480 volt Ground fault protection

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mbrooke

mbrooke

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Ok, so here is what I take from this. If a facility was wired in all MC cable without conduit, there would be no need to ground fault protection at 1000amps and above. ( SCPD= Short Circuit Protective Device ) How do others interpret this?
 

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I think I see what you are saying. The suggestion in the paper is that fault conditions cause the conductors to move, which reduces the magnitude of the fault current, preventing OCPD from opening. If you have MC cable then the conductors in the cable are pretty tightly bound together and thus couldn't move, eliminating this dynamic reduction in current flow.

Not sure I buy it, since at 480V you can have an arc through the air between rigid bus bars. Not sure how you would wire everything (including inside your equipment enclosures) using MC :)

-Jon
 
winnie said:
I think I see what you are saying. The suggestion in the paper is that fault conditions cause the conductors to move, which reduces the magnitude of the fault current, preventing OCPD from opening. If you have MC cable then the conductors in the cable are pretty tightly bound together and thus couldn't move, eliminating this dynamic reduction in current flow.

Not sure I buy it, since at 480V you can have an arc through the air between rigid bus bars. Not sure how you would wire everything (including inside your equipment enclosures) using MC :)

-Jon
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Yup- the movement makes and brakes the arc- that making and braking may not trip the thermal part of the breaker fast enough. 1000 amps and up, because the NEC has determined these are services capable of supplying enough fault current to significantly move conductors in conduit during faults.
 
Have you ever seen a ground fault in a switchboard, GF upstream of the main and major damage that results or downstream of the main with GFPE and limited damage (assuming the GFPE operates). I have seen countless major "blow-ups" and they happen in any electrical installation and material type. Properly function GFPE minimizes the damage.
 
brian john said:
Have you ever seen a ground fault in a switchboard, GF upstream of the main and major damage that results or downstream of the main with GFPE and limited damage (assuming the GFPE operates). I have seen countless major "blow-ups" and they happen in any electrical installation and material type. Properly function GFPE minimizes the damage.
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Is it because the ground fault can't trip always trip a regular breaker magnetically? Does the same level of damage occur at 120/208?
 
I put this together a few years back on another forum, and if there are glaring errors please let me know as I am NOT an engineer, just an old electrician that has tried to understand his trade a little..


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 DISTRIBUTION 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 than 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.
 
This is pure gold, I learned some new concepts I was not aware of.

All this leads me to ask... why do they stop at 1000amps? Wouldn't an arc have impedance on say a 600amp service? :?
 
Fulthrotl said:
there are a number of people on here, when they post, i stop and read carefully.

brian is one of them. :happyyes:
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Indeed! He is brilliant, and well ahead of his time. I sometimes like to think of him as electrician, engineer, designer, code expert, testing expert, and several other respected professions.
 
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