Is it ok to use armored cable (bx) as a ground?

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  #1  
Old 07-30-04, 02:15 PM
sk545
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Is it ok to use armored cable (bx) as a ground?

My house has hybrid type of wiring, some receptacle outlets have NM wiring with a ground which have a 3 prong receptacle installed. Others have a armored cable (bx) type of cable which have a 2 prong receptacle installed. Is it ok for me to change these 2 prongs to a 3 prong without rewiring? As i understand it, armored cable sheath provides a method of grounding, but is it good enough for for 15Amp receptacles? I live in US, Massachussetts.

I am pretty sure its a armored cable bx, but is there a definite way to tell? All i see is a metal spiral sheeth covering a black conductor and a white conductor. Those conductors are in turn covered with cloth like material. All of the outlets are installed in a metal outlet box, not plastic.

Thanks.
 
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  #2  
Old 07-30-04, 02:52 PM
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BX cable can provide grounding, but only if connected properly. And depending on the vintage of BX, it may or may not be good enough grounding for today's standards. Without some sophisiticated test equipment, it's usually not possible to judge the quality of the grounding. But a simple ohmmeter will at least allow you to check for some grounding.

So there is no clear cut answer to your questions. If the ohmmeter indicates at least some grounding, I might go ahead and put in 3-hole receptacle. But I would also put GFCI protection for anything except things that you really should avoid it for (computers, anything on a surge suppressor, refrigerators, freezers, sump pumps, iron lungs, smoke detectors, etc.).
 
  #3  
Old 07-30-04, 04:34 PM
sk545
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thx for answering. I am thinking of getting a electrician to check the wiring, but a question comes to mind: Will the electrician have to check every single outlet i want converted or just one to tell me if it was ok to use the BX for ground?

But a simple ohmmeter will at least allow you to check for some grounding.
I have a multimeter. Is that the same thing? If so, how does one use it to test for grounding? I found the following in a faq:

One moderately safe approach is to connect a 100W lightbulb
between hot and the ground you wish to test. The lamp should
light fully. If you have a voltmeter, test the voltage between
the ground and the neutral. You should see less than 2 volts.
If the voltage is much higher, or the lamp dims, disconnect it
quickly - the ground may be overheating somewhere. The ground
should be checked for poor connections.

ftp://rtfm.mit.edu/pub/usenet/news....al-wiring/part1
Does that sound around the lines of what you were thinking?
 
  #4  
Old 07-30-04, 07:23 PM
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To conduct a simple test for grounding:
  1. Shut off the breaker.
  2. Set your multimeter to the lowest ohm setting it has (the scale with the omega).
  3. Pull a receptacle out of the box without disconnecting any wires.
  4. Put your two probes on the silver screw (or white wire) and the metal box.
  5. If it reads close to zero ohms, then you have at least some kind of grounding.
 
  #5  
Old 07-30-04, 07:54 PM
sk545
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Set your multimeter to the lowest ohm setting it has (the scale with the omega).
Ok, i'll try that, however i would like to know if i am doing it right. The multimeter i have is this one:

http://files.photojerk.com/sk545/IM000472.jpg

I take it that the omega sign is that weird looking horseshoe in the bottom left hand corner? If it is, then that part of the multimeter has these: 2000k, 200k, 20k (blanked out by light in the pic above), 2000, and lastly 200. So i set it 20k and follow your directions, correct? Also, i hope the red and black wires are connected in the correct holes in the multimeter since it has 3 holes...
 
  #6  
Old 07-30-04, 09:07 PM
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Set it on 200 (lowest). 20k (i.e., 20,000) is neither the lowest nor the highest.

Yes, the "horseshoe" is the Greek capital letter omega, for ohms.

Yes, your probes are in the right holes.
 
  #7  
Old 07-31-04, 10:41 AM
sk545
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Here are the results:

Your method:

I get 01.4 ohms as the voltage, its close to zero but is it good enough?

The other lamp method:

The 100watt bulb glows very well without any flickering whatsoever. I get 00.3 ohms (the scale of multimeter set on ACV 200) when i test it by probing the neutral and ground as it says. Well below 2 ohms.

At this point, i thougt it was safe to put in a 3 pronger. I did it to one of the outlets only and will do the rest after the electrician checks out the wiring.

A question about multimeter: Does it matter which probe goes into which hole? Like, can i use the red and black probes interchangeably?

Thx.
 
  #8  
Old 07-31-04, 10:52 AM
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Yes, it sounds like you have a ground, so I'd say the 3-hole receptacles are okay. Of course, we don't know that the ground isn't bootlegged to the neutral somewhere, but you can't know everything.

Does it matter which probe goes into which hole? Like, can i use the red and black probes interchangeably?
The probes are interchangeable.

I get 01.4 ohms as the voltage
You got 1.4 ohms as the resistance, not the voltage.

I get 00.3 ohms (the scale of multimeter set on ACV 200) when i test it by probing the neutral and ground as it says. Well below 2 ohms.
You got 0.3 volts, well below 2 volts, not ohms.
 
  #9  
Old 07-31-04, 11:10 AM
sk545
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oops, sorry about that. Thx for clarifying.



Since i did these methods on only one outlet, does it mean that these results are fine to go by for other BX type outlets in the house? Or do i have to do all of this for each outlet separately?
 
  #10  
Old 07-31-04, 11:58 AM
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I'd repeat the test at each outlet as I replaced it.
 
  #11  
Old 07-31-04, 12:27 PM
sk545
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will do then. At what ohm over zero is the ground considered weak/bad? I am getting 1.4, so i am wondering where one draws the line.
 
  #12  
Old 07-31-04, 01:24 PM
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#14 copper wire has a resistance of about 0.003 ohms per foot. So your reading of 1.4 ohms (if accurate) is equivalent to #14 wire over about 460 feet. That would be 230 feet to the panel on the grounding wire and 230 feet back on the neutral wire. It's probably not that far to your panel, so something is introducing a bit more resistance, probably because the BX doesn't provide as good a connection as #14 copper and/or because the connections aren't that great. But it's not too bad. Keep in mind that these calculations are pretty rough, because the multimeter isn't putting enough current through that connection to understand exactly what would happen if you had a fault current.

When you put that 100-watt bulb between hot and ground, you were running about 0.8 amps over the grounding wire. Then you measured about 0.3 volts between neutral and ground. That suggest a total one-way resistance of about 0.36 ohms, which suggests a distance of only about 120 feet. At least were in the same order of magnitude here. But this test is only valid if absolutely everything else was turned off on this circuit. If anything else was running on the circuit, the voltage on the neutral would not have been zero, and the test would be inaccurate.
 
  #13  
Old 07-31-04, 01:46 PM
sk545
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But this test is only valid if absolutely everything else was turned off on this circuit. If anything else was running on the circuit, the voltage on the neutral would not have been zero, and the test would be inaccurate.
Hmm, are you referring to your test or the bulb test when you say the above?
 

Last edited by sk545; 07-31-04 at 02:05 PM.
  #14  
Old 07-31-04, 02:44 PM
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I made that comment in the paragraph that was discussing the bulb test. With my test, the breaker is off, so of course everything on the circuit is off too.
 
  #15  
Old 07-31-04, 03:56 PM
sk545
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oh right, i forgot about that.

I'll do the bulb one again with everything off, and report back.
 
  #16  
Old 07-31-04, 06:20 PM
sk545
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I get the 0.5 volts instead of 0.3 after making sure nothing was turned on. So it went up very little.

I got sort of a off topic question:

What would happen if i were to accidently screw on the wires on the receptable in the wrong place? In other words, if i srewed the black wire on the silver screw and the white wire on the brass srew of the receptable and turned on the circuit? Will it blow up something or the breaker would just trip? I am asking since it seems like a common mistake.
 
  #17  
Old 07-31-04, 09:02 PM
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Your 0.5 volt reading makes the two tests correlate very well, within the range of precision of the equipment.

Reversing the black and white wires is called "reverse polarity". It does not affect the operation, but does affect the safety. Every homeowner should own one of those $8 outlet testers that has three lights on it. And every homeowner should use it to test every receptacle in the house for proper wiring.
 
  #18  
Old 07-31-04, 11:06 PM
sk545
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you mean like this one:

LINK

I have one like that, and i do get two green lights in it when i plug it into the outlet i am working on. Whats strange is that the tester for some reason doesn't go in all the way into the outlet. Other three prong plugs fit in fine, but the tester just goes in half way and no more. Still, even with half way in, i get 2 green lights. Weird, might be a radioshack problem...
 
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