Variac Tripping MCB Issue - Page 1
Forum section: electronics / general

author: Tesladog
author_rank: Newbie
timestamp: October 07, 2025, 04:24:43 pm
content: Hi Everyone,I have a 15A variac that I have a mcb tripping issue with. Inrush current is not a problem here as I have compensated for that. The variac is wired in line with a soft-start relay and a 20A mcb. Switching the variac on is fine when it isn't attached to any load. If I connect a load between the brush and the neutral it immediately trips the mcb. It also trips the mcb if I try to take a voltage measurement between the brush and the neutral using a multimeter.

author: Stray Electron
author_rank: Super Contributor
timestamp: October 07, 2025, 08:45:33 pm
content: You do realize that Variac's are auto-transformers so the secondary is not isolated from the primary in the way that most transformers are, don't you? If your MCB also detects leakage, that could be what's causing it to trip. If you have an isolation transformer around you could try driving the Variac from it and seeing if it still trips your MCB. If I'm right then I don't think that it will.

author: Tesladog
author_rank: Newbie
timestamp: October 08, 2025, 08:55:15 am
content: Yes, I do know that variacs are auto-transformers and that the secondary is not isolated from the primary. I have several other variacs in the same type of set-up to control hv transformers, which is what the one that I am having issues with is set up for without any mcb problems. I have been running like that for years.Why does measuring the voltage from the brush (output) to neutral cause the mcb to trip? I have done exactly the same with several other variacs and no tripping occurs.

author: theHWcave
author_rank: Regular Contributor
timestamp: October 08, 2025, 09:46:23 am
content: Does measuring the live input connectionat the variacagainst neutral also trip the MCB?Does it trip - measuring between brush and neutral whena) the brush is positioned over the live input to the variac (assuming as with many variacs that you can turn further for "over voltage" at the brush)?b) the brush is positioned at or close to the neutral end of the variac?

author: Stray Electron
author_rank: Super Contributor
timestamp: October 08, 2025, 05:00:30 pm
content: Quote from: Tesladog on October 08, 2025, 08:55:15 amIt might just depend on how much current it takes to trip the mcb, particularly current flowing through ground. It usually takes VERY little current flowing through the ground circuit to trip a modern cb. it's also possible that there is some resistance in your neutral line and that that is causing some current to flow through the earth ground. That's why I suggested powering the whole rig through an isolation transformer to find out. If there is current flowing through the ground circuit then using an isolation transformer should stop it but you will have also have to isolate the frame of your variac from ground while you are testing and that means that there might be some for voltage on it if you have leakage somewhere.In the US, the neutral and ground are supposed to ONLY be connected back where power enters the building and nowhere else. The ground in the wiring and at the outlets is strictly a protective circuit and no current is supposed to flow through it under normal conditions but I don't know that's handled in your jurisdiction.You could also try bypassing your mcb with a small fuse, strictly as a test, and see if you can then measure the output of the variac without blowing anything. Or bypass your mcb with a (fused) amp meter and see how much current really is flowing through it, both when measuring the output voltage and when not measuring it.

author: themadhippy
author_rank: Super Contributor
timestamp: October 08, 2025, 05:56:50 pm
content: Quotethinks someone is getting mcb mixed up with rcd/gffi/ .

author: Tesladog
author_rank: Newbie
timestamp: October 09, 2025, 08:41:34 am
content: Thanks for the info, but I am in the UK and it is an MCB and not an RCD.

author: themadhippy
author_rank: Super Contributor
timestamp: October 09, 2025, 10:05:24 am
content: Quotejust a thought you 100% certain its JUST an mcb and not an rcbo? to trip an mcb requires at least the rated current,in this case over 20A to be drawn through the device,and multi meters certainly dont draw 20A,well not unless you got em set wrong.Even an rcd/rcbo requires at least around 20-30ma to trip,much more than a bog standard multimeters load

author: Tesladog
author_rank: Newbie
timestamp: October 09, 2025, 12:21:28 pm
content: Tried the measurements that you suggested and it only trips when the measurement is attempted between the brush and the neutral and it doesn't matter where the variac knob position is when this happens.

author: SteveThackery
author_rank: Super Contributor
timestamp: October 09, 2025, 01:48:06 pm
content: Quote from: Tesladog on October 09, 2025, 08:41:34 amThis is mighty puzzling. There's no way a multimeter could blow a main circuit breaker without also blowing your eyebrows off, and even then only if you'd left it on a current range.There is something more going on than the OP has revealed. There must be.

author: Tesladog
author_rank: Newbie
timestamp: October 09, 2025, 04:02:40 pm
content: Thanks guys for all of your input, but as this issue is very puzzling, I have decided just to buy another variac.I fitted one of my other (same type) variacs into the circuit and no issue with mcb tripping, so I guess there is some peculiar fault with the variac.

author: MathWizard
author_rank: Super Contributor
timestamp: October 11, 2025, 03:22:50 am
content: Here in Canada with no RCD involved, my Auto-Trans and isolation transformer together, would sometimes trip the 10 or 15A breaker. There was also a fridge and a few small SMPS on that breaker (like for LED bulbs and a modem), but I never really worked out the details of what caused the tripping.I seem to remember it happening, when plugging the AT, into the IT, with no load or DUT attached. I think I even unplugged the fridge, and it would still happen sometimes. I was thinking maybe an in-rush NTC might help, but I never got around to trying it.I think my Hakko FX-888D soldering iron, and clone 858 hotair station together, would also sometimes trip this breaker, one of these days I'll have to try and scope or measure what's going on. For those 2, I'm guessing it was also the fridge running. The house wiring was all new, but maybe something's not right.

author: theHWcave
author_rank: Regular Contributor
timestamp: October 11, 2025, 10:43:13 pm
content: Just to confirm: if the brush is right above the point where the L is connected to the variac, measuring between brush and N trip the MCB but it does not when you measure between L and N (using L at the terminal of the variac) ?Also if the brush is at the very bottom as close as possible to N, measuring between brush and N trips the MCB but not when measuring between N and N (using N at the terminal of the variac)?That is one strange variac ! Some pictures would be good...

author: AVGresponding
author_rank: Super Contributor
timestamp: October 12, 2025, 08:35:36 am
content: Quote from: MathWizard on October 11, 2025, 03:22:50 amThat's an inrush current issue. The OP is getting nuisance tripping when measuring the voltage of some part of an already on DUT with a DMM. There is something very strange happening, as any current large enough to trip an MCB which is presumably supplying a socket ring, would absolutely vaporise the internals of a DMM.

author: EC8010
author_rank: Frequent Contributor
timestamp: October 12, 2025, 02:57:02 pm
content: Years ago, switching on a pair of 10A Variac (used as quiet lighting dimmers) would sometimes trip our 32A MCB. The reason was that Variacs don't have much in the way of primary inductance, so their switch-on surge can be rather large, depending on which part of the mains cycle you hit.

author: Stray Electron
author_rank: Super Contributor
timestamp: October 12, 2025, 04:30:04 pm
content: Quote from: EC8010 on October 12, 2025, 02:57:02 pmAgree. But that's not what the OP says is happening. He specially says that the MCB is tripping only when he attempts to measure voltage on the Variac. Other than the MCD (RCD?) tripping because of some kind of leakage in the Variac, the only thing that I can think of that could cause the tripping is that the OP has his meter set to measure current and not voltage. But the OP seems like he has enough knowledge to know how to use a volt meter so that only leaves the possibility that his MCB has some kind of built in RCD. I would like to know more about his Variac and the MCB that he's using; how old are they and how long has he had them, where did they come from, has he used this type before, etc etc. The OP does say that he has larger Variacs and that they don't trip but I think this one (and/or the MCB) must be different in some way.I'd specifically like to know if the MCB that he's using came with the Variac and they were pre-wired together and if he's using them in an UN-Modified manner.Not that we haven't already seen mis-wired Variacs coming from China on this forum.But until the OP provides more information about the Variac and the MCBanddoes some testing, we're just guessing here.

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 12, 2025, 07:38:30 pm
content: I'm going to guess a mechanical problem where applying even a small force on one of the terminals causes it to short.

author: AVGresponding
author_rank: Super Contributor
timestamp: October 12, 2025, 08:09:11 pm
content: Quote from: NiHaoMike on October 12, 2025, 07:38:30 pmThat seems eminently plausible