The variac dials a voltage between 0 - 120v. I have no idea if the thing is current limited. It has, IIR, a 5amp fuse in it. We usually have it set to about 11 - 12v. This makes the piano wire get nice and hot. The device is actually fairly large, and we usually have one person on each end lightly pulling it through the foam.
Presumably the guy on the neutral side should be safe if he were to touch the wire. But what about the guy on the hot side?? What’s the safest way to use this? I imagine that if we could be perfecly insulted from ground (like by standing on a platform) the thing should be safe.
But maybe this doesn’t matter at all - if the device only produces a voltage of 11 - 12v, maybe that’s not even enough to worry about, in terms of body resistance and how much current it can push through?
So basically, I’m looking for an analysis from the electrical guys in terms of is this thing safe, and what could be done (if anything) to make it safer?
12 volts isn’t anything to worry about. The main concern would be burns from the hot wire, but there’s no way to protect from that and still have the thing useful.
The only danger is if someone cranked the variac up to a higher voltage. The way to prevent that is to get rid of it-- find out the voltage you need for the cutter to work right, and then make a transformer that runs straight off the wall socket that delivers that voltage and current on the secondary side (-although I already know that it can be useful to have an adjustable option).
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Ive taken full 120v household current a few times you really dont have anything to worry about. It hurts like hell but its a lot less dangerous than my power tools. Standing on anything but a metal floor youll be well insulated from the ground. Assuming all of the connections, switches and contacts are covered so that you cant touch them youll be fine. The only danger from electric shock should be from the hot wire, which you shouldnt be touching anyway.
I’d wire in a GCFI. That way, if someone does manage to make contact to mains voltage (which he could, since a Variac is an autotransformer and has no isolation from the mains) the device will trip if more than 5 mA is unaccounted for.
Worst advice ever. Please refrain from giving electrical advice in the future. There are many circumstances under which household mains voltage can be lethal. You don’t don’t screw around with this stuff.
Yeah, I suppose the GCFI idea would be best; but its not within my power to go ahead and wire one up.
What’s the difference between a GCFI and a GFI?
DougC: Unfortunately, yes, we pretty much have to have an adjustable option. Depending on what foam we’re using, how it needs to be cut, etc. we need to be able to adjust the voltage.
Actually, it’s GFCI–blame my dyslexic typing. The full abbreviation stands for Ground Fault Circuit Interruptor. GFI means the same thing, only word “circuit” has been left out.
I would put an isolation transformer before the variac. Or you could consider using a step-down transformer so long as the secondary is isolated from the primary. With an autotransformer such as a variac it is quite possible to have only 12V across the output leads yet still have full line voltage, or very close to it, to ground.
Galvanic isolation would be the safest thing, IMO. So I would first look into installing a 120 to 24 VAC step-down transformer between the Variac and wall outlet. (Just make sure the transformer can handle the power.) If that doesn’t work out for some reason, then I’d go with Q.E.D.’s suggestion and install a GFCI outlet. They’re only $7 or so, and take a whopping 10 minutes to install.
What if instead of running the current through the cutting wire you ran it through a smaller totally covered heating element in the device that would heat the wire but not transfer current to it.
If there is a TV/appliance repair shop going out of business in your area, you’re all set. That’s where you’ll find isolation tranformers (no step up or down). Maybe you can scrounge one. Then you’d have no need to alter your current design (assuming it is otherwide safe).
Well, yes, and no. A variac is a type of transformer called an autotransformer. Let’s see if I can draw one:
1o-----3
3
3
3
2o-----3
3
3o-----3
For a variac, 2 is the sweeper, 1 is the hot line input and 3 is the common. As you can see there is no isolation between the 1-3 input and the 2-3 output.
I don’t know how many times this subject has been covered on this board, but I suppose it bears repeating since there are clearly plenty of posters who lack respect for electricity. As Q.E.D. correctly pointed out, the advice quoted is lousy – almost to the point of criminality.
Irrespective of voltage, the current required to kill a person is measured in milliamps. A common household outlet provides 15 to 20 full amps of current. Do the math.
The OP expressed a legitimate concern, and cavalierly discounting the potential danger, a la** DougC** and NevarMore, is simply stupid.
Can I jump on the bandwagon and say this is extremely bad advice.
The majority of fatal shocks would be from 120 volt circuits as this is the most common voltage here in the US.
Even those that get a shock from a 208-240 branch circuit are most likely to receive a 120 volt shock as it is more common for shocks to be phase to ground.
I have received my share of 120 and 277 volt shocks and luckily I am still here, but I would never say that these voltages are nothing to worry about.
The isolation transformer would be the way to go or at the least the plug in GFCI protection.
Remember that a GFCI will not prevent a shock it can only shorten the duration of the shock.