Trying to convert 2 prong to 3 prong outlets (BX wiring)

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Old 06-06-11, 03:36 PM
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Trying to convert 2 prong to 3 prong outlets (BX wiring)

My house was built around 1957 and I have the old 2 prong outlets. My wiring is BX.
I thought BX was supposed to ground itself back to the panel but when I use a circuit tester it indicates open ground. I plug the 3 prong circuit tester into an adapter while testing. The confusing thing is that if I connect the adapter wire to the center screw in the outlet then plug the tester into the adapter I get a solid ground indication. I need to have solid grounding for my computer equipment and my entertainment equipment so I currently use the adapter method but it makes access to the other outlet plug extremely awkward and impossible for another adapter arrangement below it. If the adapter wire gives true ground indication on both my tester and my suppressors can I assume that I can simply pigtail a ground from my new 3 prong outlet to the metal outlet box?
 
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Old 06-06-11, 03:45 PM
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Originally Posted by Clicker2 View Post
I thought BX was supposed to ground itself back to the panel but when I use a circuit tester it indicates open ground.
It did when originally installed, but the steel sheath corrodes and causes intermittent or open ground. Newer BX includes an aluminum bonding strip tacked to the inside of the steel spiral to maintain continuity. If your BX does not have this bonding strip it is not considered a legal ground anymore.

I need to have solid grounding for my computer equipment and my entertainment equipment
Code allows you to install a retrofit ground wire #12 copper from the panel ground bar to the outlet box you want to be grounded. You can fish it through the studs just like you would with romex cable.

If the adapter wire gives true ground indication on both my tester and my suppressors can I assume
The problem is that the tester may show a good ground because it is using an extremely small current to test the integrity of the steel BX sheath. In a real short circuit or power surge situation you need the ground to handle a lot of amps and the corroded steel simply can't do it. In fact a short circuit on old BX can cause the sheath to heat up red hot without tripping the breaker.
 
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Old 06-07-11, 05:53 AM
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Originally Posted by ibpooks View Post
In fact a short circuit on old BX can cause the sheath to heat up red hot without tripping the breaker.
I can say I have first hand knowledge that this can happen. Bad BX running under my house in a crawlspace caused a fire. This is exactly what the investigator said, that the insulation degraded causing a short to the armor, and it glowed hot enough to ignite the floor joists without tripping the breaker. In order to get a new certificate of occupancy the house had to be completely rewired and all BX removed.
 
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Old 06-07-11, 10:21 AM
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Originally Posted by JerseyMatt View Post
In order to get a new certificate of occupancy the house had to be completely rewired and all BX removed.
Wow, GFCI breakers would have been about 100x cheaper.
 
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Old 06-07-11, 03:25 PM
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Originally Posted by ibpooks View Post
Wow, GFCI breakers would have been about 100x cheaper.
So I gather that you are saying the GFCI is sensitive enough to detect a voltage change due to a short while the breaker is unable to detect the voltage change. Is there any way I could use GFCI and ground my equipment directly somewhere other that back to the box (which would cost me quite a bit since the room in question is over a garage under the house and I dont think I can snake through). I dont think I saw a breaker bar at the box could I be missing something? It is breakers by the way and looks fairly recent. I moved into the house due to a family members passing so I dont have a solid history on what was done when.
 
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Old 06-07-11, 03:50 PM
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Originally Posted by Clicker2 View Post
So I gather that you are saying the GFCI is sensitive enough to detect a voltage change due to a short
A GFCI detects an imbalance of current with a trip threshold of about 5mA (0.005 amps). GFCI protection is a legal remedy for personnel protection on ungrounded circuits. It does not provide protection to equipment from surges, etc that a true ground would.

ground my equipment directly somewhere other that back to the box
The code only recognizes a ground wire back to the panel as a legal method.

(which would cost me quite a bit since the room in question is over a garage under the house and I dont think I can snake through).
Could you run along the outside of the house? The wire could be tucked inside a siding lap for example.

I dont think I saw a breaker bar at the box could I be missing something?
The ground bar is inside the panel box where the other bare and sometimes white wires land.
 
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Old 06-07-11, 09:11 PM
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The other thing with older BX cables when you try to change the junction box there is a golden rules with them you CAN NOT use plastique boxes with bx cable due you loose the bonding { grounding } very easy and if you have to use the plastique box then you will have to get a bonding lockring to bond it together. { that is one of very few legit way to deal with old school BX's }

But normally just run new MC { Metal Clad } they will have it own grounding conductor and you will not have to worry about the BX bonding/grounding issue.

Merci,
Marc
 
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