bond bushings

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gserve

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Is a bond bushing required by code to be installed on an offset nipple where one end is screwed into a myers hub mounted on the main disconnect enclosure and the other end installed into the bottom of a meter socket? These are service entrance conductors from the bottom of the meter to the top of the main disconnect(line side). Code reference please. Thanks
 
Re: bond bushings

The short answer is yes. See 250.92(B)(4). Although if the meter socket didn't have concentric or eccentric KO's than a bonding locknut or bonding wedge lug could be used instead of the bonding bushing.

[ March 16, 2005, 08:47 PM: Message edited by: infinity ]
 
Re: bond bushings

I say no. Only one end the raceay needs to be bonded. 250.92(B)(2) covers this example as one side of the offset is threaded into a hub.
 
Re: bond bushings

Since the original question stated that the entry into the service disconnect was via a Myers hub I would stand by my original answer. A Myers hub is a fitting that seals a conduit entry in a wet location. In this instance the hub is entering the top of a service disconnect. Since Myers hubs are usually supplied with a locknut that is not of the bonding type than bonding on one end of the offset nipple would be required. It could be at either the meter pan or the service disconnect as per 250.92(B)(4).
 
Re: bond bushings

bphgravity...I see in the code that bonding is only required on ONE end of metalic raceways enclosing service conductors, and its purpose is for a low impedance path to open primary overcurrent devices in case of a ground fault.

But, doesn't this metalic raceway also provide a parallel path for the neutral current? What if there was a neutral connection failure where it connects to the ground bar in the service disconnecting means and the neutral current were to flow through the metalic raceway via the bonding screw back to the source?

On the end of that raceway,where there isn't any bonding, and the locknut wasn't making a good connection, wouldn't there be high resistance path that could cause arcing and possibly a fire where the neutral current were seeking this path to the source?

shortcircuit2
 
Re: bond bushings

The utility grounded conductor is terminated in the meter enclosure. This connection is bonded to the enclosure. The grounded conductor then continues to the service disconnect where it is terminated to a connection that is to be bonded to the service equipment enclosure. If the racway between the two enclosures is metallic, it needs to be bonded per a method as inidcated in section 250.92(B). (2) indicates a raceway is considered bonded by utilizing a threaded boss.

That's it your done. There is no requirement for both ends of the raceway to be bonded. A low-impedance path is provided by one end bonding.
 
Re: bond bushings

I'm surprised to see you up this late Bryan.

[ March 18, 2005, 12:39 AM: Message edited by: physis ]
 
Re: bond bushings

bphgravity...I understand that bonding is only required by the NEC on 1 end of the metalic raceway between the meter can and the service disconnect.

But in my last post I described a situation that could cause a fire...

If the grounded conductor connection in the service enclosure that is bonded to the enclosure were to fail for some reason, lets say aluminum wire that has corroded over a period of time or a termination that was not tightened by the electrician and eventually lost connection (which I have seen)...

Wouldn't the neutral current seek an alternate path through that bonded metalic conduit that was only bonded on 1 end, and if that same electrician had a rather loose locknut on the end that wasn't bonded, there would be arcing at that locknut where the neutral current were seeking a path back to the source, and that would possibly be a fire hazard, wouldn't it???

shortcircuit2
 
Re: bond bushings

Originally posted by shortcircuit2:
I see in the code that bonding is only required on ONE end of metalic raceways enclosing service conductors, and its purpose is for a low impedance path to open primary overcurrent devices in case of a ground fault.
Click to expand...
What overcurrent device do you expect service raceway bonding to open?

Very unlikely it will open the fuse on the high voltage side of the POCO transformer and it will not open the service disconnect.

All the bonding will do is keep the raceway at the same potential as the meter and panel enclosures.


Originally posted by shortcircuit2:
bphgravity...I understand that bonding is only required by the NEC on 1 end of the metalic raceway between the meter can and the service disconnect.

But in my last post I described a situation that could cause a fire...
Click to expand...
ShortC2 what are you looking for here? :)

Bob
 
Re: bond bushings

The utility grounded conductor is terminated in the meter enclosure. This connection is bonded to the enclosure. The grounded conductor then continues to the service disconnect where it is terminated to a connection that is to be bonded to the service equipment enclosure. If the racway between the two enclosures is metallic, it needs to be bonded per a method as inidcated in section 250.92(B). (2) indicates a raceway is considered bonded by utilizing a threaded boss.
Click to expand...
I guess I still don't completely understand services. There are two things here that don't look right to me.

Wouldn't there two be MBJ's?

Isn't the neutral being used as a bonding conductor?
 
Re: bond bushings

Sam any meter or CT can that I have worked on has the 'neutral' terminal solidly connected to the meter enclosure.

On a PVC or SE service this works well, there are no parallel paths for unbalanced neutral current and the meter enclosure is grounded by the 'neutral' which at that point is really an MGN (Multi-ground-neutral.) This is expressly permitted by 250.142(A)

When you use a metal raceway between the bonded meter and panel enclosures you have created a parallel path for unbalanced neutral current.
 
Re: bond bushings

I've been looking everywhere for this code, thanks Bob.

But then I'm reading it and I can't find permission to do that. 250.142(B) Ex. comes close.

This just seems to break the rules, I don't get it.

Edit: I had to fix the code ref.

[ March 18, 2005, 12:05 PM: Message edited by: physis ]
 
Re: bond bushings

Originally posted by physis:
I've been looking everywhere for this code, thanks Bob.

But then I'm reading it and I can't find permission to do that. 250.142(B) Ex. comes close.
Click to expand...
Sam permission to do which, use the neutral as a grounding means or create a parallel path?
 
Re: bond bushings

Sam I agree it does sound like it breaks the rules, but the rules are different on the supply side of the service disconnect, or on the supply side of the MBJ as another way to look at it.

250.142 Use of Grounded Circuit Conductor for Grounding Equipment.

(A) Supply-Side Equipment. A grounded circuit conductor shall be permitted to ground non?current-carrying metal parts of equipment, raceways, and other enclosures at any of the following locations:

(1)On the supply side or within the enclosure of the ac service-disconnecting means
Click to expand...
The permission to ground the meter enclosure with the grounded circuit conductor is right there in 250.142(A)(1). :)
 
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