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Old 04-12-2007, 07:51 PM   #1
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Default Addressing Ground Fault Issues

Electricians are often faced with having to close/shut/reenergize Main Line Switches (either bolted pressure Switches High Pressure Contact switches or Circuit breaker) after opening due to a Ground Fault. When care is not taken in closing a Main Line Switch the electrician exposes himself to possible dangerous sustained arcing faults with personal injury or equipment damage for the facility owner with the liability for the damage. What I have listed below is the approach I have utilized over the years for a safe, effective fairly fast method to restore power to a facility.

What I have outlined below describes Ground Fault on Main Line Switches (I use this term loosely to include various styles of devices), but the type of systems discussed and the procedures for locating faults can be utilized on distribution systems with multiple levels of Ground Fault Protection.

WHAT IS GROUND FAULT PROTECTION OF DISTRTIBUTION EQUIPMENT:
  • The Ground Fault Protection (GFP) system is designed for equipment protection, NOT PEOPLE PROTECTION as some may think.
  • GFP was first adopted into the NEC in 1971 NEC article 230.95. The reason for this new Article was the increase in sustained arcing ground faults resulting in system burn down that accompanied the increase in the use of 480/277 distribution systems.
  • The basic NEC rule for the mandatory installation of GFP is on Main Line Switches 1000 amps and larger and more than150 volts to ground.
  • Setting for the GFP relay is a maximum of 1200 amps with a maximum of 1.0 second delay.
  • While arc faults occur at all voltage levels sustained arcing ground faults occur at a voltage above 370 VAC. The peak voltage of 208/120 VAC system to ground is 169 VAC, while for a 480/277 VAC system the peak voltage is 391 VAC above the 370 VAC threshold. 120 X 1.414=169 and 277 X`1.414=390 (numbers are rounded off).
  • The nature of this sustained arc is the impedance of the arc is high and the fault does not generate enough current to operate the OCP. But this arc has damaging energy and can burn down switchboards, turn busways into a mass of molten metal and KILL IN the proverbial flash of less that a second.
  • Switchboards AIC ratings are designed for the worse case fault, this is a bolted phase to phase fault, in reality this type of fault, bolted; is rare. Studies have shown most faults start as ground faults and then if the OCP does not clear the fault they may become phase to phase but not bolted.
TYPES OF GROUND FAULT PROTECTION SYSTEMS:
  • There are 3 basic types of GFP in use these are Zero Sequence, Ground Return and Residual.
  • Early GFP systems utilized the Ground Return. This was a Current Transformer (CT) usually with a 600 to 5 ratio mounted on the neutral to ground bond. The 5 amp secondary operated a dash pot and this in turn operated to open the Switch. There were several problems with this system, the first being the neutral ground bond exist at the main service and at the utility transformer; this dual path desensitized the GFP system as there were two ground return paths to the source. Additionally the dash pot had inherent time delay. Then there was the Main Line Switches, early Switches utilized linear motors and circuit breakers utilized motor operators, both considered slow. Therefore, these early systems had three factors that could delay their operation under ground fault. Ground Return type GFPs are still in use with the newer style relays, typically this type of GFP is utilized with double ended Switchboards (two or more feeds).
  • Manufactures were working on large window CTs (12’-36’X8-16”) to provide a faster safer more reliable cost effective solution. These CTs encompass the phase and neutral/grounded bus/conductors. This system is referred as a Zero Sequence system, this system coupled with the advent of new electronic relays and Shunt trips for circuit breakers and spring opening bolted pressure Switches resulted in VERY FAST clearing of Ground Faults. Zero Sequence systems measure all current leaving and all current returning to the Main source the sum of these is effectively “0” amps (there may be a small amount of leakage current). If there is a ground fault the current takes an alternate path (multiple ground paths) bypassing the Zero Sequence CT and result in current on the CT secondary, when this current reaches the preset current threshold of the GFP relay and meets the time delay the contacts close to operate the Main Line Switch or CB. These CTs are actually called current sensors this has to due with the secondary output or the ration is not given in ration like standard CTs. I am no CT expert and state this here only for educational purposes.
  • Residual GFP systems utilize 2 to 4 CTs, wired in such a manner to a GFP relay that mirrors operation of the zero sequence system.
  • While different manufacture offer different settings typically the basic GFP relay has settings between 100-1200 amps, with delay from instantaneous-1.0 seconds.
WHY MAIN LINE SWITCHES TRIP:
  • Modern Switchboards have incorporated many optional protection schemes in addition to standard OCPs and GFP there is blown fuse protection, normally 3-KAZ fuses in parallel with the main fuses if the a main fuse blows the parallel KAZ blows and a spring actuator pops up to operate a microswitch resulting in operation of the Main Line Switch shunt. In addition the following or a combination of the following may be installed, phase relays, under voltage relays, and reverse phase relays to offer loads on the distribution system protection.
  • So prior to chasing a Ground Fault, one has to determine if the Switch operation was a result of a blown fuse, UV or phase loss. While the blown fuse operation is simple to find, it is obvious when you look at the KAZ and additionally some Switchboards have blown fuse indication lights on the front of the Switchboard. Phase loss and UV operation can be more difficult to diagnose, if the power is restored when you arrive on site.
  • Some GFP relays have indicators that will not allow you to close the Main Switch without resetting the indicator, others allow you to close the Main Switch with the indicator still showing a GFP operation and other GFPs systems have no indication of operation, leaving you to guess the reason the Main Switch opened.
  • In order of operation based upon personnel experience for Main Line Switch opening.
    • Electricians tracing circuits short a 408/277 VAC branch circuit, or electricians or others as part of construction short a 480/277 VAC circuit. Electricians that are still tracing circuits in this method are NOT electricians. *1 see below
    • Faulty equipment shorts to ground; number one here is motors, with cooling tower fan motors being number one, HVAC compressors number two. *2 see below
    • An actual ground fault with water leading the list for the cause of the ground fault.
    • A defective GFP relay or other protection device, defective CT or an open in the CT secondary wiring.
    • A fault in 208/120 VAC distribution in a properly installed system will not operate a 480/277 GFP
  • *1 In a typical office building the majority of branch circuit breakers are 20 amp and 30 amp, if the GFP relay is set at the 100, 200, or 300 amp setting a fault on a 20 or 30 amp branch circuit can result in an operation of the GFP opening the Main Line Switch. A standard molded case circuit breaker will operate in the instantaneous range at 6 to 10 times the rating of the CB 10X20=200 or 30X10=300, plus or minus 35% accuracy. So if the GFP relay is set low a fault in any branch circuit can result in a GFP operation. IMO this is not good safe coordination, I feel 400 amps should be a minimum (BUT I AM NOT AN ENGINEER).
  • *2 With motor faults and large equipment faults it is almost impossible to coordinate the GFP with the OCP for the utilization equipment. Assuming the same 6-10 times instantaneous rating of the OCP a 100 amp OCP would operate on a ground fault at around 1000-1200 amps which may be at or above the GFP relay settings (maximum setting 1200 amps). A 200 amp OCP with a ground fault can result in fault current of 1200-2000 amps well above the operation of the GPR relay. This is the price of safety with a 480/277 VAC system with GFP.
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.



Last edited by brian john; 04-13-2007 at 04:16 PM. Reason: Additional information
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Old 04-12-2007, 07:58 PM   #2
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Wow! Fantastic post. Thank, Brian. That post will be sticky for a while, if you don't mind.

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Old 04-12-2007, 08:01 PM   #3
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I have pictures and drawing to add when I get to the office.
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Old 04-12-2007, 08:05 PM   #4
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Pics and drawings would be helpful!
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Old 04-12-2007, 09:01 PM   #5
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Brian John:

I just want to say thank you for wonderful post with this set up.

one quick question if that ok i can copy and print this so i can pass around with few other electrician whom deal with larger PB system ?


otherwise that is very wonderfull advise what to do in a safe manner which you pin point it pretty clear with it

Thanks !!

Merci , Marc
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Old 04-13-2007, 05:11 AM   #6
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Brian

Top class post. I read the post three times and even after translating the US shorthand I found it faultless. Many thanks for a job well done.

Frank
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Old 04-13-2007, 06:04 AM   #7
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Quote:
Originally Posted by brian john View Post
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.
Great post. One question: What level of PPE do you wear prior to this step, and has it changed since OSHA has adopted NFPA 70E?
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Old 04-13-2007, 08:29 AM   #8
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What level of PPE NEVER HIGH ENOUGH...Please feel free to print and pass out.

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Old 04-13-2007, 04:18 PM   #9
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An ITI (NOW GE) GFP Relay




ASEA BROWN BOVERI GFP Relay



Split Core Window CTs 10"X24" and 10"X36"


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Old 04-13-2007, 04:19 PM   #10
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Time mak Phase Failure Relay




Taylor Phase Guard relay, one of two phase relays commonly in use in 480/277 VAC switchboards.



A small high current test set utilized to test GFP systems, Depending on the impedance of the test leads the maximum current from this set is about 3000 amps.


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Old 04-13-2007, 04:36 PM   #11
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I need to get my picture posting skills down, I also believe my auto focus went on my camera today..


Blown Fue Indicator device with type KAZ fuses.




Westinghouse GFP relay


POOR QUALITY due to backdrop reflection, the realy has a very reflective from cover.

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Old 04-25-2007, 08:21 PM   #12
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I agree excellent info! thanks just curious looks like some classic equiptment my guess vintage late 50's thru 60's, is that slate board with those fuse holders with brass screws..........
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Old 06-17-2007, 05:32 PM   #13
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Actually all are current protective devices, the Blown Fuse device was recently purchased for an upcoming job.
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Old 06-17-2007, 05:52 PM   #14
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Quote:
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Actually all are current protective devices, the Blown Fuse device was recently purchased for an upcoming job.
That's the one, put in series with the real fuses that has a little pin that pops out of the top, to trigger that little switch on the top? Prety neat. I've only read about those, and never saw one for real, except your picture.
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Old 06-11-2008, 10:56 PM   #15
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Out of curiousity how often do you see flashover occur in service entrance equipment @ 208/480?

I live in a high lightning area and typically faults of this nature are related to a high potential, short duration transients which exceed the insulation properties of the switch gear. Once this happens the arc caused by the transient creates an ionized path to ground or cross phase which allows 60Hz current to flow until an OCP or ground fault protection device operate. Faults of this nature can be high impedance in nature which may not allow fault current to exceed the rating of the OCP - ie a sustained fault.
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Old 06-12-2008, 07:49 PM   #16
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In my expierence 1 of of a 1000 may be lighting related,50% or more are water related, the rest loose connections.

At 208/120 VAC the peak of the sine wave is insufficient to sustain an arc and thus extinguishers and no arcing sustained ground faults with related burn downs like you see in 480/277 distribution.
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Old 06-12-2008, 10:58 PM   #17
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Agree - I've seen some 480 switchgear that looks like someone went to work on it with a blowtorch.
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Old 09-21-2008, 11:36 PM   #18
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Default Ground Fault Issues

The main problem in the Electrical line is ground fault.There are some types of fault occurs in the supply line.For example, Line to Line fault -When two lines are interrupt over in a single supply then there is a line to line fault occurs.
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Old 09-22-2008, 08:32 AM   #19
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Thanks for the informative post. Having gone on a couple of these calls before , I now realize that I have been diagnosing these issues cocky and dangerously. You are 100% correctwith all steps.
I will make the adjustments next time.
You might have saved me from a bad situation some day. Thanks again
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Old 09-24-2008, 01:23 AM   #20
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Default Re: About Ground Fault

Now a days the main issue in electric field is ground fault.It mainly occurs due to interrupt between two lines.
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