ECN Forum Forums Code Related Discussion NEC & other Code issues neutral & ground - isolated

Does anyone know the real reason a sub-panel has the neutral isolated from the ground in the panel, but back at the main panel it is tied together?

Because a 'grounding' conductor differs from a 'grounded' conductor in that it does not carry current.

the exception being that service entrance conductors are allowed the dual usage of both.

See the 1999 NECH Commentary in Section 384-20:

384-20.
A separate equipment grounding conductor terminal bar is required to be installed and bonded to the panelboard for the termination of feeder and branch-circuit equipment grounding conductors. Where installed within service equipment, this terminal is bonded to the neutral terminal bar, as illustrated in Figure 384.5. Any other connection between the equipment grounding terminal bar and the neutral bar, other than allowed in Section 250-32 is not permitted. If this downstream connection occurs, current flow in the neutral or grounded conductor would take parallel paths through the equipment grounding conductors (the raceway, for example) back to the service equipment. Normal load currents flowing on the equipment grounding conductors could create a shock hazard. Exposed metal parts of equipment could have a potential difference of several volts created by the load current on the grounding conductors. Another safety hazard could be created by this effect where subpanels are used, since arcing or loose connections at connectors and raceway fittings, etc., could serve to create a potential fire hazard.

>Exposed metal parts of equipment could have a potential difference of several volts created by the load current on the grounding conductors.
And then the question becomes one of how many milliamps could flow.

And I insist that with a three wire electric clothes dryer, there are enough mA to be fatal.

If the neutral & ground were tied together somewhere on the interior of a facility, the grounding system would then carry a portion of the neutral current and a few problems would arise. For one, an unsuspecting person may open up a ground connection, become in series with the open connection, allowing the current to flow through him. This could be fatal. Another thing is that a loose connection anywhere on a raceway that serves an EGC would arc and potentially cause a fire. The safest place to bond is at the service equipment. Making this connection is necessary to allow ground fault current to flow back to the utility transformer with a low enough impedance to either operate the primary fuses or to allow the fault to burn clear with minimal arcing.

You guys already nailed this one, how 'bout an example. Sub-panel noodle and ground NOT isolated, neutral (aluminum, no penetrox) became high impedance, sub-panel fed insta-hot water heater (connection between water pipes and ground) now we had neutral (overcurrent) imposed on plumbing pipe. Cast iron DWV (That's Drain, Waste, Vent just in case) now a wet guy in shower touches cold or hot water spigot with big wet toe on drain and learns new dance in shower. Fixed this 20 years ago, and code became MUCH clearer after I drew the circuit for myself.

I know of an open neutral incident where the current flowing through the plumbing system generated enough heat to melt solder and cause major water damage. It sounds hard to believe that with water in the pipe the solder would melt, but that appears to be the case.

Redsy,
Boy I've heard of that, but never seen it, the only thing close is a welder who had clamped onto a pipe (with water in it) and the ground path was bad, did the same thing, melted the solder off the joints. What a mess, but a NEAT mess.

It's especially hard to believe when you have tried to solder a pipe that has a little water in it and the darn solder won't flow.

hmmm,
would anyone here consider the bond at the X-former, and isolation throughout the premisis ( all service equipment..) a cleaner method?