Explain This!

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!

On Sun, 9 Dec 2007 01:42:11 -0700, evo wrote (in article ):

What forum was that?

| 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.

which would clarify why nobody has taken a stab at explaining it yet

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 .

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.

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

The rest of what he says is also nonsense.