Question Section 250.56

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JohnJ0906 said:
In most cases there would be too much earth resistance for this to be an effective ground fault path. See 250.4(A)(5) last sentence.
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Unless of course they use a few of these babies.

47b7cc35b3127cceb2ff5070498a00000026100UaNmbRs1Ys



That's a 2" 10 foot long copper pipe with a 4/0 wire cad welded to the pipe. The pipe has small holes in it. The cap is removable and the pipe gets filled with salt.

The hole is augered out, the rod set, the hole is filled with water, then they pour betonite into the hole. The salt in the pipe will drawn moisture to the pipe and keep a resistance somewhere between 5 and 10 ohms.

Each rod cost about $600. On the residential job I am working on they will have 5 of these rods spaced around the perimeter of the building connected with 4/0 copper.

I don't know what else they are doing but the cost of the system is $25,000. I bet it will clear a fault.
 
iwire said:
Nope, not at the voltages we normally deal with, do the math.
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I have no idea-- what is the formula.? Would you need zero ohms to clear a fault.

$25000, I hope it does something.
 
Dennis Alwon said:
I have no idea-- what is the formula.? Would you need zero ohms to clear a fault.
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It depends on the voltage.

You know the math.

The voltage to ground will be most likely be 120 or 277 volts.

You mentioned 10 ohms of resistance.

277/10=27 amps of fault current, that may well trip a 15 or 20 amp circuit but not very quickly.

120/10=12 amps of fault current, that will not clear even a 15 amp circuit assuming no other loads.

Now if it was utility voltage things would be different

13,800/10=1,380 of fault current and would open power company over current protection.
 
iwire said:
It depends on the voltage.

You know the math.
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Yes I know the math but I also said between 5 and 10 ohms. if they get 5 ohms 120/5=24 amps. That would probaly blow a 15 amp breaker but you are right it may not clear a ground fault very quickly.

The whole purpose of this system is for lightning protection anyway. The faults will clear in a normally operating sytem, (hopefully)

BTW. I have seen a 15 amp breaker trip with a 18 amp load on it. Someone wired a new house and they didn't think about the chanderlier having 800 watts on it. When all the other lights on the circuit were on it would blow the breaker after a period of time.
 
iwire said:
Dennis, the bottom line is they are not there to clear faults and yes they might but I would not say they will. :smile:
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Sorry-- I was agreeing with you that's why I said they were installed for lightning protection. My first statement was a bit oversighted but I was impressed with those babies. Mea Cuppa, mea cuppa
 
The 2008 code should make it clear that bonding and grounding are two different concepts with two different purposes but are inextricably linked.

Bonding clears faults.
Grounding handles outside overcurrents (lighting, hv lines) and stabilizes the voltage. That zero voltage at the neutral point is only zero in reference to itself, until it is attached to the ground. Then all zero voltages on different pieces of equipment will have a voltage gradient (or difference) so small that it becomes safe.

At least that's MY take on it.

--Jerry
 
JerryWright said:
That zero voltage at the neutral point is only zero in reference to itself, until it is attached to the ground. Then all zero voltages on different pieces of equipment will have a voltage gradient (or difference) so small that it becomes safe.
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Actually, bonding accomplishes that. EGC's (are supposed to) effectively interconnect non-energized parts of equipment, with or without an earth electrode.

The electrode system is 'supposed to' include Mother Earth in that 'mesh' of bonded equipment. The above discussion points out the considerable futility of this.

Lightning dissipation is usually considered the only real advantage of the earth connection in low-voltage systems, and mostly in medium-voltage systems.
 
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