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-   -   Breaker is off - circuit still hot? (http://www.diychatroom.com/f18/breaker-off-circuit-still-hot-14614/)

Potomac101 12-20-2007 05:27 AM

Breaker is off - circuit still hot?
 
Situation:
I wanted to put a ceiling fan in a bedroom where only an overhead light had existed. Shutting off the breaker at the main panel cut off the light, but I still received a shock. This happened again when I replaced a bathroom fan.
Is electricity flowing through the ground wire?
Any ideas about how to correct this?
Thanks,
Potomac101

NateHanson 12-20-2007 06:31 AM

How many wires in the box? Where'd you get the shock from? The ceiling box? Or one of the insulated wires?

AllanJ 12-20-2007 07:37 AM

When a circuit is turned off, it is possible for there to be induced voltage from live circuits whose wires happen to run next to the dead circuit wires inside walls.

It takes only a few thousandths of an ampere to electrocute someone so such induced voltage can result in a dangerous current flow.

Use a voltmeter to measure between hot and neutral and also between hot and ground to make sure you actually killed the circuit. It is possible you may actually measure voltage. If you get far less than line voltage, it is safe to short hot and neutral and ground while you are working on that dead circuit and that will make things safe.

(Be sure to remove the alligator clip wires or whatever else you used to short the circuit wires before turning the power back on.)

Linemen do short the hot conductor(s) to ground when working up on poles with the power turned off.

HouseHelper 12-20-2007 07:42 AM

You may be getting a shock from a shared neutral. This could be a legal installation (multiwire circuit) or an illegal one (neutral from another circuit tapped). How old is the house? Do you have black and red wires attached to adjacent breakers?

220/221 12-20-2007 05:17 PM

Quote:

When a circuit is turned off, it is possible for there to be induced voltage from live circuits whose wires happen to run next to the dead circuit wires inside walls.

Not enough to actually feel.


The power is likely coming from the neutral via another circuit on a MWBC or something miswired /backfed elsewhere in the circuit....or a static shock from the carpet :laughing:

Andy in ATL 12-20-2007 08:24 PM

Quote:

Originally Posted by AllanJ (Post 82030)

It takes only a few thousandths of an ampere to electrocute someone so such induced voltage can result in a dangerous current flow.

Educate me about induced voltage deaths or injuries in a dwelling unit.

Andy

RichyL 12-21-2007 09:50 AM

Could be a double fed circuit, although I would say it is most likely the neutral of a mwbc (multiwire branch circuit), in which case youll have to look for the breaker that feeds the other circuit sharing this neutral and shut it off as well, to kill the circuit entirely. In most cases the multiwire circuit will be entering your panel via a 3 wire w/ ground. (black red white bare) And as 220/221 pointed out hopefully you are not doing the moonwalk on your carpet before touching mettallic objects. Happy holidays

Potomac101 12-22-2007 05:35 AM

This is a problem that I've ignored when it happened. I was putting a receptacle in the attic the other day and was reminded of the situation. I worry that I'm leaving a booby trap for a future owner of the house.

BEDROOM: The electricity comes into the room at the overhead light (now light/fan). From there it runs to the various receptacles and to the switch.

BATHROOM: The electricity comes into the room at a receptacle (GFCI) to a switch to the fan.

The house is 18 years old. For the most part it was set up w/ one 20 AMP breaker per room - 12/2 w/ ground - 15 AMP receptacles. I'm not familiar w/ multiwire branch circuits, but I don't think that there are any 3 wire circuits (no red wires) going to bedrooms/bathrooms,

I think that the shock came from a wire - not the box.

In the future I'll check circuit w/ a voltmeter and short out the hot, neutral, and ground wires, but that might not help the next guy.

Getting to the wiring behind the light/fan in the bedroom - well, I'd like to avoid that if the problem can be solved working w/ the bathroom fan and the circuit breaker.

I don't want to start taking everything apart during the holidays. Would you keep the ideas coming with the idea that I'll tackle this after the relatives leave?

QUESTIONS:
If there are no mwbc's, how do I check for a "miswired/backfed" somewhere else in the system?
What's a "double fed circuit?"

I appreciate your seeing me through on this.

Have a great Christmas.

Potomac101

Andy in ATL 12-22-2007 05:56 AM

A double fed circuit is when two circuits ON THE SAME PHASE are connected together accidently. This could have happened when the house was initially wired, or later on. When you turn off one breaker, the other continues to feed the circuit.

ALWAYS check wires that you are going to be working on with a voltmeter. A "non-contact" tick tester can give false readings.

I have recieved strong static shocks when working in boxes I knew were dead and didn't like it one damn bit.:laughing:

RichyL 12-22-2007 02:51 PM

Is the box you were working in metal or plastic? Did you see anywhere on the circuit where the neutral was tied to the grounds? Also how many wires are in that box and describe the splices. Nuetral or hot picked up from another circuit or grounded neutral is my guess. Finding out which wire shocked you or if it was in fact the box would be helpful but i dont expect you to take one for the team. There are a number of scenarios that could be causing this though and for us to answer your questions you will have to accurately answer some of ours. These kinds of problems are usually solved by process of elimination. Let us know when you get that voltmeter and we can go from there. Have a happy holiday:)

220/221 12-22-2007 05:47 PM

If everything is as you described it, it's not "double fed". The light wouldn't have gone off when you turned the breaker off.


Double fed.

Circuit A is a bedroom.

Circuit B is the bedroom next to it.

If someone inadvertantly put a jumper between the outlets between the rooms, those two circuits would be tied together.

If both of those circuits were tied to seperate breakers on the same phase, you could turn of A and it would still be powered by B all the way back to the breaker.

If the two circuits were on opposite phases there would be a phase to phase short.


It's hard to speculate on the cause of the "shock" that happened long ago. You have to investigate while you have things apart. There are SO MANY screwey things people can do to mess up their electrical systems.

Here is a possible. Lets say someone wanted to add an outlet under a switch but the switch had no neutral (suicide switch). They used the hot from that circuit but grabbed a neutral from another circuit. Power to circuit A was off but the return current from neutral B could be trying to make it's way back to the source.

Electricity is odd and the more homeowners/handymen mess with it the worse it gets.


PS. I have gotten my share of "shocks" when there was no voltage and no static involved. Just a funky funny bone kind of feeling when I tweak my wrist or something installing switches/outlets. Psychological???

Andy in ATL 12-22-2007 08:07 PM

I've almost jumped off ladders when my bones have cracked just right.:laughing:


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