Plug in neon receptacle tester

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MD84

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What would be the impedance of a neon receptacle tester?

I ask because of some troubleshooting I did today. I was disconnecting some equipment so a customer could move a machine. One item was a 120v receptacle. I checked that it was energized with my fluke 87 hi z dmm and read 120v. I turned off the circuit breaker and then checked again. I still had 109v. I thought it was capacitive coupling. This particular branch circuit did come out of a panel in which all the conductors leave in a common 2" conduit. That supported the thought of capacitive coupling. I did not have a low z meter but I did have a plug in tester. I thought it would have a lower impedance then the 87 and perhaps it would not light up.

It did light up and it showed hot and ground reversed. This was with the breaker off. I turned the breaker on and it now showed hot and neutral reversed.

I tried testing with a non contact voltage tester. It lit up on the neutral with the breaker on or off. It lit up on the hot only with the breaker on.

I traced the wires and found no improper terminations. There was one abandoned conductor in the conduit which was un-terminated and insulated. I checked the panel and I am guessing there is no SBJ. This is a SDS 480:208Y. Neutral to all three lines voltage were balanced. All three lines to ground voltage were imbalanced. I think there were two lines with 200v to ground and one with about 0.

I am thinking that with no SBJ the neutral point was elevated above ground. I did not have time to open the transformer. I will check that tomorrow.

Any thoughts on these readings with these instruments? Confirmation of my theory? I will confirm no SBJ and perform bonding tomorrow. I am thinking this will correct the abnormal results.
 
The neon lamps in those testers are ballasted with a resistor. In operation, the impedance is roughly the same as that of the resistor alone, which is typically 50-100k. That gives a couple milliamps of current flow. However, the bulb will not light at all until a breakdown voltage of about 70-100V is across it. This pretty well rules out "phantom" voltages from capacitive coupling in conduits. However, the capacitive coupling and leakage between transformer windings can be much greater. It sounds like that's what you're seeing. That SDS needs to be bonded.
 
No SBJ a common occurrence?

No SBJ a common occurrence?

Thank you. That is good information.

It is really surprising to me how many SDS I have found which are not bonded. Checking for good a good balanced source is the first thing I do regardless of troubleshooting an issue or installing new equipment. I would think that some of these systems would have been realized before I came across them.

Risking derailing my own thread I am curious how often others are finding systems which are not bonded. I am also curious just how dangerous an unbonded system like this can be. I think it would be less dangerous than a piece of equipment which has no EGC that is powered by a bonded system.

An unbonded or floating system can have an ungrounded conductor contact equipment enclosures without tripping the circuit. These metal parts are now energized. A person could touch that equipment and have a potential across them yet there is no solid path back to the source through ground. I realize there would be some voltage exposed due to capacitive coupling. How much though? Lethal? I would guess it would depend on the size of the source, the length of the conductors, their proximity to grounded objects and equipment and et cetera.
 
MD84 said:
Thank you. That is good information.

It is really surprising to me how many SDS I have found which are not bonded. Checking for good a good balanced source is the first thing I do regardless of troubleshooting an issue or installing new equipment. I would think that some of these systems would have been realized before I came across them.
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There is a ton of bad electrical out there. It is probably the most forgiving of all the forms of energy we use.

Risking derailing my own thread I am curious how often others are finding systems which are not bonded.
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Not many for me.

.003
I am also curious just how dangerous an unbonded system like this can be. I think it would be less dangerous than a piece of equipment which has no EGC that is powered by a bonded system.
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If everything is bonded together properly and voltage is 480 or below I don't know that one is any more or less safe than the other. I agree with you that a circuit with no EGC in a solidly grounded system is worse than a first fault on an ungrounded system.

An unbonded or floating system can have an ungrounded conductor contact equipment enclosures without tripping the circuit. These metal parts are now energized. A person could touch that equipment and have a potential across them yet there is no solid path back to the source through ground.
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The metal parts have now been equalized with ground potential. Same thing as putting that screw through the neutral buss into the metal case of a service disconnect or bonding the XO of a transformer to its c*-ase.

I realize there would be some voltage exposed due to capacitive coupling. How much though? Lethal? I would guess it would depend on the size of the source, the length of the conductors, their proximity to grounded objects and equipment and et cetera.
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We live in a world where most everything is solidly grounded rather than ungrounded whatever the reasons. I think shocks from capacitive coupling are more of a problem on ungrounded systems, but I don't know for sure. I don't think about it too hard; I find it easier to rest in the wisdom of people smarter than me who worked all this out either on purpose or by trial and error.
 
MD84, are you talking about intentionally ungrounded systems like 480 ungrounded delta or are you tailing about systems where the installer screwed up?
 
MD84 said:
I am talking about a system in which the installer screwed up. Where you have a system that should be grounded but it is not.
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With that and re-reading your first post I would say that is a hazardous condition that should be corrected ASAP. And yes, I have run into this quite a few times.

I think Action Dave was thinking ungrounded delta.

The issue I see as hazardous is the fact the neutral voltage will not stay at the same potential as EGC and that presents shock hazards to uses and maybe equipment damage. Keep in mind the line to line and line to neutral voltages will remain normal.


As as far as an ungrounded conductor faulting to metal parts and energizing those parts, as Dave explained that will not happen. The metal parts are, or should be, grounded and will stay at ground potential.

But now the neutral to ground voltage will be the line to line voltage on two phases and zero volts to the faulted phase.


In any case it must be corrected.
 
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