Bonding of Metallic Cable Armor

[timm333]

I have a question about the bonding of metallic armor of cables as per NEC. For armored power cables: we connect the armor to the “main” ground on both ends of cable . For armored signal cables: we connect the shield wire to the “isolated” ground at only one end of cable.

For armored signal cables: should the armor be connected to the “isolated” ground or to the “main” ground? Also for armored signal cables: should the armor be connected to ground at one end of cable or at both ends? Thanks

 

[roger]

There are article 320 and 330 cables, they are different as far as EGC's are concerned. Look at both types to determine what you have.

 

[timm333]

We have multi-conductor MC cables so article 330 applies. The cables have Aluminium interlocked armor. Armor is not used as EGC. It looks that article 330 does not give information for bonding of armor.

In general the shield (screen) of "armored signal cable" is connected to the isolated-ground (only on one end of cable.) So it would make sense to also connect the armor of "armored signal cable" to the isolated-ground (only at one end of cable). What do you guys think about this?

 

[hbiss]

I think what the NEC requires.

Armor is not used as EGC. It looks that article 330 does not give information for bonding of armor.

330.6 Listing Requirements. Type MC cable shall be listed.
Fittings used for connecting Type MC cable to boxes, cabinet,
or other equipment shall be listed and identified for such use.

The armor is bonded by the connectors to the metallic box which in turn is bonded to the green EGC. If your MC is the type that does not have a green EGC but instead a heavy bare aluminum conductor under the armor, the armor indeed does serve as the EGC. The connectors bond the cable armor to the box just like type AC.

-Hal

 

[timm333]

Our multi-conductor cables have a green EGC (not the bare aluminium conductor under armor).

I was just wondering if we connect the armor of signal cable to ground on both ends, then ground loop current might flow. (Just like ground loop current flows in the shield/drain wire when both ends of shield/drain wire are connected to ground.)

The cable has gland on both ends. Maybe it would be better that for signal cable: grounding-locknut is used on the gland on one end of cable, and no grounding-locknut is used on the gland on the other end of cable.

 

[hbiss]

It sounds like you are using MC cable for a non-electrical application in a way it was not intended. The NEC would not apply in that case, and you are on your own to come up with a design that works for you.

-Hal