3 phase bolted fault or ground fault?

[topgone]

I usually use the definite time element when protecting the equipment from ground faults! Even if there is an inverse time setting, it is always good to have backup protection that is based on the thermal capacities of the equipment!

 

[steve66]

Hi all,
I have a question regarding the picture I attached... it's an example from Cooper Bussmann's SPD Handbook (2005 edition). the left diagram shows how the #10 EGC is not properly rated for the 50,000 amps of short circuit current. #10 wire is only rated for 4,300 amps at a 1 cycle opening time of a OCPD. however, the diagram on the right shows the #10 wire is now properly protected by using current limiting fuses that limit the current down to 3,300 amps.

My question is... it seems like the 50,000 amps of current they are using as an example seems to be the 3 phase current you get assuming all phase legs are bolted together. shouldn't this number be the ground fault current instead, since we are determining the amount of fault current going along the EGC?

Thanks!

I don't see where they specifically state what the 50,000 amps is. I believe it simply means there is 50,000 amps available to flow from any line to the neutral. So I'd say you are correct that the single line to ground (SLG) fault current is the one to use.

I love the note below the disconnect with the circuit breaker - A 2/0 ground wire for a 60 amp disconnect. How often do you actually see that in the wild?

 

[topgone]

I don't see where they specifically state what the 50,000 amps is. I believe it simply means there is 50,000 amps available to flow from any line to the neutral. So I'd say you are correct that the single line to ground (SLG) fault current is the one to use.

I love the note below the disconnect with the circuit breaker - A 2/0 ground wire for a 60 amp disconnect. How often do you actually see that in the wild?

But, isn't SLG bigger than the three-phase fault at the terminals of a generator? I would agree that a three-phase fault will be greater than an SLG if the case given was a fault down the lines of a distribution line or when the system is grounded thru a neutral resistor!

 

[Grouch1980]

I don't see where they specifically state what the 50,000 amps is. I believe it simply means there is 50,000 amps available to flow from any line to the neutral. So I'd say you are correct that the single line to ground (SLG) fault current is the one to use.

ok, that's fair actually... I can see that it may mean from any line to the neutral.

 

[Grouch1980]

Just to throw a wrench into the engine... the same example is in the 2017 version of the SPD Handbook. However, the example includes this excerpt as well, see the attachment.

The attachment says good engineering practice says you should calculate 3 phase faults as well as phase to ground faults when analyzing the withstand rating of an EGC. In the words of Foghorn Leghorn... boy, i say boy, why is a 3 phase fault even considered? it won't flow through an EGC.

 

[steve66]

The attachment says good engineering practice says you should calculate 3 phase faults as well as phase to ground faults when analyzing the withstand rating of an EGC. In the words of Foghorn Leghorn... boy, i say boy, why is a 3 phase fault even considered? it won't flow through an EGC.

Well, it is Cooper Bussman, and they want to sell fuses. So they do have some motivation to make the issue look as bad as possible for circuit breakers (hence the 2/0 ground wire for a 60 amp circuit breaker). I'm not saying any of this is incorrect, but just keep the source in mind.

That said, there are also double line to ground faults (LLG). These can be higher than the single ground fault (LG), or even higher than the 3 phase bolted fault current.

Anyhow, LLG faults may sound a little bogus. After all, once the second line is shorted to gnd, the two lines are also obviously also shorted together. So we would normally expect much of the fault current to ignore the ground, and just flow on the line conductors. For example, if we have a 200 amp feed with 3/0 phase wires and a #6 ground, then yes, most of the LLG fault current is going to flow back on the 3/0 wires, and much less will flow on the #6 ground wire. That's just ohms law, because the resistance of the #6 is much higher than the 3/0.

But consider their example with a 60A circuit breaker, #10 phase wires, and a 2/0 ground. Now almost all the LLG fault current will flow on the ground wire since its resistance is much less than the #10 phase conductors. Now the LLG fault current will probably give the highest fault current on the ground wire.

If there are double line to ground faults, it seems like there should also be triple line to ground faults. But I can't find any references anywhere online to "Triple Line to ground faults". Is that because the sum of the three currents from all three phases to ground would actually be zero? They would cancel each other out? (I suddenly can't figure out how that doesn't happen with 3 phase bolted L-L faults? But that's probably best left for another post.)

So the only explanation I can really come up with is that they are just including the 3 phase faults to approximate LLG faults, which might be higher than the LG faults, and which might be kind of hard to calculate manually.