shocking situation

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readydave8

readydave8

re member
Location
Clarkesville, Georgia
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electrician
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?
 
If the central AC compressor circuit and the old 3-wire electric clothes dryer circuit are both coming out of the service center with the service disconnect, 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?
 
130504-2118 EDT

readydave8:

If the pole transformer is anywhere within a reasonable distance, then connect a long wire to the transformer ground rod. For the moment it is assumed that the connection from the transformer center tap to the ground rod is a low resistance and that there is no large current flow in this ground conductor. This ground rod point will be your voltage reference for measurements. Be careful because you do not know where or what the problem is. You might use two screwdrivers spaced about 12 ft apart to measure ground voltage near the pole transformer before getting too close.

Use a meter like a Fluke 27 as your test voltmeter.

Measure the voltage to the air conditioner enclosure with and without the dryer connected. Does it repeat your prior results?

Measure the voltage to the main panel neutral bus with and without the load. What are the results?

Repeat to the main panel ground bus. Again what are the results?

These results may already tell you the problem. If not, then the next steps will have to be determined based on these results.

.
 
Visually check the overhead service drop.

Visually check the overhead service drop.

Visually check the overhead service drop.


After storms I have seen the neutral wire burnt in half usually somewhere under where a tree limb has fallen.

Always look for tree limb damage.

Especially anywhere near tornado alley.
 
rt66electric said:
Visually check the overhead service drop.


After storms I have seen the neutral wire burnt in half usually somewhere under where a tree limb has fallen.

Always look for tree limb damage.

Especially anywhere near tornado alley.
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Took a look at service drop 1st when I got there, no readily apparent damage.
 
al hildenbrand said:
If the central AC compressor circuit and the old 3-wire electric clothes dryer circuit are both coming out of the service center with the service disconnect, 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?
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I did not think to check AC EGC conductor at service end, I would expect it to have voltage if dissconnected from ground bar, basically this would be similiar to one probe on AC and the other stuck in ground?

I thought about neutral, but still don't understand why turning off breaker to dryer makes voltage go away, as I said cord is showing infinite between either of the hot blades to ground/neutral blade, so I assume I'm not reading voltage at AC that is going through resistance in dryer to ground?
 
al hildenbrand said:
If the central AC compressor circuit and the old 3-wire electric clothes dryer circuit are both coming out of the service center with the service disconnect, 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?
Click to expand...

I agree for the OP, as I have advocated before. Draw it out, and/or look at the facts. 10V on the case of the AC unit could be an ungrounded case and a high resistance ground fault internally on the AC unit. Nope, assuming the dryer unplugging causes it to go away and also easily verifiable, but definitely requires verification.

Or if the grounding equipment conductor is good to the panel, then the only other possibility is a 10V potential difference between the entire grounding system of the panel and earth, not just the case of the AC unit. That would confirm the failure of the grounding electrode system. Now you have one definite problem. But 10V? Either somewhere in the electrical system you have a high resistance ground fault, or you have a 10V potential between the bonded neutral and ground to the earth. Since the 10 volts disappears when the "unintentional bonding" of the neutral and ground goes away, that means you have 10V of potential between the neutral and earth. The path being, through the service, meter wire and then along the utility company's earth bond. That, Watson is it.
 
readydave8 said:
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?
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There could be significant capacitive leakage between the dryer heater element or other live parts and the frame of the dryer. If there is a break in the EGC, the current present on the "grounded" metal of the dryer would be transferred by the EGC to anything else (like the A/C) which is connected to the same EGC. But if that stretch of EGC is not actually grounded, the leakage current would instead appear as a phantom voltage on the EGC. If you put a load on the EGC or use a low impedance voltmeter, the 10 volts should go away. But the underlying problem is a bad bonding path to ground.
A combined EGC/neutral connection to the dryer socket might be contributing to the problem, but the underlying cause is a break in the bonding path.
If the problem involved a bad neutral, then you would also measure ~10 volts neutral to ground at different places, and the voltage appearing on the ground when the dryer is plugged in would be the result of a bond at the dryer.
Bottom two lines though are that
1. since opening the breaker to the dryer makes the voltage go away, the source of the leakage current has to be the dryer itself. Capacitive leakage will not show up on your DC ohmmeter.
2. There is missing bond connection somewhere between both the dryer and the A/C and the panel ground or the ground rod connection has failed.
 
Strathead said:
I agree for the OP, as I have advocated before. Draw it out, and/or look at the facts. 10V on the case of the AC unit could be an ungrounded case and a high resistance ground fault internally on the AC unit. Nope, assuming the dryer unplugging causes it to go away and also easily verifiable, but definitely requires verification.

Or if the grounding equipment conductor is good to the panel, then the only other possibility is a 10V potential difference between the entire grounding system of the panel and earth, not just the case of the AC unit. That would confirm the failure of the grounding electrode system. Now you have one definite problem. But 10V? Either somewhere in the electrical system you have a high resistance ground fault, or you have a 10V potential between the bonded neutral and ground to the earth. Since the 10 volts disappears when the "unintentional bonding" of the neutral and ground goes away, that means you have 10V of potential between the neutral and earth. The path being, through the service, meter wire and then along the utility company's earth bond. That, Watson is it.
Click to expand...
10 volts does not go away only when dryer unplugged to interrupt unintentional bonding, also goes away when dryer breaker turned off and remains plugged in, maintaining unitentional bonding.
 
readydave8 said:
I did not think to check AC EGC conductor at service end, I would expect it to have voltage if dissconnected from ground bar, basically this would be similiar to one probe on AC and the other stuck in ground?
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Actually, I'm suggesting starting with the AC EGC conductor at the service end connected to the panel terminal.

I like Gar's suggestion, in Post #3, to grab a "known earth reference" and to test from that to the EGC and the neutral at the circuit breaker panel. It sounds like the whole mass of EGCs and neutrals in the house may be "unstuck" from connection to earth.
 
GoldDigger said:
There could be significant capacitive leakage between the dryer heater element or other live parts and the frame of the dryer. If there is a break in the EGC, the current present on the "grounded" metal of the dryer would be transferred by the EGC to anything else (like the A/C) which is connected to the same EGC. But if that stretch of EGC is not actually grounded, the leakage current would instead appear as a phantom voltage on the EGC. If you put a load on the EGC or use a low impedance voltmeter, the 10 volts should go away. But the underlying problem is a bad bonding path to ground.
A combined EGC/neutral connection to the dryer socket might be contributing to the problem, but the underlying cause is a break in the bonding path.
If the problem involved a bad neutral, then you would also measure ~10 volts neutral to ground at different places, and the voltage appearing on the ground when the dryer is plugged in would be the result of a bond at the dryer.
Bottom two lines though are that
1. since opening the breaker to the dryer makes the voltage go away, the source of the leakage current has to be the dryer itself. Capacitive leakage will not show up on your DC ohmmeter.
2. There is missing bond connection somewhere between both the dryer and the A/C and the panel ground or the ground rod connection has failed.
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I did not think it was phantom voltage 1. call was in response to HVAC tech feeling a shock 2. my Greenlee 6701 volt-cont will trip a gfci when going from hot to ground, I assumed that meant it would also pull enough current to not show phantom voltage, and have been using it to keep my digital meters honest? Volt-conn showed voltage, it has lites for different voltages so can't really say exactly how much-- probably between 0-12
 
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I repaired a similar type scenario a few weeks ago.

Customers dryer was shocking anyone who touched when in service/running.

Long story short, the EGC to receptacle connection had become separated, rendering the dryer frame without an EGC.

I have followed up with the home owner recently and dryer works fine now with no issues.
 
readydave8 said:
I did not think it was phantom voltage 1. call was in response to HVAC tech feeling a shock 2. my Greenlee 6701 volt-cont will trip a gfci when going from hot to ground, I assumed that meant it would also pull enough current to not show phantom voltage, and have been using it to keep my digital meters honest? Volt-conn showed voltage, it has lites for different voltages so can't really say exactly how much-- probably between 0-12
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If it is not phantom voltage and it goes away when the dryer breaker is turned off, then has to be leakage that does not show up on an ohmmeter. If not stray capacitance, then possibly surge protection. Since it does not require the dryer to be turned on, it will not be current through motor or heater or even timer.
If it were a problem with partially open neutral and voltage on the neutral it would not change just from turning off the dryer breaker and yet still appear with dryer off. Unless the dryer CB is also feeding another load somewhere else! (Bootlegged receptacle, for example.) That could make it a combination of MWBC/high resistance neutral with open EGC. Two faults at the same time is not as probable, but is not impossible either.
 
readydave8 said:
I did not think it was phantom voltage 1. call was in response to HVAC tech feeling a shock 2. my Greenlee 6701 volt-cont will trip a gfci when going from hot to ground, I assumed that meant it would also pull enough current to not show phantom voltage, and have been using it to keep my digital meters honest? Volt-conn showed voltage, it has lites for different voltages so can't really say exactly how much-- probably between 0-12
Click to expand...

OK, one thing I forgot to speculate about in the first post is a shock from 10V potential. While not impossible, it would take some fairly extenuating circumstances for this. Think of a 9v battery and a tongue:happyno:!

Next what is the range of your meter? That could explain an infinity reading from hot to ground at the dryer. Some of those handhelds for filed work only go up to 600 ohms. So 601 ohms would be infinity!

.

Lastly, IMO with the information you gave it must be something wrong with the dryer period. If it were me, I might, run a wire from the dryer frame to the earth and see if I am getting the 10 volts, but you must have a problem with the bond from the neutral ground boding point and earth as well, and every piece of metal in the system must be energized with the same 10 volts, unless there is a resistance in the grounding that is common to the AC unit and the dryer, but not the rest of the house.

I will repeat one thing and go a little further. I have stated this silly item here before. I call it the "one little electron" system. Draw it out. know that electricity is a closed loop. It MUST start at a source, usually a transformer, battery, UPS, or generator for us. Staring at the source, imagine you are one little electron. Your mission should you accept it is to leave the source and return to the source, by any means available. Every time you come to an open switch you turn around and try to find another way, or you throw the switch and continue. You travel through loads, but it tires you out. Lastly it takes imagination or a stretch to remember that there are usually parallel paths, and you are usually trying to deal with only a portion, so you just have to ignore those parallels that don't affect you.

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
 
Strathead said:
Lastly it takes imagination or a stretch to remember that there are usually parallel paths, and you are usually trying to deal with only a portion, so you just have to ignore those parallels that don't affect you.
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But in this particular case, if you have the opportunity to choose between parallel paths and none of those paths lead directly and without obstruction to the return side of the supply you started at, you also have to consider what would happen if you took one of the other paths instead! You just might arrive at an exposed metal surface without being completely tired out yet. :)

I like the technique.
 
GoldDigger said:
But in this particular case, if you have the opportunity to choose between parallel paths and none of those paths lead directly and without obstruction to the return side of the supply you started at, you also have to consider what would happen if you took one of the other paths instead! You just might arrive at an exposed metal surface without being completely tired out yet. :)

I like the technique.
Click to expand...

All kidding aside though, the exposed metal surface is a block the switch is the contact and the work develops the shock, which is when you give it that last little effort and zap the c$%p out of someone!

Between this and equating electricity to water pressure=voltage flow= amperage pipe size and restriction = resistance I believe I have had great success over the years getting people to grasp electrical concepts that otherwise had eluded them.
 
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