Three-prong question

[Richard Roshak]

Did an inspection on a 55+ year old home today. BX wiring and two prong outlets, however there were 7 outlets which were three prong. These tested grounded utilizing two different testing methods, however when I removed the outlet cover, there was no grounding wire. I removed the outlet from the metal junction box retested and the outlet tested ungrounded. I screwed the outlet back into the junction box and it tested as being grounded. Is there a way to test these older system to determine proper grounding without removing the outlet cover? How would you properly ground these outlets? How is the metal frame on the outlet part of the grounding screw and and the grounding slot?

[Jerry Peck]

Richard,

A complex question which has a complex answer, yet a simple answer too.

The simple answer is that BX (which is really AC cable) does not provide a recognized or approved ground. Yes, sometimes it provided a measurable ground, but not an approved or recognized ground.


These tested grounded utilizing two different testing methods, however when I removed the outlet cover, there was no grounding wire.


I removed the outlet from the metal junction box retested and the outlet tested ungrounded.


I screwed the outlet back into the junction box and it tested as being grounded.

Because there is no approved ground, the armor covering was serving as a ground.


Is there a way to test these older system to determine proper grounding without removing the outlet cover?

You have just inserted and used the key word ... "proper". There is no "proper" ground, thus there is no need to remove the outlet cover to check.


How would you properly ground these outlets?

By re-wiring with a method which has a proper ground in it.


How is the metal frame on the outlet part of the grounding screw and and the grounding slot?

All are connected together, thus the grounding slot is connected to the strap, the grounding screw is in at tab protruding out from the strap, the strap contacts the metal box to which the metal AC cable armor is attached, and, with all the conditions being right, you will read a ground at the receptacle outlet.

However, let's examine how BX (AC cable) is made: AC cable is made by wrapping a metal strip spiral wound around the inner conductors ... think of a coil spring compressed, such used in retractable ball point pens, now take that 1" long spring and stretch it out - how long is that spring wire now? Quite long, isn't it?

That is the length of the grounding path in the AC (BX) cable as it is spiral wound around the conductors inside. That is also steel and not copper, so its resistance is quite a bit higher than the resistance for the copper conductors inside, now increase that resistance by the tremendously long length of that conductor if you were to un-wind the covering and stretch it out ... quite a lot of high resistance conductor you would have there.

But we are not down yet - there is still one more thing which needs to be addressed: The coil. That ground conductor is wound like a coil, and when a high current flows through a coil it creates impedance, and at some point it becomes a choke coil, which means it "chokes off" all current passing through the coil ... which is just the opposite of what you would want when you had a high ground fault current flowing. You would want the least resistance, the least impedance possible.

[Speedy Petey]

However, let's examine how BX (AC cable) is made: AC cable is made by wrapping a metal strip spiral wound around the inner conductors ... think of a coil spring compressed, such used in retractable ball point pens, now take that 1" long spring and stretch it out - how long is that spring wire now? Quite long, isn't it?

That is the length of the grounding path in the AC (BX) cable as it is spiral wound around the conductors inside. That is also steel and not copper, so its resistance is quite a bit higher than the resistance for the copper conductors inside, now increase that resistance by the tremendously long length of that conductor if you were to un-wind the covering and stretch it out ... quite a lot of high resistance conductor you would have there.

But we are not down yet - there is still one more thing which needs to be addressed: The coil. That ground conductor is wound like a coil, and when a high current flows through a coil it creates impedance, and at some point it becomes a choke coil, which means it "chokes off" all current passing through the coil ... which is just the opposite of what you would want when you had a high ground fault current flowing. You would want the least resistance, the least impedance possible.
Actually, AC cable has a bonding tracer run along with the conductors. This should be familiar to most here. This tracer is simply wrapped back on the spiral sheathing. This ensures that the grounding path is linear along the cable length instead of spiraling around as Jerry described.
The sheathing of real AC cable with the bonding strip IS a legitimate grounding conductor when used with the proper connectors.
This cab be found in NEC 250.118 (8) & 320.108

[Jerry Peck]

Actually, AC cable has a bonding tracer run along with the conductors. This should be familiar to most here. This tracer is simply wrapped back on the spiral sheathing. This ensures that the grounding path is linear along the cable length instead of spiraling around as Jerry described.
The sheathing of real AC cable with the bonding strip IS a legitimate grounding conductor when used with the proper connectors.
This cab be found in NEC 250.118 (8) & 320.108

Now-a-days, yes, but not back when AC cable was BX (which is AC cable), and which is what I was referring to - back then when BX was, well, ... "BX".

I am trying to recall when that bonding strip came into being in AC cable but am coming up blank on its introduction, nonetheless, that was after "BX" was gone.

[Vern Heiler]

Richard,

However, let's examine how BX (AC cable) is made: AC cable is made by wrapping a metal strip spiral wound around the inner conductors ... think of a coil spring compressed, such used in retractable ball point pens, now take that 1" long spring and stretch it out - how long is that spring wire now? Quite long, isn't it?

That is the length of the grounding path in the AC (BX) cable as it is spiral wound around the conductors inside. That is also steel and not copper, so its resistance is quite a bit higher than the resistance for the copper conductors inside, now increase that resistance by the tremendously long length of that conductor if you were to un-wind the covering and stretch it out ... quite a lot of high resistance conductor you would have there.

But we are not down yet - there is still one more thing which needs to be addressed: The coil. That ground conductor is wound like a coil, and when a high current flows through a coil it creates impedance, and at some point it becomes a choke coil, which means it "chokes off" all current passing through the coil ... which is just the opposite of what you would want when you had a high ground fault current flowing. You would want the least resistance, the least impedance possible.

Just a little "Devil's Advocate" here. The spiral wrapped sheathing would have to be insulated, as coil wire is, for these conditions to exist. With all of the coils shorted together the effective cross section of the conductor most likely would offset the difference in conductivity. I'm not saying an additional grounding conductor is not a good idea. Just that it may not be as dangerous as first thought.

[Jerry Peck]

Just a little "Devil's Advocate" here. The spiral wrapped sheathing would have to be insulated, as coil wire is, for these conditions to exist. With all of the coils shorted together the effective cross section of the conductor most likely would offset the difference in conductivity. I'm not saying an additional grounding conductor is not a good idea. Just that it may not be as dangerous as first thought.


Actually, the current does not reliably travel as you are expecting it to - even with the metal spiral wound covering being uninsulated, which is why the newer AC cable has that bonding strip in it.

If it acted as you stated, there would be no need for that bonding strip.

Testing showed it not to be as you described, which is why the bonding strip was added.

BX Cable (http://www.seatekco.com/bx.htm)

http://www.nema.org/stds/eng-bulletins/upload/NEMA-Termination-of-Bonding-Strip-on-Type-AC-Cable.pdf

[Bill Kriegh]

I believe the ground strip in the cable showed up in the late 50s early 60s time frame. As with most upgrades the existing stock wound up being used for years.

Having been in more than one attic with a glowing sheath on old AC/BX cable I can tell you the old stuff does have issues.

Although not a perfect test, hooking a several hundred watt load to the hot and supposed ground and checking voltage drop may confirm lack of a ground strip - but won't necessarily prove it exists.

[Speedy Petey]

I believe the ground strip in the cable showed up in the late 50s early 60s time frame. My guess as well.

Considering the age of the house there is a chance that it is real AC cable.

[Joe Tedesco]

Type AC Armor Clad Cable "BX" is the common trade name for Type AC.

"BX" was G.E.'s Sprague Electric division's trademark.

I believe that the internal bonding strip rule was added to the 1959 NEC.

For the present rule See 320.100 in the 2008 NEC.

[Jerry Peck]

Considering the age of the house there is a chance that it is real AC cable.

I think it is probably "BX" armored cable, not the real Type AC with the bonding strip/conductor - given the age of the house and the basically agreed upon introduction of Type AC with the bonding strip/conductor.

However, we will not know ... unless someone wants to go there and take it apart. :eek:

[John Steinke]

AHEM.. There's a need to correct a few things.

Let's start with the first part ... vocabulary. Just what is meant by "BX?" I can think of at least three separate methods that I've heard referred to as "BX," and the rules differ for each of them. The rules have also changed over time.

So ... the first thing to do is to determine exactly what wiring method was used.

If AC, or 'armored cable' was used, the jacket DOES provide the ground path you need, and all is well.

If "flex" was used - flexible tubing that had wires pulled into it by the contractor, using the jacket as ground was probably recognized at the time, but would be limited to 6 ft. lengths (or it would need a ground wire) today.

If MC, or "metal sheathed cable" was used, it came with a ground wire in it, and that needed to be used - not cut off. (Please note that there are newer types of MC that change this rule considerably).

Using the yoke, or strap, of the device as your grounding connection to a metal box is a practice that was recognized, without limitation, just a decade ago. Today, nearly all circumstances would have you attach a ground wire directly to the device.

[Jerry Peck]

Let's start with the first part ... vocabulary.


John,

Continuing with the vocabulary part ...



If AC, or 'armored cable' was used, the jacket DOES provide the ground path you need, and all is well.

There is what is known as BX, which was "armored cable", but not Type AC, which is also known as "armored cable". My 1905 NEC has a section covering "armored cable".

Then there was Type AC, which is also known as "armored cable", and the term "type AC" first shows up in my 1928 NEC. My 1925 NEC does not use the term "type AC", it only uses the term "armored cable".

Then there was Type AC as we know it today, the 1959 NEC includes two new section for "armored cable", also listed as "Type AC", one of these two new section is the requirement for the bonding strip.

Thus, when referring to "armored cable" one must recognized the different types of "armored cable", even that "type AC" (also stated as "Type AC") has different construction, and that the modern version of armored cable / Type AC was effective only at 1959, and as Bill (I believe it was Bill) stated, they were allowed to use up stock of the older Type AC / armored cable, and, to complicate matters, many AHJ did not adopt the 1959 NEC right away, meaning that requirement did not become effective until the 1959 (or a newer) NEC was adopted. Basically, it would have been the manufacturers who made the change for areas which did not adopt the 1959 NEC as their new production of Type AC / armored cable would all be the newer style, whether or not any local jurisdiction did or did not require its use.

Complicated things a bit? Probably.

Cleared things up a bit? Hopefully.