto?)transformer is feeding
balanced three phase loads and that the secondary side neutral has
somehow come in contact with the system ground: Disconnect switches
don't always break all phases at exactly the same time. They certainly
open phases at different points w.r.t. their current waveforms. This wil
result in a momentary current imbalance which, if returned through
ground (instead of neutral) can cause ground fault protection to
operate.
> I would love to hear other theories before I divulge mine.
>
> Hint: my explanation involves the electrics of a classic car!
--
Paul Hovnanian mailto:Paul@Hovnanian.com
------------------------------------------------------------------
The ark was skippered by amateurs, the Titanic was skippered by
professionals.
| |
| fearfakter 2008-01-11, 8:25 pm |
| auto transformers are mainly used to adjust the voltage levels to be
balanced in three phase systems and to isolate the secondary from the
primary to protect sensitive componants from surges and grounding damage. If
dc motor control boards are used in this machine that are single phase they
need to have there suppling phases balence finely. my guess is if the
people that built the thing or where it came from had an unbalenced
substation and the auto transformers were adjusted for there situation and
yours is not the same then you might have to change taps or something to
correct the problem. Not knowing what is what there is is really difficult
to say with any accuracy but this should be investigated.
hope this is some help
cheers
fearfakter
"evo" <evo@sc.rr.com> wrote in message
news:475baa5e$0$9830$4c368faf@roadrunner.com...
>I recently posted this to an electricians' forum, it currently has 158
> views: but no takers
>
> A problem easily repaired today is much more difficult to explain what
> occurred because of it. We recently relocated a machine that included a 70
> KVA Autotransformer & a primary disconnect.
> The call-back was not due to problems running: But because when they
> switched off the X-former's disconnect, it tripped out the overhead
> bus-duct, (ground-fault!, at the switchgear).
> I found that the X-former's secondary neutral was landed on machine
> ground,
> (a ground wire from the X-former had fallen out of sight in the conduit to
> the machine).
> Though the white tape should have caused further investigation, the
> electrician's oversight was still understandable since:..
> 1) the neutral lug on the machine was obviously not used,..
> 2) both would have a common point in a normal X-former.
> After connecting properly & allowing production to resume...
> My challenge is why did the bus fault out while disconnecting?
>
> I would love to hear other theories before I divulge mine.
>
> Hint: my explanation involves the electrics of a classic car!
>
>
| |
| Salmon Egg 2008-01-12, 3:25 am |
| On 1/11/08 4:35 PM, in article 47880b6e$0$4991$4c368faf@roadrunner.com,
"fearfakter" <bknight@stny.rr.com> wrote:
> auto transformers are mainly used to adjust the voltage levels to be
> balanced in three phase systems and to isolate the secondary from the
> primary to protect sensitive componants from surges and grounding damage. If
> dc motor control boards are used in this machine that are single phase they
> need to have there suppling phases balence finely. my guess is if the
> people that built the thing or where it came from had an unbalenced
> substation and the auto transformers were adjusted for there situation and
> yours is not the same then you might have to change taps or something to
> correct the problem. Not knowing what is what there is is really difficult
> to say with any accuracy but this should be investigated.
Transformers that isolate ARE NOT (AND CANNOT BE) AUTOTRANSFORMERS!
Autotransformers are cheaper and lighter just because they can use
conductive current transfer instead of coupling current via mutual coupling.
Bill
| |
| Don Kelly 2008-01-12, 3:25 am |
| ----------------------------
"Salmon Egg" <SalmonEgg@sbcglobal.net> wrote in message
news:C3AD8614.1B64F%SalmonEgg@sbcglobal.net...
> On 1/11/08 4:35 PM, in article 47880b6e$0$4991$4c368faf@roadrunner.com,
> "fearfakter" <bknight@stny.rr.com> wrote:
>
>
> Transformers that isolate ARE NOT (AND CANNOT BE) AUTOTRANSFORMERS!
> Autotransformers are cheaper and lighter just because they can use
> conductive current transfer instead of coupling current via mutual
> coupling.
>
> Bill
>
The rest of what he says is also nonsense.
--
Don Kelly dhky@shawcross.ca
remove the X to answer
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