Using a Variac Safety concern

[veryworried]

Hi, I am using a variac to control my fan as it's too loud at night.  If I use an RCD device will that protect me if there is a fault?

Thanks.

[Per Hansson]

Yes, a normal variac offers no isolation at all, it is basically works like a large resistor (or better rheostat) so your RCD will work fine!
P.S: There are isolating variacs too (they work like a normal transformer) but they are not the norm when you speak of a "variac"

[veryworried]

Yes I've read about the isolation ones and lots of conflicting articles about it.  I think mine is just a standard variac with the copper windings and brush.  It is a stand alone unit that's plug and play no wiring necessary.

Thanks for your expertise on the subject.

[metrologist]

I was looking for illustrations that show how variac is dangerous but did not find them. Sometimes I don't see the issue, like in this case, as long as things are wired properly.

[CaptDon]

Actually works more like an autotransformer with a variable tap position.

[eugene]

A variac is a variable autotransformer that does not provide the isolation of a conventional transformer.

But is that a problem? Do you need isolation? To put it another way, using the fan with a variac does not provide LESS isolation that using the fan without a variac.

If the variac itself is potentially faulty then fix or replace it, but the basic design is not, in itself, hazardous.

[Zero999]

It will be fine. There's no need to worry. It's not dangerous. You could also try lamp dimmer to reduce the speed, but it might be noisy.

[veryworried]

Yes I did use a cheap plug in dimmer off ebay, but after a while my fan stopped working and had to buy another one.  As I understand it, the cheap one chops up the sine wave which causes the hum, so I opted for something more stable and consistent (auto transformer).

But I do have another concern that worries me.  If the auto transformer is turned on and nothing else is connected to it, will it overheat and cause a potentially dangerous situation ?

I'm going to be fitting a 1 amp fuse to the input & output plugs that are connected to it as I only need it for a fan.

[eugene]

ut I do have another concern that worries me.  If the auto transformer is turned on and nothing else is connected to it, will it overheat and cause a potentially dangerous situation ?

An autotransformer with no load is no more dangerous than an ordinary transformer with no load. But there's no reason to trust me. Turn it on with nothing plugged in. Then, using your own hand, touch it to see how hot it gets.

[mag_therm]

Yes I've read about the isolation ones and lots of conflicting articles about it. 

Yes, Confusing info. In Australia where I was and I suppose in other countries with 240 V outlets, the electrical courses included the variac use.
In old days, extension leads were often hand wired, so the neutral prong was not really standard.

Using a variac to get from 240V to, say 120V  or 24 VAC can raise a false sense of safety because the uninformed can probe on 24 VAC thinking it is safe.
But the "0V" terminal might be at 240 V wrt ground earth, with the other terminal at 216V wrt ground.

Often the variac has input to tap to allow setting higher than input voltage.
I have one like that here in USA, it allows up to 140 V out  with 120 V in. Also the socket is old type, unpolarized, no connection diagram.

Using a GFCB (RCD) or isolation transformer upstream of variac is a good plan. I have dedicated GFCB for all outlets on electronic bench and another for the outside workshop.

[wizard69]

NO!   Simply not enough information here, we don't even know what you mean by "protect me" nor do we know what sort of fan you are talking about.   An RCD protects you from leakage to ground!   If that is what your concern is then you may be good.   On the other hand if you are running a fan in a Brown Out condition you can have significant overheating in the motor which depends upon the motor.   This can lead to a fire hazard.

Hi, I am using a variac to control my fan as it's too loud at night.  If I use an RCD device will that protect me if there is a fault?

Thanks.

[mag_therm]

Wizard makes a good point about checking first for overcurrent at low voltage on some motors.

I have two fans here , one a more recent box fan and one an smaller,older bedroom type. Both have the basic impedance limited motors.
That is , XL >> R
I tried both on full speed setting  on the bench variac here  with a PEM current transformer.
Box fan draws 1.5 A at 120 V, falling almost linearly with voltage, to 0.5 A at 60 V, and runs usably down to about 25 V
Small fan draws 0.53 A rms at 120 V, falling linearly down to 30 V where it is too slow to make a usable breeze.

[veryworried]

Sorry I'll try and include more information

this is the fan im using:

https://www.amazon.co.uk/Senelux-Floor-Fan-Electric-Circulator/dp/B0957W3P4W/ref=sr_1_3?crid=2ROKYSH81JA67&keywords=air+circulator+fan+20&qid=1659814897&sprefix=air+circulator+fan+20%2Caps%2C64&sr=8-3

this is the RCD im using:

https://www.screwfix.com/p/masterplug-13a-fused-plug-through-active-rcd-adaptor/63731

The transformer plugs directly into the masterplug RCD.  All plugs fitted with 1 amp fuse.

What I want to know is will it trip and sever the connection to protect me from death if there is a fault.

thanks for your time.

[veryworried]

Actually I think I'm in over my head here.  I'll just scrap it and use a weaker desk fan to keep me cool.  wtf was I thinking this is insanely dangerous.

I'm going back to playing on my computer where I feel safe and forget this entire thing ever happened.

[Zero999]

It's not dangerous. I lived with my parents' for 25 years and they had no RCD in their house and I was fine.

[David Hess]

Hi, I am using a variac to control my fan as it's too loud at night.  If I use an RCD device will that protect me if there is a fault?

I do the exact same thing and it is completely safe.  The variac does not provide an isolated output like a transformer would, but the fan is intended to work with a non-isolated source anyway.

[james_s]

Actually I think I'm in over my head here.  I'll just scrap it and use a weaker desk fan to keep me cool.  wtf was I thinking this is insanely dangerous.

I'm going back to playing on my computer where I feel safe and forget this entire thing ever happened.

It's not dangerous at all, I mean we don't even use RCDs over here except in wet locations, normal indoor receptacles don't even have them. A variac also will not defeat an RCD, they do not provide isolation, if you can even get one that does isolate it will be bulkier and it will specifically say so on it because that would be such an unusual feature.

[Zero999]

One thing to note is, whilst an autotransformer doesn't defeat an RCD, it will make it less sensitive. Suppose the variac is set to 50% and 10mA leaks from the secondary to earth, only 5mA of extra current will flow through the primary, so the RCD will only see 5mA of imbalance. Reducing the sensitivity of the RCD at lower voltage in this case is no big deal, especially given the shock risk is also lower.

EDIT: This isn't true. See post below.

[David Hess]

One thing to note is, whilst an autotransformer doesn't defeat an RCD, it will make it less sensitive. Suppose the variac is set to 50% and 10mA leaks from the secondary to earth, only 5mA of extra current will flow through the primary, so the RCD will only see 5mA of imbalance. Reducing the sensitivity of the RCD at lower voltage in this case is no big deal, especially given the shock risk is also lower.

It is still detecting the total leakage current to Earth, which is the part that matters, no matter what how the variac is set.

An isolation transformer defeats a RCD if the secondary is grounded because it diverts the leakage current before the RCD can detect it.  If the secondary is floating, then there should be no leakage current to detect and if the isolation transformer fails, then the RCD would detect any leakage current and operate normally.

[Zero999]

One thing to note is, whilst an autotransformer doesn't defeat an RCD, it will make it less sensitive. Suppose the variac is set to 50% and 10mA leaks from the secondary to earth, only 5mA of extra current will flow through the primary, so the RCD will only see 5mA of imbalance. Reducing the sensitivity of the RCD at lower voltage in this case is no big deal, especially given the shock risk is also lower.

It is still detecting the total leakage current to Earth, which is the part that matters, no matter what how the variac is set.

No it isn't. The RCD/GFCI just senses the imbalance between the phase and neutral conductors.

[james_s]

The lower the voltage out of the variac, the lower the risk of shock or injury so that should help to offset it. Either way I wouldn't be concerned, as long as you're not using the fan while in the bath or standing in a puddle outside it shouldn't matter at all. As I mentioned earlier we don't even use RCDs in the first place in North America except for damp locations.

[David Hess]

One thing to note is, whilst an autotransformer doesn't defeat an RCD, it will make it less sensitive. Suppose the variac is set to 50% and 10mA leaks from the secondary to earth, only 5mA of extra current will flow through the primary, so the RCD will only see 5mA of imbalance. Reducing the sensitivity of the RCD at lower voltage in this case is no big deal, especially given the shock risk is also lower.

It is still detecting the total leakage current to Earth, which is the part that matters, no matter what how the variac is set.

No it isn't. The RCD/GFCI just senses the imbalance between the phase and neutral conductors.

What remains after the imbalance is the leakage.

[Zero999]

One thing to note is, whilst an autotransformer doesn't defeat an RCD, it will make it less sensitive. Suppose the variac is set to 50% and 10mA leaks from the secondary to earth, only 5mA of extra current will flow through the primary, so the RCD will only see 5mA of imbalance. Reducing the sensitivity of the RCD at lower voltage in this case is no big deal, especially given the shock risk is also lower.

It is still detecting the total leakage current to Earth, which is the part that matters, no matter what how the variac is set.

No it isn't. The RCD/GFCI just senses the imbalance between the phase and neutral conductors.

What remains after the imbalance is the leakage.

The autotransformer steps the voltage down and the current up, so the current imbalance in the supply conductors will be less than the leakage from the secondary.

Fortunately as mentioned above, the shock risk goes down, as the voltage is reduced, so it's not that much of a problem, but it's something to be aware of.

EDIT: This isn't true. See post below.

[David Hess]

The autotransformer steps the voltage down and the current up, so the current imbalance in the supply conductors will be less than the leakage from the secondary.

Fortunately as mentioned above, the shock risk goes down, as the voltage is reduced, so it's not that much of a problem, but it's something to be aware of.

The RCD measures the residual current across both conductors (common mode) after the point where ground is attached to neutral.  If the output of the autotransformer was dialed to zero, connecting the output to neutral, and a leakage of 10 milliamps was still present, then the common mode measurement would still be 10 milliamps instead of 0, presumably all flowing through the neutral.

As pointed out that would be weird since the output voltage is so low, but neutral gets pushed around by return currents and the output impedance could be low.  Lowering the output voltage should lower leakage, but there are circumstances where it does not and an RCD should not care.

The differential measurement of the return current from hot, through neutral, gets halved by the autotransformer in your example, but the common mode leakage through ground does not and the later is what the RCD measures.

[Ground_Loop]

Actually I think I'm in over my head here.  I'll just scrap it and use a weaker desk fan to keep me cool.  wtf was I thinking this is insanely dangerous.

I'm going back to playing on my computer where I feel safe and forget this entire thing ever happened.

I really hope you're kidding.  It would be a shame to let the ever noisy safety "experts" get under your skin.