Neutral Grounding

[kwired]

I would tend to concur. But if it was important enough to not run new wire And I was going to use it as an egc then I would feel a little better about it after slappin' a megger on it.

I just did 3 of these in a domitory. They had 2 blacks and a green in the conduit and when they got new machines they just slapped the green on the neutral terminal and left the grounding strap on. Well, they had the old surface mount 3 prong receptacle and the neutral got smushed when replugging it in and didn't connect as it should. So the residence were coming down to the concrete laundry room floor barefooted and getting hammered when they turned on that machine and touched it.

fortunately I did have 1" conduit, plenty of wire, and plenty of time and only about 25 feet to go. I ended up just changing all 3 on that floor with new recep's, plug cords, and pulling the jumpers off of the equipment. I realize that not everyone has this luxury though and just hope the OP makes it a safe installation.

Megging the AC sheath will not tell you anything about how much impedance it will have when carrying a heavy current from a ground fault.

A megger is for testing insulation integrity and not for measuring low resistances.

I also think you would have had people getting shocked on the concrete floor even if some other wiring method was used - use of the neutral for equipment grounding was the problem here, and I have seen this before on code compliant (3 wire) installs of ranges or dryers that have grounded surfaces nearby. Just one more reason why they eventually decided we need 4 wires to these appliances instead of 3.

 

[Sparky3141]

Yeah, I see your point w/ the megger. I've never used one. I was thinking that if you put 500 volts on it that you might see a breakdown of continuity but remembering that it's only battery powered and won't pruduce any amperage to speak of, it does sound kinda silly.:dunce:

As far as people getting shocked on that system though; it's been that way for decades and I hadn't heard of any prior complaints.

 

[Curious nerd]

In another thread, Jim Dungar dug up a study of old wiring which included resistance measurements on old armored cable. http://ewh.ieee.org/cmte/pses/ffat/support/RESAReport.pdf, section 3.2.1. It was downright common to find aged sheathing measured at 2 or even 3 ohms per 100 feet (that's obviously without a bonding strip). A fifty foot run could still clear a fault at that rate if nothing else went wrong.

 

[winnie]

Another related point which the NEC is silent on is just what constitutes an effective ground fault path, and does the sort of breaker feeding the circuit make a difference in this judgement.

An AC cable without bonding strip might have high impedance and not put a normal breaker into the 'instantaneous trip' range...but it would certainly trip a GFPE breaker.

-Jon