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Surprise Shock

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E

Eddie_T

Member
Location
Southern mtns near eastern continental divide
Occupation
PE (retired)
I was troubleshooting a 3-way switch circuit for a friend. It was in a detached building classified as a garage but more of a man cave. The electrician that wired the building left wires too short to reach receptacle and switch screws so back stab connections were used. I was trying to determine if there was a switch problem, improper installation or a loose carrier wire. I pulled the disconnect in the sub panel, the lights went off and I was trying to get some slack in the wire when I got a shock. I could measure voltage on all three switch screws but didn't have a suitable wire to ground the circuit (I was afraid of frying a meter lead). I closed it up deciding to think it through. My guess is energy storage in the fluorescent circuitry but lack any experience there. I am going to try to make a coat hanger wire tool to release the stab connections to permit switch removal if necessary. If I do remove a switch I plan to add lengths of wire (maybe stranded) using wire nuts so that the switch screws can be used.
 
drcampbell

drcampbell

Senior Member
Location
The Motor City, Michigan USA
Occupation
Registered Professional Engineer
I use a voltage detector. I might also use inline splices, but not until after I've used a voltage detector.

The inductors that are common in older fluorescent fixtures will dissipate their stored energy less than a second after being switched off.
The capacitors in newer fixtures will generally be on the downstream side of rectifier diodes and shouldn't be able to backfeed energy to their line connections.
fluke-voltage-detector-1lac-a-ii-500x250.jpeg
 
E

Eddie_T

Member
Location
Southern mtns near eastern continental divide
Occupation
PE (retired)
drcampbell said:
I use a voltage detector. I might also use inline splices, but not until after I've used a voltage detector.

The inductors that are common in older fluorescent fixtures will dissipate their stored energy less than a second after being switched off.
The capacitors in newer fixtures will generally be on the downstream side of rectifier diodes and shouldn't be able to backfeed energy to their line connections.
View attachment 2553573
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I plan to get a tester as pictured. I am hoping I can devise a way to use one to tell me when service is restored and my generator is in use.

I will carry a Ne2 tester next time it's much faster than the meter. I can't imagine any other source for the voltage and the bus bar in the sub panel wasn't hot when I checked it (which was later). I will be cautious if I work on it again. I left it in a configuration where either switch will turn the lights on and off from it's location only. So it appears that only one carrier wire can be enabled.
 
K8MHZ

K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
Occupation
Electrician
You may have power coming from a different source other than the sub panel you shut down. I have seen stranger things happen. You may have to pull the main and go from there. Much better than getting a shock.

There are some sparkies that don't like the non contact testers. They are actually a great tool once you know how to use them and get a feel for what you can trust.
 
P

paulengr

Senior Member
K8MHZ said:
You may have power coming from a different source other than the sub panel you shut down. I have seen stranger things happen. You may have to pull the main and go from there. Much better than getting a shock.

There are some sparkies that don't like the non contact testers. They are actually a great tool once you know how to use them and get a feel for what you can trust.
Click to expand...

They are good for detecting the PRESENCE of voltage. Above 1000 V they are used for detecting absence, too. Below 1000 V the multimeter is superior as it is less susceptible to false negatives. It is required to use a multimeter for safety (test before touch) as part of LOTO below 1000 V.
 
Little Bill

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Semi-Retired Electrician
If the 3-way circuit happens to have one of the switches in the house (or other building), the line voltage might be coming from the house panel and not the sub. Or, since the OP said he pulled the disconnect in the sub (sounds like a fuse) it could be a neutral back feeding from another circuit. That would either be a shared neutral or someone tapped the closest neutral for the lights.
 
Fred B

Fred B

Senior Member
Location
Upstate, NY
Occupation
Electrician
Eddie_T said:
I was troubleshooting a 3-way switch circuit for a friend. It was in a detached building classified as a garage but more of a man cave. The electrician that wired the building left wires too short to reach receptacle and switch screws so back stab connections were used. I was trying to determine if there was a switch problem, improper installation or a loose carrier wire. I pulled the disconnect in the sub panel, the lights went off and I was trying to get some slack in the wire when I got a shock. I could measure voltage on all three switch screws but didn't have a suitable wire to ground the circuit (I was afraid of frying a meter lead). I closed it up deciding to think it through. My guess is energy storage in the fluorescent circuitry but lack any experience there. I am going to try to make a coat hanger wire tool to release the stab connections to permit switch removal if necessary. If I do remove a switch I plan to add lengths of wire (maybe stranded) using wire nuts so that the switch screws can be used.
Click to expand...
Why would you be afraid of frying a meter lead?
Came in behind a handiman that the ho had running wiring until he couldn't figure out why things weren't working. And I had a similar experience to OP and it tested back to neutrals tied with another circuit in attic jbox. With power on a second circuit, I had 30V to ground on off circuit, apparently back feed through several light fixtures. The Neutral was open on the live circuit and trying to use the neutral on the off circuit that wasn't connected but became live when made contact to ground.
 
Fred B

Fred B

Senior Member
Location
Upstate, NY
Occupation
Electrician
paulengr said:
They are good for detecting the PRESENCE of voltage. Above 1000 V they are used for detecting absence, too. Below 1000 V the multimeter is superior as it is less susceptible to false negatives. It is required to use a multimeter for safety (test before touch) as part of LOTO below 1000 V.
Click to expand...
I've had false positives alot from a ticker, never had a false negative, KOW, false positives I've traced to circuit wire proximity to a live circuit wire. It seems to be reading induction from the other wire. But always use multimeter to confirm just as you say.
 
E

Eddie_T

Member
Location
Southern mtns near eastern continental divide
Occupation
PE (retired)
Little Bill said:
If the 3-way circuit happens to have one of the switches in the house (or other building), the line voltage might be coming from the house panel and not the sub. Or, since the OP said he pulled the disconnect in the sub (sounds like a fuse) it could be a neutral back feeding from another circuit. That would either be a shared neutral or someone tapped the closest neutral for the lights.
Click to expand...
As stated in the OP it was in a separate structure. I didn't see a fuse in the disconnect pull out but I really didn't examine it.
 
Little Bill

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Semi-Retired Electrician
Eddie_T said:
As stated in the OP it was in a separate structure. I didn't see a fuse in the disconnect pull out but I really didn't examine it.
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I'm not understanding what you mean by "pull-out" at the subpanel. Is there a pull-out disconnect that feeds the sub? What I meant by "sounds like a fuse" is I thought maybe the sub was a fused panel and one of the pull-outs for dual fuses was used to feed the lighting circuit.
 
E

Eddie_T

Member
Location
Southern mtns near eastern continental divide
Occupation
PE (retired)
Little Bill said:
I'm not understanding what you mean by "pull-out" at the subpanel. Is there a pull-out disconnect that feeds the sub? What I meant by "sounds like a fuse" is I thought maybe the sub was a fused panel and one of the pull-outs for dual fuses was used to feed the lighting circuit.
Click to expand...
It's actually just a non-fused disconnect panel with pull out disconnect.
 
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