GFI Nescience Trips

[JMK 0]

Hi All - I have a situation where the GFI outlet for testing control panels constantly and continuously trips. The outlet is a 120 VAC, 20 Amp general purpose receptacle outlet derived from a 120/208 VAC, 3 PH system. (Photo Attached) This area of the shop is used as a test lab for small 120 VAC control panels, and when the servo amps are powered on the GFI will trip. I tried to use inline filters, UPS, and ferrite cores on the amps’ power leads. The only work around… use a non GFI outlet. However the safety dept. frowns on eliminating the protective device. Have any of you come across this before? Your insight is appreciated. JMK

[JRoss 84]

Yes. The electronics in a lot of drives cause leakage that the GFI circuit recognizes as a hairdryer in a bathtub. Only workaround I've ever heard was non-GFI outlets.

[drforsythe 15]

I second the reply from JROSS. It is a common problem. We use non-GFCI outlets on our 110v 'portable' servo applications.

[JMK 0]

Thanks for the replies. Just for grins, we hooked up a scope... There is a about a 2 amp differencial current, which the GFCI sees as a ground fault leakage... Definitely a hair dryer in the bathtub! JMK

[JMK 0]

For those interested: I had to do some research to apprise the safety folks… NEC 590.6 Requires GFCI Protection for Personal on all 120 VAC, 15, 20, and 30 Amp receptacle outlets installed in a temporary wiring scheme. However, the exception in 590.6(A) doesn’t make it a requirement in industrial establishments. One needs to comply with the Assured Equipment Grounding Conductor Program illustrated in 590.6(B)(2). Basically, all branch circuits need to be installed in accordance with the requirements of article 250. There must be continuity testing of the grounding system for all cord sets, receptacles, and equipment involved in the temporary install. Written records must be kept of the testing… Testing occurs before initial use, upon discovery of damage and repair, and for periods exceeding 3 months. Just an FYI – The 2011 NEC cites “Maintenance” and “Repair” to “Equipment” in the temporary wiring specifications outlined in article 590. Temporary wiring doesn’t apply to construction sites anymore. JMK

[paulengr 44]

Can't be done. There is a fraction of time on a drive where there is a conduction path to ground. Best you might be able to do is use an isolation transformer. That way through the transformer you re-establish an entirely new ground plane. All your ferrite beads and such are a total waste of time because they are addressing the wrong issue. If you really, really still want GFI, you have two choices. You can use a CBCT (core balanced CT) or even better, connect a resistor to the ground leg of the transformer and do high resistance grounding. This allows you to control the maximum ground fault current and simultaneously allows you to easily monitor and trip on it. CBCT's do the same thing but don't restrict maximum current draw, and don't require a separate neutral (that's what a GFI is at it's core, a CBCT).

[larry818 0]

Can be done. www.bender.org These guys are the only game in town with UL listed GFCIs with active filtering. Tolerant of choppy stepper drivers and VFDs. The NEC not only requires a GFCI, but a "listed" one, so you can't just roll your own.

[paulengr 44]

Correct. But it does not specify design details on how you achieve it. You can use listed components in an assembly to do something. See UL 508/508A. In power distribution, component based designs such as using Listed CT's/PT's/CBCT's, and Listed protection relays (Bender sells these and is one of the most popular brands) is the standard way of doing things. This happens because there are so many variations on how to do things that it isn't practical to sell single-use items. For instance, you can buy protection relays that function as trip units for circuit breakers and also perform motor overload protection (not only the usual version but also voltage/current imbalance, phase loss, jams, etc.). In low voltage you don't see this as often because you can usually just buy a single box that does everything. Bigger market, more options, lower margins.

[larry818 0]

Code requires that the gfci be listed for "personnel protection", 508A doesn't get you that.