connection between grounded and grounding bar in residen

[rcarrier]

I am seeking explanation,clarification and guidance on the following Residential Service scenarios:#1 From utility meter enclosure, a 3-wire cable is used,an additional grounding electrode conductor is connected to the grounded conductor. This 3-wire cable enters the service (breaker)panel,there is a "tie-bar" between the neutral (grounded)conducor bar/buss and the grounding bar/buss, and the grounding bar bonding screw is used to bond these to the metal enclosure. yes/no?
scenario#2 From service (breaker)panel mentioned above,a 2-pole breaker is used to feed a "sub-panel" in an attached garage. Does the "tie-bar" between the neutral and grounding bars/buss in the panel in the garage get used ...or is it removed? ...and where is this stated in article 250?
Scenario #3: Meter enclosure is a combination meter/breaker because of long distance to service/breaker panel. Now a 4-conductor cable is used because of Art.250-142(B).In the service/breaker panel,Does the "tie-bar" between the neutral and grounding bars get used or removed? Thanks!

 

[charlie b]

Re: connection between grounded and grounding bar in residen

Scenario #1: Yes.

Scenarios #2 and 3: Removed.

You yourself cited the relevant NEC article: 250.42.

The reasons that there is a difference are (1) That current is always seeking a path back to its source, and (2) That current will take every available path. The function of the equipment grounding conductors (EGC), that ones that connect to the ground bar in the panel, is to carry fault current. If a fault occurs with a piece of equipment, such that a hot conductor comes into contact with the case or other external metal part, any person who touches that equipment is going to get a shock. The shock can be enough to kill, but the current will not be high enough to cause the breaker to trip. However, with the EGC creating a path from the case back to the ground bar, then via the ground screw or bonding jumper to the neutral bar, the current in this path will be high enough to trip the breaker. This will terminate the event before the person can receive a fatal shock. That is why the ground and neutral buses are connected at the main service disconnecting means ? to complete the current path from the fault point back to the source.

However, if you also connect the ground and neutral at a subpanel, then there will be two paths for current to flow back to the source during normal operation. Current will be flowing in the neutral most of the time (unless the loads running at the moment are perfectly balanced among the phases). But with the ground and neutral tied together both at the main panel and at the subpanel, the EGC will be in parallel with the neutral wire. Therefore, the EGC will carry current. This will cause the external metal parts of each and every component that has an EGC its to become energized. You could not safely touch anything in the facility.

 

[twistin214]

Re: connection between grounded and grounding bar in residen

if we could add scenario #4 to a detached garage with 3 wire/no egc. do we refer to 250.32(B)(2)and bond ground bar-nuetral and drive a ground rod.thanks and no other metallic paths.

 

[roger]

Re: connection between grounded and grounding bar in residen

Originally posted by twistin214:
if we could add scenario #4 to a detached garage with 3 wire/no egc. do we refer to 250.32(B)(2)and bond ground bar-nuetral and drive a ground rod.thanks and no other metallic paths.

Yes

Roger

 

[roger]

Re: connection between grounded and grounding bar in residen

Actually, I should have pointed out that one rod may not be enough, (per the NEC) read 250.32(A), 250.50, and 250.56

Roger

 

[bphgravity]

Re: connection between grounded and grounding bar in residen

The bare conductor in a 2-wire SEU cable is permitted to be used as a grounded conductor when serving a separate structure. See 338.10(B) Exception. In this case, the rules of 250.32(B)(2) must be met.

 

[charlie b]

Re: connection between grounded and grounding bar in residen

A side question was posted on this thread by a DIY. I have removed that side question and all responses that were related to it.