3 wire services

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Old 03-10-11, 02:34 PM
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3 wire services

I am wondering what the point is in still allowing three wire services while banning 3-wire feeders for the obvious safety reasons. The way I can see it, the same thing can happen with a 3-wire service as what can happen with a three wire feeder, the neutral breaks, and the electricity is going to want to flow through the casing, wanting to take any path to get back to the neutral on the pole xfrmr, which is always grounded. Wouldn't it be smarter for the codes to do everything in their power to stop allowing the bond at the first disconnect? I want to push it back to the pole xfrmr lugs personally.
 
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Old 03-10-11, 02:42 PM
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The point of the bond needs to be at the first over-current protection device, which is almost always the main breaker panel. If the bond was upstream of the main breaker, you would be uncertain of the main breaker's ability to clear a ground fault. Furthermore, individual service drops do not have over-current protection so a fault could run well into the 1000s of amps before the primary fuse on the transformer opened or the drop melted. This is why it is essential to ensure the main breaker can clear a ground fault and keep that kind of fault current outside the house.
 
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Old 03-10-11, 04:22 PM
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If the bond was upstream of the main breaker, you would be uncertain of the main breaker's ability to clear a ground fault.
How would that happen?
 
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Old 03-10-11, 04:35 PM
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Because the increased impedance of the service drop neutral would limit short circuit current which would in turn increase the breaker's response time. You want the lowest impedance ground as possible between the fault and the overcurrent protection device to ensure the quickest response.

This is similar to the reason why rusty conduit is a lousy ground. If a hot wire shorts to a rusty pipe, the pipe may not carry enough current to trip the breaker immediately, but it will carry enough to turn the conduit into a heating element.

When you add several hundred feet of aluminum or steel cable service drop neutral to the fault path, it can substantially slow the main breaker's response time.
 
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Old 03-10-11, 07:29 PM
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How would a ground wire have more impedance than a neutral wire, with current flowing through it that can be on the same phase, or like my town, 4/0 feeding 7 houses, 3 0f which have 200A services? I don't seem to be getting this.
 
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Old 03-11-11, 11:51 AM
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Justin; so, the question is: Why is something like a dryer circuit now a 4 wire system, when the service lateral is still a 3 wire system? I'd suggest it's a cost/risk assessment question.
If the overhead sustains an open neutral, yes, that's pretty bad. The homeowner would soon see items not working correctly; the question remains: is it dangerous? Depending on the quality of the earth ground, the local ground potential will drift up as a function of the unbalanced load. Not typically dangerous, though, as the bonding short attaches to home plumbing and local earth. So, the actual risk of a serious shock is still fairly low, though admittedly, not zero, especially with plastic plumbing and just one or two ground rods.
Compare that to an open neutral on a 3 wire dryer feed. Now, the homeowner is exposed to a live steel cabinet, fed by a 120 v line thru a timer and motor to his/her body then their bare foot to concrete ground. That will give a serious jolt.
My overall take is that a 4w service lateral would be somewhat safer, at a pretty high cost of the additional conductor. Which, I might add, is just as likely to fail as the current carry neutral, with no recognition of the defect.
 
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Old 03-11-11, 12:41 PM
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The NEC is not a debateable subject . The NEC requires that the Neutral Service Conductor be Grounded at an accessible point on the "Line" side of the Service Dis-connect , forming the "Grounded Circuit / System Conductor".

The Grounded Circuit Conductor is required to be isolated from Ground on the "Load" side of the Service Dis-Connect because if there was a break in this Conductor , and it was Ground-connected , the circuit current would flow thru the Grounding path which could be the metallic surface of a armored cable ,or the metallic surface of a raceway; not all Grounding paths are copper conductors equivilent in size to the circuit conductors.
 
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