Safety ground on a meter base

[stevenal]

Got a call from the head state electrical inspector this morning. Seems he objects to a drawing of mine as being non-standard. The drawing in question shows the service neutral passing through the meter base on to the service disconnect where the main bonding jumper connects it to the enclosure and the grounding electrode system. From here a bond wire returns to the meter base to bond the enclosure.

The standard method according to him is to connect the service neutral to the meter base enclosure on its way through to the service. No extra bond wire is used.

Seems to me this violates NEC 250.6 since unbalanced load current is passing through the conductor used to ground the meter base enclosure. I also believe this method to be unsafe, since the load current combined with a poor connection could case excessive touch or step potentials.

Is this method really standard? Any other comments? Thanks.

 

[davidbeach]

Steve, if the neutral is grounded at the service disconnect, and assuming a reasonably short distance between the meter and the service disconnect, how would you generate a voltage on the meter base using the inspectors method that wouldn't also be imposed on the meter base via your method? Voltage drop in the neutral conductor means that there has to be some difference in "ground" potential between the transformer and the service disconnect. Either means of connection will result in similar voltage differences between "ground" at the transformer and the meter base housing.

I don't see anything particularly wrong with your connection, and it might well be easier to work with a smaller bond conductor than to tap the neutral, but I'm also not sure it gains a whole lot.

On the other hand, if there is a significant distance between the meter base and the service disconnect, you could well have a lower potential at the meter base than using the inspector's connection.

 

[stevenal]

David,

All we need is some load current in the neutral, the two enclosures within a reachable distance from each other, and a resistive connection at either end. Hand to hand touch potential with the inspector's method, and no fault is required. My method has no current (normally) in the bonding conductor. Even if connections are poor, no difference in potential can occur without some current to cause a drop. I see your point regarding hand to foot potentials.

 

[davidbeach]

Steve,

You're right, I wasn't thinking hand-to-hand potentials, and I'm also picturing the meter base on an exterior wall and the service disconnect on the inside and within one stud space. The voltage drop in the neutral due to load current, while non-zero, will be very small; is it enough to be a problem? In your connection, it could depend where the resistive connection is, in the right place your bond wire could be holding the meter base at essentially the same potential as the inspector's connection. The resistive connection could also be elsewhere and have the meter base at the same potential as the local ground.

Is this the battle you want to have with the state? I've had dealings with inspection agencies where there was no way I was going to allow them to win, and there were others where I knew I was right, but in the interest of maintaining good relations I just let it go.

 

[stevenal]

David,

Hey I'm always ready for a good fight, just ask my wife. Point taken, though.

Anyone care to speak of the code issues? Or what really is standard practice?

 

[dpc]

FWIW, the NEC (250.24) would allow the grounding electrode conductor to be connected to your grounded service conductor within the meter base enclosure instead of inside the service disconnect enclosure.

Your approach seems preferable to me, but is the neutral insulated from the meter base enclosure on its way through?

Does EUSERC have any guidance on this?

I wouldn't count on logic or scientific reasoning to have much of an impact on the person you are dealing with.

 

[stevenal]

Thanks DPC, that section was just pointed out to me. Seems at odds with 250.6. Yes neutral is insulated from enclosure on its way through.

I asked EUSERC previously if their standards addressed grounding details. Their answer was no, so I didn't buy them.

 

[dpc]

Stevenal,

From April/May 2002 necDigest published by NFPA:

"The number of bonding bushings required for a given installation will depend on the bonding functions that have to be accomplished. For example, given an installation where rigid metal conduit is used to enclose the service-entrance conductors from a self-contained meter enclosure (socket) to the service panelboard, the use of a single bonding-type bushing or locknut on either end of the conduit complies with the requirement to bond all of the non-current carrying metal parts. The construction of the meter enclosure provides a direct connection between that enclosure and the grounded service conductor. That connection bonds the meter enclosure. At the panelboard, the required main bonding jumper connects the enclosure and all equipment grounding conductors to the grounded service conductor. That connection effectively bonds the panelboard enclosure. At this point, the raceway is all that remains to be bonded, and a single bonding connection to one end of the service raceway effectively bonds that raceway. Any of the means specified in 250.92(B) can be used to bond the service raceway and, as discussed, bonding-type bushings and locknuts can be used to meet this bonding requirement. "

This seems to be imply that your neutral conductor at least **can be** bonded directly to the meter enclosure and satisfy NEC.

 

[dpc]

http://www.naelectric.com/pdfs/contractHandbk/NESH_ch4.pdf#search="grounding meter enclosure NEC"

If you can open this link, scroll down to section on grounding of meter bases. Seems to agree with the inspector, at least for hot sequence.

 

[stevenal]

Thanks DPC,
One locknut at one end only? Seems sloppy. And the lack of a locknut doesn't prevent continuity, it just doesn't ensure it. So conduit and neutral are in parallel when the big trucks roll by? I don't understand code committees.

 

[dpc]

Well, I'd never rely (solely) on a conduit system for a safety ground, but it's still allowed by NEC. Kauffman/GE showed back in the 1960s that steel conduit is an unreliable grounding conductor.

The NEC committees have representatives from manufacturers and trade association who resist changes that might have a negative impact on their products. Like a lot of things, it's an imperfect process.

 

[waross]

Re one locknut.
I believe that the reference is to a grounding type locknut, That is, either a grounding type bushing must be used or one of the locknuts must be a grounding type.
put another way;
Either the bushing or ONE of the locknuts must be a grounding type.
respectfully