Thread: electrical ground question

Did an inspection yesterday on a house built in 1932. The electrical system has reportedly been completely upgraded. Used my simple plug-in tester and most outlets tested fine (a couple had reversed hot/neutral, and no GFCI, but those are other issues). The panel in the basement was indeed pretty new. Had mostly flexible armored cable running from it, and the cables enclosed two fabric wrapped wires and no separate ground wire. I recognize that the metal armoring can act as a ground... assuming it is continuous. Wanted to make certain I wasn't seeing false grounds, so I opened a couple of outlets. Two wires, nothing attached to the ground screw. So my potentially dumb question is how are the outlets reading as grounded? Is it simply picking up a ground from the box and the armoring with no attachment to the ground screw needed? Thanks in advance!

Wired with a BX cable and a metal box. Ground screw with outlet bracket attached to metal box making the connection to the BX cable back to the service panel.

To often the electrical system is said to have been completely upgraded when they actually they are just saying that the service panel has been replaced and possibly the service cable has also been replaced.

BX does not provide a suitable ground path.

The receptacles should be treated as though they are not grounded, either two-wire ungrounded-type receptacles should be used (two-prong receptacles) or, because the receptacles have been replaced, the receptacles need to be GFCI protected if the receptacles are the grounding-type (two-slots with ground).

While I have not seen it myself, I have heard and read where electricians have seen the outer armor covering glowing red from a fault.

Visualize the outer armor covering this way - it is spiral wrapped, right? So, 'unwrap' the armor covering of a one foot piece (buy a 1 foot piece of MC cable or 3/8" (or 1/2") flexible metal conduit and unwrap it) ... now visualize how long that ground path is over a 40 foot run !!!!

Steel is not as good of a conductor as copper, has a lot more resistance, now increase the length multiple times to account for the spiral wrap and that is a lot of resistance in that armor covering ... no wonder it will glow red and not trip a breaker, it is nothing more than a long strip heater like in a portable electric heater you use (and see the strips glowing red hot).

'Newer older' armored cable has flat ribbon strip which is run with the wires and connects each spiral wrap with the adjacent spiral wrap, making the ground path as short as the length of the cable.

Newer AC cable typically has a round aluminum wire run the length of the cable with the wires, the wire touches each spiral wrap but is more resistant to damage than the ribbon strip.

And, no, neither the ribbon strip nor the wire is required to be connected to ground. The use of a proper connector to the cable makes the connection to the armor, the internal grounding wire simply reduces the ground path to a straight path the length of the cable instead of spiral path the length of the spiral wrapped armor.

Originally Posted by Jerry Peck

BX does not provide a suitable ground path.

The receptacles should be treated as though they are not grounded, either two-wire ungrounded-type receptacles should be used (two-prong receptacles) or, because the receptacles have been replaced, the receptacles need to be GFCI protected if the receptacles are the grounding-type (two-slots with ground).

While I have not seen it myself, I have heard and read where electricians have seen the outer armor covering glowing red from a fault.

Visualize the outer armor covering this way - it is spiral wrapped, right? So, 'unwrap' the armor covering of a one foot piece (buy a 1 foot piece of MC cable or 3/8" (or 1/2") flexible metal conduit and unwrap it) ... now visualize how long that ground path is over a 40 foot run !!!!

Steel is not as good of a conductor as copper, has a lot more resistance, now increase the length multiple times to account for the spiral wrap and that is a lot of resistance in that armor covering ... no wonder it will glow red and not trip a breaker, it is nothing more than a long strip heater like in a portable electric heater you use (and see the strips glowing red hot).

'Newer older' armored cable has flat ribbon strip which is run with the wires and connects each spiral wrap with the adjacent spiral wrap, making the ground path as short as the length of the cable.

Newer AC cable typically has a round aluminum wire run the length of the cable with the wires, the wire touches each spiral wrap but is more resistant to damage than the ribbon strip.

And, no, neither the ribbon strip nor the wire is required to be connected to ground. The use of a proper connector to the cable makes the connection to the armor, the internal grounding wire simply reduces the ground path to a straight path the length of the cable instead of spiral path the length of the spiral wrapped armor.

Thanks!

I'll also add that when most homes are "rewired", way too many times the wiring for the light switches and fixtures and wiring through the walls is never touched. Many times you can use your non contact voltage sniffer to detect old wiring.... We all have a non contact volt sniffer, don't we?

Originally Posted by Tom Iovanne

Did an inspection yesterday on a house built in 1932. The electrical system has reportedly been completely upgraded. Used my simple plug-in tester and most outlets tested fine (a couple had reversed hot/neutral, and no GFCI, but those are other issues). The panel in the basement was indeed pretty new. Had mostly flexible armored cable running from it, and the cables enclosed two fabric wrapped wires and no separate ground wire. I recognize that the metal armoring can act as a ground... assuming it is continuous. Wanted to make certain I wasn't seeing false grounds, so I opened a couple of outlets. Two wires, nothing attached to the ground screw. So my potentially dumb question is how are the outlets reading as grounded? Is it simply picking up a ground from the box and the armoring with no attachment to the ground screw needed? Thanks in advance!

While current BX cable does provide a ground conductor through the armor, it is an expensive alternative to NM-B (Romex). If BX was run inside the walls, then NM-B could have been used. Not to mention, BX requires special box connectors. It is ok that they went to the extra expense for BX, but gained nothing over NM-B.

Originally Posted by Jimmy Roberts

While current BX cable does provide a ground conductor through the armor, it is an expensive alternative to NM-B (Romex). If BX was run inside the walls, then NM-B could have been used. Not to mention, BX requires special box connectors. It is ok that they went to the extra expense for BX, but gained nothing over NM-B.

There is no current BX cable, it would be AC cable or MC cable.

I think the conclusion was that the old wiring (BX) was not replaced when the house was 'completely rewired', only the panels were replaced.

That is quite common.

The OP did not say what was used.

I introduced the thought that the wiring was BX due to age of house. Not Tom Iovanee.

My thought was that the original wiring was left in place and original would create OP condition for his question. Not saying saying it meets code, just what is going on with ground at wall outlet and tester.

Originally Posted by Tom Iovanne

Thanks!

Rely on it. But let me ask my next dumb question... how are you able to detect old wiring with it? Isn't voltage voltage, regardless of what kind of wire it's on?

Originally Posted by Tom Iovanne

Had mostly flexible armored cable running from it, and the cables enclosed two fabric wrapped wires and no separate ground wire.

Originally Posted by Garry Sorrells

The OP did not say what was used.

I introduced the thought that the wiring was BX due to age of house. Not Tom Iovanee.

Actually ... ... Tom did say what was used.

Tom "described" BX ... you out a name to what he described.

Originally Posted by Tom Iovanne

Rely on it. But let me ask my next dumb question... how are you able to detect old wiring with it?

Not sure what you are referencing in the above.

Isn't voltage voltage, regardless of what kind of wire it's on?

Yes.

The only way to know what voltages are where is to use a volt meter and measure the voltage. MOST residential wiring is 120 volt / 240 volt, occasionally ... er ... rarely ... will you find 3-phase in a single family house.

There were a few areas in Coral Gables in South Florida where the service was 120 volt / 240 volt with a 3-phase drop for the air conditioner - that is the only place that 3-phase drop went to. Most of those have now been abandoned because the AC units were typically replaced with 208/240 volt rated units and a new 240 volt circuit is run from the service equipment to the condenser unit. The air handler unit is usually near the panel inside the house and a new 240 volt circuit is run from the panel to the AHU.

Originally Posted by Jerry Peck

where the service was 120 volt / 240 volt with a 3-phase drop for the air conditioner - that is the only place that 3-phase drop went to.

Don't think you'd get 240 volts on 3-phase... it'll be 120 / 240 for single phase (two hots & a neutral) or 120 / 208 for 3-phase (three hots & a neutral).