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#172062 - 12/11/07 06:05 PM Short Circuit Current
BigB Offline

Registered: 03/31/04
Posts: 725
Loc: Tucson, AZ USA
I had a strange thing happen today. I was working on an old house, wired partly with BX. I was attempting to pull a receptacle out of a box when one of the conductors shorted to the metal box. I saw the spark, followed by the hum, but the breaker didn't trip. It actually happened twice, with one instance for over three seconds. The breakers are all recent from a recent upgrade. Now I know BX without the bonding strip is an unreliable ground fault path. But what I can't understand is the hum. For the circuit to hum like that, I would think there has to be mega current flowing. And if there is mega current flowing, the breaker should open before three seconds. So I guess what I am really asking is, how much current has to flow to make the conductors hum quite noticeably? And if there is a huge amount of current flowing from the ground fault, the grounding path must be low resistance to allow it, right?

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#172066 - 12/11/07 06:44 PM Re: Short Circuit Current [Re: BigB]
ausador Offline

Registered: 03/26/07
Posts: 49
Loc: Central Florida
Interesting question....very subjective....with no possible correct answer.

House is wired partly with the circuit in question wholly BX back to the panel with proper and tight connectors?

Not all breakers trip you know, sure U.L. says they do but it doesn't mean it is so. I was foreman on a 12 million dollar addition/remodel to a sears store. You could take any hot from any panel and weld it to the box from the fault current and then sit there and watch it hum.... (btw before we pulled grounds in all the conduits the fault current to ground could be as little as 4 amperes)

26 panels in this building, the grounding conductor rang at 3 ohms to earth from the farthest. Explain it? can I?..except to speculate about 40 year old breakers and how they were built....

An audible hum in my experience can happen far below the trip threshold of most breakers but only when the ground reference is compromised otherwise the fault would trip the breaker almost instantly.

Wires can start "singing" with as little as 5 amperes of current when they are not balanced with a neutral enclosed in the same conduit.

Anyone working with the early hydraulic elevators will tell you about hearing the wire "slapping" against the inside of the pipe each time the motor cycled. (the wires actually try to straighten out each time the load is applied, but are restricted by the bends in the conduit.)

anyway if you choose to give more information we can try to give a better answer...but right now all I can say is that I doubt there is any "mega" current flow going on. If there is you should verify it with an amprobe, then you will have a documented fault and ask the place to pay you to fix it.....:)

#172069 - 12/11/07 07:13 PM Re: Short Circuit Current [Re: BigB]
KJay Offline

Registered: 11/27/07
Posts: 763
Loc: MA, USA
I’ve see that happen before a few times. If that were a Cutler Hammer CH series breaker, you could probably sit there listening to it hum and buzz like an arc welder indefinitely until somewhere in the circuit the BX sheathing turns into a glowing hot red/orange heating coil and sets the surrounding structure ablaze. I know of at least two older buildings that burnt down like that. The farther away from the panel the worse it gets. A lousy, indirect, high impedance path to ground.

#172074 - 12/11/07 08:04 PM Re: Short Circuit Current [Re: KJay]
Gmack Offline
Junior Member

Registered: 05/14/06
Posts: 30
Loc: Michigan
It is not the CB,s problem. It is just allowing a "load" as any other such, as a 1500 watt heater, transformer, ballast, whatever.

The "hum" or noise is the same as with mentioned loads,

The service upgrade would just make the BX hum all the more.

BX clad is an impeadance/load and not an EGC.

So the breaker doesnt care. Until overload occurs. No that is subjective depending on many variables

#172075 - 12/11/07 08:11 PM Re: Short Circuit Current [Re: Gmack]
BigB Offline

Registered: 03/31/04
Posts: 725
Loc: Tucson, AZ USA
So that answers my question, I didn't think you would hear the conductors humming at a load lower than the OCPD's trip threshold.

The residence in question had an upgrade and some new romex circuits added, however they just re fed the old BX circuits with romex/j boxes.

This house is a nightmare, I am doing the work for an organization that assists low income homeowners.

First time I ever saw eyeball trims installed in the ceiling, held up with baling wire to the rafters, the socket spliced in with a flying splice. These are just trims mind you, with NO CANS.


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