240v in standard switch boxes.

[360Youth]

I'm new here so if this has been posted before, or old news sorry. This is something I noticed a few years ago that I have started passing on to electrician and electrician wannabees for their own safety. How many bpeople realize how many household switchboxes have 240volts in them at any given time. I was working on a mult-gang box once (don't remember if I was trouble shooting, or what) and put my meter across two of the switches (S3 and S3 or S3 and SP) and got 240v where the S3 was from a circuit elsewher in the house. It is something I have been mindful of ever since. I even go to the extent of installing warning labels on th inside of cover plates if i feel like it warrants it. How many others have come accross this?

 

[George Stolz]

This is why it's very important to ensure a circuit is off before working on it.

A related code section (it doesn't apply in this case, but worth a look anyway) is 404.8(B).

 

[petersonra]

what difference does it make? 240V is not appreciably more dangerous than 120V.

 

[Dennis Alwon]

There is always a possibility that switches are fed from 2 circuits in one box. That's why we have multimeters. If the two circuits are installed on a double yoke (stack type switch) then the circuits must be installed on a double pole breaker so that both poles will be open when you shut it off. The code does not require that if the circuits are on different yolks. You could potentially have 4 circuits in a box with 4 switches, unlikely but possible.

 

[infinity]

what difference does it make? 240V is not appreciably more dangerous than 120V.

I would say that for the same resistance of the human body it's twice as dangerous.

 

[iwire]

I would say that for the same resistance of the human body it's twice as dangerous.

I agree.

But now I ask what are the chances of getting a 240 volt shock while working in that box.

120 or 240 are still 120 to ground and IMO few people actually get line to line shocks, you almost have to try to do that.

 

[infinity]

I agree.

But now I ask what are the chances of getting a 240 volt shock while working in that box.

120 or 240 are still 120 to ground and IMO few people actually get line to line shocks, you almost have to try to do that.

Quite true, but I think that is inaccurate to say that there isn't a greater danger from a 240 volt shock as compared to a 120 volt shock. Due to simple Ohm's law it could be the difference between life and death.