HOW TO SAFELY RESET GFP PROTECTED MAIN LINE SWITCHES:
Try to determine if the Switch opened from GFP operation or other system protection device.
Ask if there is any on going construction.
Ask if the Switch opened at dusk, outside lighting can be an issue especially after periods of rain.
Check the GFP settings time and current, write them down.
Have someone (because it requires a lot of walking) shut off all downstream devices at the level below the Main.
Once you are sure the power is off, open and test the bus with a tester, you are sure works (HAVE YOU TESTED YOUR TESTER). It is possible for Switches to hang up BE CAREFUL.
Inspect the bus for any water and signs of a fault.
Megger the bus phase to phase and phase to ground, if the reading is low check that there are no relays connected to the bus, bus taps or other loads you are unaware of. Normally we test the bus at 100 VDC first to avoid damaging protective relays such as Phase or UV relays, then retest at 1000 VDC. We’d like a reading above 50 megohms, 5 can be acceptable, .5 as recommended by some IS NOT ACCEPTABLE, IMO.
Open the next downstream OCP and then megger the feeders. Continue in this manner.
With HVAC equipment, motor loads, large duct heaters and other utilization equipment you will need to get to the load conductors for the equipment, do a visual on the equipment looking for signs of a fault as you test.
Check for blown fuses and opened CBs on the 480/277 VAC distribution as you work your way through the building.
Hopefully you will located the source of the fault, if you don’t the problem may be with the relay and require testing, and this is more than just pushing a button. A high current test set is required.
If you have done all of the above, with exception of item 12, you may have to close the Main Switch without resolving the issue, THIS IS CONTRAVERSAL and may violate local and federal rulings, BUT IMO this is sometimes the only solution when all other avenues have been exhausted.
Set the GFP relay to 100 amps (or lowest setting) and the time delay to the lowest time setting Instantaneous, .1 or minimum are typical minimum settings.
READ THE SWITCH OPERATION Instructions, the number of Service calls we receive for damaged switches due to improper operation exceeds a level that would be obtained if Electricians would READ THE INSTRUCTION.
With all downstream OCPs open, all covers installed on the Switchboard, all cover screws installed tight, you dressed in proper safety gear and no one else in the room; close the Main Switch in such a manner that places you in as safe a position as possible.
If the Switch holds and it should if you have done all the above steps as noted. Move on to the next level OCP, megger as noted.
If the conductors megger OK, close these devices one at a time allowing any equipment that may need to start time to start.
If the GFP operates at any point and you gave equipment sufficient time to start you may have located the fault.
Reclose the Main Switch per step 13.c. and start closing all the OCPs leaving the one with the fault off.
Bring the building on line and return to the OCP with the suspected fault and isolate the source of the fault.
One issue to be aware of is; large dry type transformers have inrush currents that can trip GFPs relays due to the inrush current are not Zero Sequence.
Once you have isolated the fault, remember to reset the GFP settings to the “As Found Settings”, reset the GFP indicator (if present).
If after this you have not located any problems, the GFP relay should be left set at the lower settings, the GFP tested at all presets for current, and if all checks out it may be necessary to install a 5 channel high speed disturbance analyzer to assist in determining the cause of the GFP operation.
NEVER remove the fuses or disable the GFP to prevent operation in order to restore power, you can and will be had liable for any deaths or damage.
If the cause of the GFP operation is a setting you believe is too low, ask the facility personnel for the coordination study to see what the engineer specified. Often the installing electricians leave the GFP set at the factory shipping settings which is the minimum for current and time. If a coordination study is not available (VERY TYPICAL) tell the facility representative they need to get their engineer of record to provide a setting. The normal response from engineers is what does the factory specify (NOTHING (well minimum) AS THE MANUFACTURE WANTS ANY LIABILITY IN THIS ISSUE) or he may ask what do you normally set it at. My first response is I set it where the engineer tells me, then I give him my rationale for a minimum of 400 amps .1 sec. If they accept this I make them sign documentations stating it was the facilities decisions to adjust the relay setting.
If you have followed al the steps listed and the system meggers in an acceptable range completing the last item, turning on the switch without locating the cause of the GFP operation, should not be an issue.
3-phase Wye distribution with GFP normal operation.
3-phase Wye distribution with GFP, a high impedance arcing fault occurs on A phase. The GFP CT will not sense this current as the current is taking an alternate path. GFP will pickup and operate to open the main Line Switch.
