GFI problems 1

[Jackson1]

I am retired from the power transformer testing department of a southeastern utility...I had a problem that I never got to investigate throughly. When testing transformers with LTC's (Load Tap Changers), whenever I would apply electrical power to the controls, out safety department required that we use short pigtails with GFI protection. Whenever I would energize the control, it would kick the GFI out...the only way I could test these controls was to by-pass the GFI protection. Not all but approximately 95% of the controls required this. I even devised a test scheme and simutaneously measured the current in the hot and ground wires. The best I can remember, there was a difference of approximately 40 - 60 miliamps...which is more than enough to activate the GFI protection device. What I never did find was why that much difference?

 

[stevenal]

GFIs require isolation of the neutral and ground and all return must be on nuetral. Your LTC control neutral is grounded to the transformer tank. An isolation transformer might keep the safety people happy.

 

[Jackson1]

Thanks stevenal for your comments. One thing I wanted to add...the GFI would trip just as soon as I energized the control (before operating control). Could this have been partially caused by static capacitive current flow? There are hundreds of different configurations to these controls. They go from purely mechanical on the older units to completely solid state on the the new units.

 

[stevenal]

Don't think so. GFIs can sense and trip for a neutral/ground bond when no load current is present. A small voltage is impressed on the neutral by the device. If current flows on the neutral, unmatched on the hot, the bond is sensed and the device trips.

 

[droog]

To expand.

GF protectors are available in various models. By GFI one usually means a device for personnel protection. Like stevenal says, these devices trip under grounded neutral conditions (even if there is no load), as well as grounded hot line. The reason for this is that people could suffer hazardous shocks if they serve as ground paths for either one of the hot or neutral wires. Also, the trip levels for personnel protectors are usually lower (5mA in USA, 10mA in Europe).

There are GF products rated for equipment protection. These products have no grounded neutral protection, and the range of trip levels varies according to applications and manufacturer's specs.

 

[buzzp]

What droog describes is classified by UL as class I and class II GF detectors. Class II is for personnel protection and class I for equipment.

 

[jbartos]

Suggestion: Visit

http://www.allanson.com/tech_support/sgfpart.pdf

etc. for more info

 

[rodar]

Jackson1
I think you have the answear there. Capacitive coupling
between the device under test and a grounded metal test
table could cause the unbalenced current in the GFI.
Particularly if switched on when the sine was at the peak.
Or is there internal to the control a static bleed off
resistor to frame.
Rodar

 

[jbartos]

Suggestion: Visit

http://www.ecmweb.com/ar/electric_gfci_basics/

for: GFCI basics

http://www.ecmweb.com/ar/electric_code_basics_3/

for: NEC Code GFCI requirements

http://www.unitedpower.com/safehome.htm

for: GFCI safety
etc. for more info