Auto transformer and GFCI protection

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Rick Christopherson

Rick Christopherson

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I want to get someone to confirm my thoughts that if the input of an auto transformer is GFCI protected, then it's outputs should also be GFCI protected.

I am looking at a point-of-use, cord-and-plug connected, 120 to 240 volt 1000W transformer. I popped the cover and confirmed it is a common-neutral auto transformer. My initial thinking is that if there is a ground fault on the output, that it should trip the supply-side GFCI if it were plugged into one. (Yes, one line at 240 is in fact tied to a common neutral, like a European system.)

Am I over looking something, or does this sound correct?
 
Rick Christopherson said:
I want to get someone to confirm my thoughts that if the input of an auto transformer is GFCI protected, then it's outputs should also be GFCI protected.

I am looking at a point-of-use, cord-and-plug connected, 120 to 240 volt 1000W transformer. I popped the cover and confirmed it is a common-neutral auto transformer. My initial thinking is that if there is a ground fault on the output, that it should trip the supply-side GFCI if it were plugged into one. (Yes, one line at 240 is in fact tied to a common neutral, like a European system.)

Am I over looking something, or does this sound correct?
Click to expand...

Yes. If you had an isolation transformer wired as an ADS, the answer would be different.
For an isolation transformer with the neutrals tied, there would still be a trip, but because of excess current in the GFCI neutral rather than missing current. :)
 
Yeah, I knew that a Separately Derived System (SDS) would not provide GFCI protection at the output.

GFCI protection is not necessarily required for this, but it is a nice bonus if it remains available. I was pretty sure about it, but just wanted to get a second set of eyes on it before I made a foolish statement. :dunce:
 
Rick...

Since both input and output neutrals are connected make sure there no:

a) 'Internal' (within AT enclosure) surge-suppressors (TSS, MOVs) connected beween the High-side and Low-side phase-conductors and neutral!

b) 'Internal' (within AT enclosure) surge-suppressors (TSS, MOVs) connected beween the High-side and Low-side phase-conductors and ground!

c) 'External' (connnected wiring to AT) surge-suppressors (TSS, MOVs) connected beween the High-side and Low-side phase-conductors and neutral!

d) 'External' (connnected wiring to AT) surge-suppressors (TSS, MOVs) connected beween the High-side and Low-side phase-conductors and ground!

Regards, Phil Corso
 
I think the protection will pass through, but there may be another issue in use. I used a 2000W toroidal autotransformer (230V to 120V) in my cafe for a coffee urn which often tripped the RCD when you plugged in the transformer. I don't know whether it was the RCD protection or the inrush of the transformer, but larger transformers like that should be left plugged in if they're cord and plug connected.
 
Whilst I do agree that any ground fault current on the output side of a auto-transformer will be seen by a GFCI on the input side .... there is a twist..... don't forget that the fault current will also need to be scaled by the inverse of the turns ratio, just as the input to output current is!

Adrian
 
Phil Corso said:
Peter...

Is the Coffee Urn's plug polarized?

Phil
Click to expand...

Yes, it was a US plug on the urn. The transformer would trip the 3P 25A RCD main in my cafe regardless of whether the urn was plugged in or not. Back then I didn't have the test equipment I do now so I couldn't check whether it was inrush or a ground fault causing the problem.
 
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