shocking situation

[Fulthrotl]

A call today was for shock from air conditioning outdoor unit. I read 10 volts from unit to probe stuck in ground.

Turning off clothes dryer breaker made voltage go away, or leaving breaker on and unplugging clothes dryer.

I expected a reading with ohmeter between either hot prong of dryer plug and ground/neutral prong but showed infinite.

Several times to confirm I unplugged dryer, saw voltage go away, plugged back in, saw it come back, same redundant checks with breaker.

I would have expected the fact that neutral and ground were together at dryer to be contributing factor, must not have anything to do with it since they are still connected when breaker is off.

Any theories?

you have two different 240 volt circuits off the same panel
with the same symptom. hm.

what else in the house is 240 volt? range? oven?
check those... betcha they have 10 volts floating on them
when the dryer is plugged in. just a hunch you are backfeeding
every 240 volt load in the house.

if you unplug the dryer, as you say, and with the dryer
circuit hot, you don't have any floating voltages, and the
dryer introduces a 10 volt float to ground.... i'd put a wire
from the AC to a ground where you are reading the delta,
and put an amprobe on it to see how much current it's drawing.
not 'cause that information will help troubleshooting, but i'm curious... ;-)

the problem is in the appliance, if it disappears when unplugged...
i suspect a short to ground on the heating element, enough
to give a tingle. probably not in the control circuit.

that's my nickels worth.

 

[readydave8]

BTW This is not any attempt to be condescending, or imply that you personally need this instruction, but instead to point out to anyone reading this who has mind blocks with tracing out circuits

Not a bit condesdending, I asked the question so that I could learn. Some of the replies included stuff I already knew, review was still helpful. Some of the replies included stuff I had forgot. Some of the replies had stuff I should have known but didn't.

 

[readydave8]

Unless the dryer CB is also feeding another load somewhere else! (Bootlegged receptacle, for example.)
.

Man, you are good. There was 120v bootlegged from 1 hot and the ground in dryer receptacle, I did not mention it because when I disconnected it, symptons did not change. But your post made me wonder if that was the only one, I didn't see a reason to crawl under house, but what if I had, and found a flying splice to another bootlegged receptacle? Whoever did the one I found knew it would "work," and may have done it again.

 

[al hildenbrand]

There was 120v bootlegged from 1 hot and the ground in dryer receptacle, I did not mention it because when I disconnected it, symptons did not change.

Until you know the internal wiring and load inside the electric clothes dryer, you can't rule out the presence of 120V load actual in the appliance. One that comes to mind is a 120V UV bulb.

I'm not suggesting that you figure the appliance internal wiring out. . . to me, the 10 V disappearing upon dryer breaker shutoff is the key that informs my opinion.

 

[GoldDigger]

Until you know the internal wiring and load inside the electric clothes dryer, you can't rule out the presence of 120V load actual in the appliance. One that comes to mind is a 120V UV bulb.

I'm not suggesting that you figure the appliance internal wiring out. . . to me, the 10 V disappearing upon dryer breaker shutoff is the key that informs my opinion.

But it is not dependent on anything in the dryer being switched on. So the light bulb, for example, is out of consideration. There certainly could be a 120 volt to neutral load, the timer motor if nothing else. But none that are energized when the dryer is off. If it has electronic controls instead, there will always be the parasitic load to power the circuit so that it can turn itself on.

 

[ggunn]

Until you know the internal wiring and load inside the electric clothes dryer, you can't rule out the presence of 120V load actual in the appliance. One that comes to mind is a 120V UV bulb.

I'm not suggesting that you figure the appliance internal wiring out. . . to me, the 10 V disappearing upon dryer breaker shutoff is the key that informs my opinion.

I'm pretty sure that in at least some electric dryers the motor that spins the drum is a 120V load.

 

[steve66]

A call today was for shock from air conditioning outdoor unit. I read 10 volts from unit to probe stuck in ground.

Turning off clothes dryer breaker made voltage go away, or leaving breaker on and unplugging clothes dryer.

I expected a reading with ohmeter between either hot prong of dryer plug and ground/neutral prong but showed infinite.

Several times to confirm I unplugged dryer, saw voltage go away, plugged back in, saw it come back, same redundant checks with breaker.

I would have expected the fact that neutral and ground were together at dryer to be contributing factor, must not have anything to do with it since they are still connected when breaker is off.

Any theories?

You mean you read 10 volts with one probe stuck in the dirt? Or one probe connected to the "ground" meaning some metal that is grounded?

If its the first one, the 10 volts is meaning less. It is probably just being picked up by capacitive coupling.

 

[readydave8]

You mean you read 10 volts with one probe stuck in the dirt? Or one probe connected to the "ground" meaning some metal that is grounded?

If its the first one, the 10 volts is meaning less. It is probably just being picked up by capacitive coupling.

One probe on AC condenser fins to get best connection, the other I started holding to ground rod (AC is beside meter base and ground rod is within reach), it was easier to just stick probe into the damp dirt next to ground rod and did not seem to change results so that's where I was reading most of the time.

 

[kwired]

Man, you are good. There was 120v bootlegged from 1 hot and the ground in dryer receptacle, I did not mention it because when I disconnected it, symptons did not change. But your post made me wonder if that was the only one, I didn't see a reason to crawl under house, but what if I had, and found a flying splice to another bootlegged receptacle? Whoever did the one I found knew it would "work," and may have done it again.

Makes sense, but doesn't really explain why unplugging dryer should make the problem go away, and still shouldn't raise the voltage on the A/C EGC if it is not part of the neutral path. There may still be a little more happening here.

With a bootleg from dryer receptacle, what else is not connected properly that may be contributing? Does A/C EGC run to either a proper EGC in a subpanel or to the service neutral bus? If it ties to neutrals someplace and a connection is compromised that may be the real source of the voltage. Is dryer receptacle three or four wire? If four wire is the bonding jumper removed from the neutral terminal in the appliance?

 

[readydave8]

Makes sense, but doesn't really explain why unplugging dryer should make the problem go away, and still shouldn't raise the voltage on the A/C EGC if it is not part of the neutral path. There may still be a little more happening here.

With a bootleg from dryer receptacle, what else is not connected properly that may be contributing? Does A/C EGC run to either a proper EGC in a subpanel or to the service neutral bus? If it ties to neutrals someplace and a connection is compromised that may be the real source of the voltage. Is dryer receptacle three or four wire? If four wire is the bonding jumper removed from the neutral terminal in the appliance?

3 wire receptacle, bonding strap seemed to be installed, once I saw it attached to neutral I stopped looking so it is possible that it is not making good connection where attached to frame of dryer.

I was under pressure that day to look like I knew what I was doing, and to not spend time that I could not justify. When the owner saw that unplugging the dryer made problem go away, we both assumed that something was wrong with (tenant's) dryer and he was satisfied.

Now I want to go back and troubleshoot based on suggestions in this thread, it may be 200' to transformer pole but I like the suggestion of running back to "clean" neutral, I can afford to spend some time at house without pay and am thinking about how to present it to owner--"I found the problem and you paid me, now I think I didn't find problem and want to work on it some more for free."

 

[GoldDigger]

3 wire receptacle, bonding strap seemed to be installed, once I saw it attached to neutral I stopped looking so it is possible that it is not making good connection where attached to frame of dryer.

I was under pressure that day to look like I knew what I was doing, and to not spend time that I could not justify. When the owner saw that unplugging the dryer made problem go away, we both assumed that something was wrong with (tenant's) dryer and he was satisfied.

Now I want to go back and troubleshoot based on suggestions in this thread, it may be 200' to transformer pole but I like the suggestion of running back to "clean" neutral, I can afford to spend some time at house without pay and am thinking about how to present it to owner--"I found the problem and you paid me, now I think I didn't find problem and want to work on it some more for free."

If the receptacle and cord are three wire, then there is no way that the dryer could be contributing to the problem just by providing an extra ground to neutral bond. Instead it proves that the source of the current is the dryer itself and that there is a problem with the neutral path from the dryer to the panel and that the leakage current is from a source inside the dryer that is connected to one of the phase lines even when the dryer is switched off. Can you confirm whether it is mechanical time or electronic controls?

 

[kwired]

The question is does all equipment grounding conductors rise in voltage or just the AC? AC maybe was more noticeable because of grounded areas around it, but all EGC's should be rising by same amount if they are connected correctly and in good condition.

There could be a loose screw connecting a segment of the neutral bus and the dryer neutral and AC equipment ground happen to be connected to this same segment. Any other EGC connected to same segment will rise in voltage also.

 

[Fliz]

the thing I suspect would cause the AC compressor EGC to have voltage on it the way you describe is both a failed service neutral, somewhere on the line side of the service disconnect, AND a failed Grounding Electrode System connection to the service center.

Is there any voltage on the AC compressor EGC at the circuit breaker end? And does it fluctuate with the dryer breaker being turned off?

I've only looked through this thread a bit but a response like this gives me quite a headache ... what are really the chances that something like this could actually happen? Not saying it can't, but aren't there other causes of such a voltage to ground? Isn't this a common problem that happens when, for example, people get a shock from the stove frame, and isn't the common cause of this is a neutral bond to the EGC that isn't at the MBJ - from a subpanel bond to neutral or elsewhere - causing unwanted current to ground on the ECG?

 

[kwired]

I've only looked through this thread a bit but a response like this gives me quite a headache ... what are really the chances that something like this could actually happen? Not saying it can't, but aren't there other causes of such a voltage to ground? Isn't this a common problem that happens when, for example, people get a shock from the stove frame, and isn't the common cause of this is a neutral bond to the EGC that isn't at the MBJ - from a subpanel bond to neutral or elsewhere - causing unwanted current to ground on the ECG?

There have been suggestions that current is leaking onto the dryer neutral, I find that hard to believe that alone is the problem. There has to be an impedance in that neutral to make the voltage rise. Naturally there is some impedance, but not enough to cause a 10 volt rise in voltage. Something somewhere that normally carries neutral current has developed some higher than usual impedance and happens to be connected to the EGC to the AC unit. It very well may be connected to other EGC's but AC is only place where a true ground reference was in vicinity to notice the rise.

If something were leaking voltage to the dryer neutral and the neutral were in good shape and had low impedance - we would either have a current limit by impedance on the ungrounded side of things or a load, or we would develop (somewhat) unlimited amount of current which would result in tripping overcurrent device in the ungrounded conductor supplying this.

If something were leaking voltage directly to the dryer neutral conductor and the neutral had a high impedance connection then I would suspect the voltage to ground on the neutral would have been much higher than 10 volts.

 

[gar]

130607-0601 EDT

readydave8:

In post #3 I provided you a measurement procedure that should allow pinpointing the region of the problem. I do not believe you have used this. If you do not want to go to the pole for a probably good reference point, then I would first use the ground bus bar in the main panel as this point.

Without taking the cover off of my panel this morning, using a Fluke 27, and using a receptacle just below the panel I measured 2 mV between EGC and neutral. Then between neutral and my ground rod at the point where it passes thru the basement floor I read 3 mV. The ground rod in my case is a 1.25" copper water pipe that supplies city water and is about 150 ft long from the city water line tap point. A rather good ground rod. Also disconnected TED 1000 so it was not introducing any 125 kHz noise into the measurement. However, reconnection did not introduction any change in the readings.

An added 10 A load on one phase increased the neutral to EGC bus voltage to 5 mV, and on the other phase reduced the reading to 1 mV.

Thus, neutral bus to EGC bus resistance is about 0.002/10 = 0.000,2 ohms or less. This was using auto ranging which has a resolution of 0.001 V.

From the main panel EGC bus you need a measurement to earth with the screwdriver about 20 ft from the house.

From the various bits of information that have been presented it does appear there is an EGC problem somewhere. But it seems that you have no real measurements that will pinpoint the problem location. Mostly there is conjecture.

.

 

[readydave8]

When I found out that turning off Dryer breaker made problem go away, but did not get ohm reading between blades on plug, I also suspected that the dryer had electronic components, I imagined that there was current leakage to ground only when power was on. At that point I did not want the responsibility of dryer repair and so did not open dryer, except cover to acess cord connections. Another possibility that I thought of later was: what if AC circuit is spliced in to dryer circuit under house, AC is source of voltage, AC has grounding issues, and so unplugging dryer cord makes voltage to ground dissapear?

Gar you are absolutely correct in that I have incomplete data and can use your proposed tests to narrow down problem.

So now the deal is, I was at house 1 hour, stopped working on problem when owner was satisfied. Do I call him and offer to work again on problem that he thinks is fixed? Wait for him to call me? Either way I plan to print out this thread for reference on future similiar calls.

Here's my "current" plan:

1. Tenant may have work done to dryer.
a. Dryer fixed, problem gone. (or new dryer).
b. Dryer not fixed, problem remains, owner calls me again and I am better prepared due to schooling received here.

2. If owner calls for other services I will definitely bring subject up, maybe influence him in to letting me do more testing.


There remains the matter of my conscience, I suspect that dryer would not be likely to cause this by itself, there must be grounding issues combined. I noticed a few other handy-man issues, went from there to his trailer park and oh boy, if only he would let me fix everything at all his properties I would have steady work for the next few months. So leaving things as is does not bother me much. I would love to hear input regarding this philosophy, don't worry, if anyone hurts my feelings I'll get over it.