gfi a variac?

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mossyhorns012

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I currently came across a industrial shop that before they send out they're motors, they test the motors on a "test rig" which has a variac transformer which enables the to control the power from 3 phase 200 volts all the way up to around 600 volts. This company is affilaited with another company across seas that had a bad electrical accident with the use of one of the "test rigs" and do to this the company is determined to get gfi protection on all of the "test rigs". My question is this, to put a gfi in before the variac would negate the protection at the motor where it is needed so the next thought would be to put the gfi in after the variac, but I have not came across a gfi that will operate with that much of a voltage difference??
 
I dont see why a standard GFCI would not work on the output from the variac.

A GFCI measures the current in each conductor and trips if these currents are not very nearly eqaul (perhaps because some current is passing to ground via a person)
The GFCI does not "know" what the voltage of the circuit is, provided of course that the voltage does not exceed the rating of the device and overstress it.

The only drawback that I can forsee, is that the internal test circuit of the GFCI wont work at a voltage much below nominal.
This test circuit consists of a push to make switch that connects a resistor between hot on the line side of the device and neutral on the load side (or the other way around) This resistor is chosen to pass the correct current, at nominal voltage to trip the device.

At a reduced voltage the current will be insufficient to trip on test, giving the false indication that the GFCI is defective.
This may be overcome by regular testing at nominal voltage, or by use of an external test resistor from hot to ground, calculated to pass the desired test current at the reduced voltage.

Very large GFCIs use a contactor or relay held in by the supply, at a voltage much below nominal these wont work because the voltage will be too low to hold in the contactor.

Smaller units latch mechanicly, just like a common MCB and should work fine on a reduced voltage.
 
The variacs I have worked with were autotransformers - with no isolation from primary to secondary. Is this an autotransformer?
 
Sounds like they have some pretty poor operating procedures. Would you like to itterate on what happened actually in this accident so we can go from there?
 
I don't know, the test rigs have no schematics and the first time I looked at this I didn't have much time. I am hopefully going back this afternoon for some deeper research.
 
the plant that had the accident was the affiliated company not this one,as far as I can tell they have a very good safety plan and procedure at this site. I have no details on the accident that happened, I just know they don't want any possibilities at they're location.
 
mossyhorns012 said:
the plant that had the accident was the affiliated company not this one,as far as I can tell they have a very good safety plan and procedure at this site. I have no details on the accident that happened, I just know they don't want any possibilities at they're location.
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Their plan and procedure doesn't include megging the motor before it's placed under a true load and real fault current:confused:
 
In the US personnel protection is done by GFCI devices at 6mA trip (Class A). In Europe their equivalent protection is done by a GFI (actually an RCD) with a setting of 30mA.

GFCI devices are not available for >240V.

If non-personnel protection is acceptable, there are many sources of 30mA GFI devices. For 480V applications, I have used devices from Technology Research Corp (TRC) out of Florida.
 
I think before you go much farther and waste a lot of time, you need more details about the other inceident. Was it a human getting shocked sceanrio? Or was it an arcing fault scanario? It makes a difference.

If you are looking for personnel protection Class A GFCI levels, good luck. In my opinion wou will never get it to hold in on a 3 phase motor circuit. There is always some leakage to ground and those are way too sensitive at 4-6ma.

If it is an arcing type GF that you want to protect against, that is much more reasonable. You can buy circuit breakers that have 30ma or 100ma GF protection built-in, just use one of those to feed your Variac. That's how I see it in the motor shops I have visited. Some of the older ones use add-on GF relays that use a shunt trip in the breaker, but now it would be simpler to just get an ELCB with it built-in.
 
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