Results 1 to 12 of 12
  1. #1
    Join Date
    Nov 2012
    Location
    Olympia WA
    Posts
    11

    Default 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!

    Crawl Space Creeper

  2. #2
    Join Date
    Dec 2008
    Location
    Maryland
    Posts
    2,809

    Default Re: electrical ground question

    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.


  3. #3
    Join Date
    Mar 2007
    Location
    Fletcher, NC
    Posts
    28,042

    Default Re: electrical ground question

    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.

    Jerry Peck
    Construction/Litigation/Code Consultant - Retired
    www.AskCodeMan.com

  4. #4
    Join Date
    Nov 2012
    Location
    Olympia WA
    Posts
    11

    Default Re: electrical ground question

    Quote Originally Posted by Jerry Peck View Post
    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!


  5. #5
    Join Date
    Mar 2007
    Location
    Spring Hill (Nashville), TN
    Posts
    5,851

    Default Re: electrical ground question

    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?

    Scott Patterson, ACI
    Spring Hill, TN
    www.traceinspections.com

  6. #6
    Join Date
    Jan 2011
    Location
    Utah
    Posts
    112

    Default Re: electrical ground question

    Quote Originally Posted by Tom Iovanne View Post
    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.


  7. #7
    Join Date
    Mar 2007
    Location
    Fletcher, NC
    Posts
    28,042

    Default Re: electrical ground question

    Quote Originally Posted by Jimmy Roberts View Post
    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.

    Jerry Peck
    Construction/Litigation/Code Consultant - Retired
    www.AskCodeMan.com

  8. #8
    Join Date
    Dec 2008
    Location
    Maryland
    Posts
    2,809

    Default Re: electrical ground question

    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.


  9. #9
    Join Date
    Nov 2012
    Location
    Olympia WA
    Posts
    11

    Default Re: electrical ground question

    Quote Originally Posted by Tom Iovanne View Post
    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?


  10. #10
    Join Date
    Mar 2007
    Location
    Fletcher, NC
    Posts
    28,042

    Default Re: electrical ground question

    Quote Originally Posted by Tom Iovanne View Post
    Had mostly flexible armored cable running from it, and the cables enclosed two fabric wrapped wires and no separate ground wire.
    Quote Originally Posted by Garry Sorrells View Post
    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.

    Jerry Peck
    Construction/Litigation/Code Consultant - Retired
    www.AskCodeMan.com

  11. #11
    Join Date
    Mar 2007
    Location
    Fletcher, NC
    Posts
    28,042

    Default Re: electrical ground question

    Quote Originally Posted by Tom Iovanne View Post
    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.

    Jerry Peck
    Construction/Litigation/Code Consultant - Retired
    www.AskCodeMan.com

  12. #12
    Join Date
    Mar 2007
    Location
    Vancouver - Canada
    Posts
    225

    Default Re: electrical ground question

    Quote Originally Posted by Jerry Peck View Post
    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).

    Joe Klampfer RHI
    www.myinspection.ca
    Pacific Home Inspections

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •