Grounding Metering Equipment

[corvalan]

I have read the following exception in the NEC:

250.142 Use of Grounded Circuit Conductor for Grounding Equipment.
(B) Load-Side Equipment.
Exception No. 2: It shall be permissible to ground meter enclosures by connection to the grounded circuit conductor on the load side of the service disconnect if
(a) No service ground-fault protection is installed, and
(b) All meter enclosures are located near the service disconnecting means, and ...

Question: Can you explain the REASON of the (a) and (b) conditions?

Thanks.

Ruben

 

[iwire]

Re: Grounding Metering Equipment

(a) Would be about false tripping the GFP.

(b) I imagine would have to do with limiting voltage drop produced potential between the meter enclosure and other bonded parts. This one is just my guess though. :roll:

 

[charlie]

Re: Grounding Metering Equipment

Ruben, IMO this is not a good idea and should be removed from the Code. Most of my colleagues want it left the way it is so I don't argue with them. With that said, I will attempt to answer your questions.

Please understand, the service is not being metered, a feeder is being metered. Grounding on the downstream side of the service disconnecting is done in accordance with the NEC. Therefore, if the grounding is permitted to be done the same as it is on the service equipment, an exception must be made and this is it.

(a) Since the grounded conductor will be used for grounding the metering, it could shunt away some of the current that would operate a ground fault device.

(b) Since you are metering a feeder, why would you be permitted to ground to the grounded conductor when you are not near the service equipment? Please refer to my introductory statement.

 

[newt]

Re: Grounding Metering Equipment

You can use neutral to ground mtr enclosures if they are located close prox. to main service.Unless it is gfi main.