Using a Variac Safety concern - Page 1
Forum section: electronics / beginners

author: veryworried
author_rank: Newbie
timestamp: August 04, 2022, 07:37:51 pm
content: 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.

author: Per Hansson
author_rank: Unknown
timestamp: August 04, 2022, 07:44:45 pm
content: 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"

author: veryworried
author_rank: Newbie
timestamp: August 04, 2022, 07:51:20 pm
content: 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.

author: metrologist
author_rank: Super Contributor
timestamp: August 04, 2022, 09:04:14 pm
content: 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.

author: CaptDon
author_rank: Super Contributor
timestamp: August 05, 2022, 12:40:03 am
content: Actually works more like an autotransformer with a variable tap position.

author: eugene
author_rank: Frequent Contributor
timestamp: August 05, 2022, 08:23:45 pm
content: 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.

author: Zero999
author_rank: Super Contributor
timestamp: August 05, 2022, 09:02:16 pm
content: 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.

author: veryworried
author_rank: Newbie
timestamp: August 05, 2022, 09:35:43 pm
content: 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.

author: eugene
author_rank: Frequent Contributor
timestamp: August 06, 2022, 01:00:27 pm
content: Quote from: veryworried on August 05, 2022, 09:35:43 pmAn 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.

author: mag_therm
author_rank: Frequent Contributor
timestamp: August 06, 2022, 01:50:42 pm
content: Quote from: veryworried on August 04, 2022, 07:51:20 pmYes, 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.

author: wizard69
author_rank: Super Contributor
timestamp: August 06, 2022, 03:29:31 pm
content: 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.Quote from: veryworried on August 04, 2022, 07:37:51 pm

author: mag_therm
author_rank: Frequent Contributor
timestamp: August 06, 2022, 04:24:27 pm
content: 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 >> RI 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 VSmall 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.

author: veryworried
author_rank: Newbie
timestamp: August 06, 2022, 08:00:00 pm
content: Sorry I'll try and include more informationthis 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=8-3this is the RCD im using:https://www.screwfix.com/p/masterplug-13a-fused-plug-through-active-rcd-adaptor/63731The 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.

author: veryworried
author_rank: Newbie
timestamp: August 06, 2022, 09:51:16 pm
content: 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.

author: Zero999
author_rank: Super Contributor
timestamp: August 06, 2022, 10:13:03 pm
content: It's not dangerous. I lived with my parents' for 25 years and they had no RCD in their house and I was fine.

author: David Hess
author_rank: Super Contributor
timestamp: August 06, 2022, 11:50:40 pm
content: Quote from: veryworried on August 04, 2022, 07:37:51 pmI 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.

author: james_s
author_rank: Super Contributor
timestamp: August 07, 2022, 12:37:32 am
content: Quote from: veryworried on August 06, 2022, 09:51:16 pmIt'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.

author: Zero999
author_rank: Super Contributor
timestamp: August 07, 2022, 07:13:40 am
content: 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. Seepost below.

author: David Hess
author_rank: Super Contributor
timestamp: August 07, 2022, 04:43:32 pm
content: Quote from: Zero999 on August 07, 2022, 07:13:40 amIt 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.

author: Zero999
author_rank: Super Contributor
timestamp: August 07, 2022, 05:05:40 pm
content: Quote from: David Hess on August 07, 2022, 04:43:32 pmNo it isn't. The RCD/GFCI just senses the imbalance between the phase and neutral conductors.

author: james_s
author_rank: Super Contributor
timestamp: August 07, 2022, 07:08:41 pm
content: 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.

author: David Hess
author_rank: Super Contributor
timestamp: August 07, 2022, 11:09:48 pm
content: Quote from: Zero999 on August 07, 2022, 05:05:40 pmWhat remains after the imbalance is the leakage.

author: Zero999
author_rank: Super Contributor
timestamp: August 08, 2022, 07:45:56 am
content: Quote from: David Hess on August 07, 2022, 11:09:48 pmThe 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. Seepost below.

author: David Hess
author_rank: Super Contributor
timestamp: August 08, 2022, 10:38:12 am
content: Quote from: Zero999 on August 08, 2022, 07:45:56 amThe 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.

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: August 08, 2022, 02:26:33 pm
content: Quote from: veryworried on August 06, 2022, 09:51:16 pmI really hope you're kidding. It would be a shame to let the ever noisy safety "experts" get under your skin.

author: Zero999
author_rank: Super Contributor
timestamp: August 08, 2022, 03:31:59 pm
content: Quote from: David Hess on August 08, 2022, 10:38:12 amYou're right. I was talking nonsense. The leakage current will find its way through the RCD, via the neutral conductor.EDIT:Here's an LTSpice simulation to help anyone else who's had a similar brain fart.

author: David Hess
author_rank: Super Contributor
timestamp: August 09, 2022, 12:28:55 pm
content: Quote from: Zero999 on August 08, 2022, 03:31:59 pmDo not beat yourself up over it. It is not an easy problem to think about. I had to think hard to come up with a way to try and explain it and I still had doubts.The same analysis applies if the transformer provides isolation, except then the isolation presumably blocks the leakage, so if the isolation fails, then the RCD is still there to work. As you pointed out, the transformer action should lower the leakage by lowering the output voltage, but the RCD still sees the total output leakage no matter what the transformer ratio is.

author: jonpaul
author_rank: Super Contributor
timestamp: August 09, 2022, 01:03:41 pm
content: Used Variacs and PowerStats since 1950s, sizes 1 ph 100 VA to 3 ph 20 kVA.Never had any problems.These are variable AUTORTRANSFORMES so the line neutral is connected to the output.No isolation.Jon

author: David Hess
author_rank: Super Contributor
timestamp: August 09, 2022, 02:47:15 pm
content: Quote from: jonpaul on August 09, 2022, 01:03:41 pmThe one problem I have had is that when the variac steps down the voltage, it proportionally steps up the current capability, which means that at low voltages it is easy to burn up the winding. So a fuse on the *output* to limit the tap current is a good idea if the load has the potential to short.

author: Zero999
author_rank: Super Contributor
timestamp: August 09, 2022, 02:57:57 pm
content: Quote from: David Hess on August 09, 2022, 02:47:15 pmThat's true. A large Variac also has the potential to damage the neutral wire and RCD, if the secondary is shorted to earth. I'd go far as to say, a fuse is a must.

author: jonpaul
author_rank: Super Contributor
timestamp: August 10, 2022, 11:04:01 am
content: never had need or used any GFI, Arc CB or the like.We use 2 kVa 1:1 isolation tras to debug mains connected SMPS.Leakage to earth is minimal.j

author: MrAl
author_rank: Super Contributor
timestamp: August 10, 2022, 11:57:34 am
content: I had learned about the variac non isolation problem a long time ago and wanted to get an isolated variac but found out they are a lot more expensive. It's too bad these are great devices to have on hand. I was even able to lower the power of a regular microwave oven so it cooks slower and so doesnt burn up meats and stuff like that. I hate the pulsing of the lower cost microwaves so i got a cheapie to experiment with.Of course the newer designs with 'inverter' technology do not pulse they are continuous so you dont need a variac.The only other thing i can think of at the moment is if the variac is run at a current level too high for it's rating the brush can get messed up.A fuse is always needed here yes, and a GFI i would think could help too if you dont have an additional isolation transformer.

author: Terry Bites
author_rank: Super Contributor
timestamp: August 11, 2022, 06:24:41 pm
content: Most Variacs are not isolated and you might experience (just the once) a fatal shock. Anything over 30V is considered dangerous.The RCD or yanky GFI will give you a reasonable chance of survival though you will have an unpleasant tingle for a few mS.Its not worth investing an expensive isolated variac or variac transformer combo for a fan.The simple way to lower the speed is with a series cap you can switch in or out. I'd experiment with "motor run" caps.https://cpc.farnell.com/c/electronic-electrical-components/capacitors/film-capacitors/motor-run-motor-start-capacitors

author: james_s
author_rank: Super Contributor
timestamp: August 11, 2022, 06:26:28 pm
content: You don't need isolation or GFCI/RCD protection for a fan anyway as long as you are using it properly and it is not damaged. There should never be any exposed connections that a person can come into contact with.

author: Zero999
author_rank: Super Contributor
timestamp: August 11, 2022, 08:18:15 pm
content: The photograph also shows a transformer with a 110VAC output, which is also centre tapped to earth.For future reference, here's one with a 230VAC output.https://uk.rs-online.com/web/p/site-transformers/4368805

author: Gyro
author_rank: Super Contributor
timestamp: August 11, 2022, 08:22:37 pm
content: Quote from: james_s on August 11, 2022, 06:26:28 pmThe poor despondent OP gave up and wandered off at reply #13 to go and buy a desk fan!

author: Terry Bites
author_rank: Super Contributor
timestamp: August 15, 2022, 04:36:16 pm
content: Oh yes. I forgot to edit it out.Take the guts out of two of these things and run them back to back. Not practical for a fan, of course.You can pick them up second hand for next to nothing at boot fairs, yard sales and the like.You can get a lot of kVA at very low cost. Even new, a pair of these things are cheaper than an equivalent isolation transformer.Doubles ths insulation resistance too and halves the capacitance.I paid £15 for a used one. New a 1 kVA for £150 vs £250. Plus spare connectors and boxes!You dont want the CTs connected to ground.Do it all at your own risk.