Dangerous fuseholder wiring in variac - Page 1
Forum section: electronics / general

author: Circlotron
author_rank: Super Contributor
timestamp: July 14, 2020, 11:21:57 pm
content: Opened up my variac to have a look inside and I noticed that the incoming mains active wire goes to the fuesholder connection that is closest to the front panel. This means that with the fuse removed, the live part of the fuseholder can be contacted by the user. Good idea to swap the fuseholder wires.

author: TimNJ
author_rank: Super Contributor
timestamp: July 15, 2020, 12:56:50 am
content: Now that's an interesting design accident that I've never considered. I've used this style of fuseholder many times in my own projects, and never once thought about that. Though, I'd say most people will *probably* unplug the thing anyway, but who knows.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 15, 2020, 01:02:26 am
content: I wonder why those variacs have a voltmeter that is too imprecise to adjust the voltage with rather than an ammeter that would actually be useful. Better yet, nowadays they should be using digital panel meters that can at least match a cheap Harbor Freight DMM for accuracy and precision.

author: Ian.M
author_rank: Super Contributor
timestamp: July 15, 2020, 01:53:02 am
content: That looks remarkably similar to the far east 'red peril' variac user 'tkamiya' tore down here:https://www.eevblog.com/forum/beginners/shopping-for-a-variac-shopping/msg3066512/#msg3066512and eventually decided to condemn for scrap as irremediably unsafe based on his findings here:https://www.eevblog.com/forum/beginners/shopping-for-a-variac-shopping/msg3068574/#msg3068574If the wiper doesn't have a slip ring but is connected by an ordinary wire wrapped a couple of turns round the uninsulated steel shaft that flexes and rubs every time the knob is turned, yours is the same as 'tkamiya's and should probably also be condemned as eventually the insulation will rub through and the shaft go live. If you want to keep it safely in service you'll need to add proper ground bonding straps between all its metalwork, an insulating sleeve on the shaft, and glassfibre sleeving over the wiper wire, (which for reliability should also be replaced with extra-flexible test lead wire of an appropriate CSA) to protect it from rubbing!

author: wraper
author_rank: Unknown
timestamp: July 15, 2020, 02:03:53 am
content: Quote from: Circlotron on July 14, 2020, 11:21:57 pmThat's basically how most of similar fuse holders are made. Swapping wires won't help much since it still will be connected through transformer anyway. You may argue that Australian mains plugs are polarized but I would not count on that.

author: newbrain
author_rank: Super Contributor
timestamp: July 15, 2020, 07:25:07 am
content: Quote from: wraper on July 15, 2020, 02:03:53 amThis.At least, the way it is now, connection to live can be broken by turning off the switch (if you don't unplug it and not even bother to turn it off, you might be eligible for a Darwin Award).In Europe, where most of the plugs are not polarized (and in some places, there's even no neutral), reversing the contacts would only make it more dangerous.

author: GlennSprigg
author_rank: Super Contributor
timestamp: July 15, 2020, 12:22:39 pm
content: Quote from: wraper on July 15, 2020, 02:03:53 amI understand what you are saying, but in Australia, (and unlike say a homemade extension cord thatmay be wired wrongly), ALL such factory supplied equipment plugs ARE correctly polarized. And unlikethe U.S. plugs, always have an Earth, (unless the device is double-insulated), which ensures the correctorientation of the pins. Our power-outlets are also 99% switched, isolating the L.H. active (hot) pin.We generally don't switch an 'outlet' in the ceiling for say an exhaust fan.

author: Ian.M
author_rank: Super Contributor
timestamp: July 15, 2020, 12:51:17 pm
content: I'll rewrite that for you: "ALL suchlegally importedfactory supplied equipment plugs ARE correctly polarized" or "ALL such factory supplied equipment plugs ARErequired to becorrectly polarized" . . .As to what's actually out there in service down under, especially on 'red peril' variacs, YMMV.However there is clear benefit in making sure its correctly wired as the O.P. proposed, both the fuse and switch are in the Line, not the Neutral, and that the incoming power cord's plug is wired correctly. While its on the bench, the O.P would be well advised to upgrade it by fitting a fuseholder (or a miniature panel mount circuit breaker if there's room and you can find one with the right current rating for an affordable price) for the output as well.Over here in the EU, external mains fuseholders that can be opened without the use of tools have been banned on new equipment for many years . . . .

author: wraper
author_rank: Unknown
timestamp: July 15, 2020, 01:49:44 pm
content: Quote from: GlennSprigg on July 15, 2020, 12:22:39 pmNo you don't. First of all, you may have incorrect mains or extension cord wiring with L/N swapped. That is not something rare. Secondly, if neutral wire breaks somewhere, then you will have line potential on neutral wire through other equipment passing current from line to neutral.

author: schmitt trigger
author_rank: Super Contributor
timestamp: July 15, 2020, 01:56:45 pm
content: "Red Peril" Variacs.That name should be trademarked.Similar to Superior Electric's Powerstats.

author: Ian.M
author_rank: Super Contributor
timestamp: July 15, 2020, 02:28:17 pm
content: 'Red Peril Variac' brand motto: "Safer than a Scariac!"

author: Benta
author_rank: Super Contributor
timestamp: July 15, 2020, 04:16:25 pm
content: I'd argue that not the wiring is bad, but that the fuse holder design is bad. Exposed metal has no place in mains circuits, there are better types on the market.Whether you get the whack directly from L or through the primary doesn't make a big difference.

author: tooki
author_rank: Super Contributor
timestamp: July 15, 2020, 04:43:49 pm
content: Quote from: GlennSprigg on July 15, 2020, 12:22:39 pmFYI, in USA, polarized 2-pin plugs are very common, ensuring correct orientation even without an earth pin. Double-insulated devices don’t need them, but they’re very common all the same.

author: GlennSprigg
author_rank: Super Contributor
timestamp: July 17, 2020, 12:48:56 pm
content: Quote from: wraper on July 15, 2020, 01:49:44 pmYes I doHowever, did you see my statement that...(and unlike say a homemade extension cord that may be wired wrongly)...And to 'tooki', yes I'm aware that a lot of U.S. plugs are now 'polarized' by way of a slightly largerpin as one of the pair, but a large amount of older plugs do not, and generally an Earth pin was rare!Back to theO.P.though, it's plain to see that the 'exposed' portion of the Fuse-Holder could obviouslycould pose a problem, even if the 'supply' was done correctly. It was a very valid point!

author: Zero999
author_rank: Super Contributor
timestamp: July 17, 2020, 09:56:15 pm
content: Quote from: GlennSprigg on July 17, 2020, 12:48:56 pmI don't think I've ever encountered an extension lead with the live and neutral swapped. Have you seen an extension lead with a neutral and earth swapped? That would work, if it had a polarised plug and was used in a circuit with no RCD present, but it doesn't mean it's safe.If the device is designed to be used in an area with polarised sockets, then it's perfectly acceptable for it to be designed with that in mind. Of course it needs to be made safe in the event the neutral or even earth conductor is broken, but that's a different matter.

author: Greg Robinson
author_rank: Regular Contributor
timestamp: July 17, 2020, 10:35:54 pm
content: Another concern, I don't see a fuse on the output?!?Thanks to transformer action, at low voltage output the input current to an autotransformer is low, but the output current can be the input/output ratio greater. Without a fused output, the brush and winding can burn out...

author: duckduck
author_rank: Frequent Contributor
timestamp: July 17, 2020, 10:44:34 pm
content: Quote from: newbrain on July 15, 2020, 07:25:07 amThat's how mine is. The fuse holder is wired in "backwards" (live end out) but to touch a live part someone would have to unscrew the fuse with the unit plugged in and turned on. I also noticed that all of the mains wiring was solderedAdd to that the perils mentioned by the guy that tore one down and I'm going to have to look at this thing closer.Sociopaths continue to build these mains-powered fire-starters because people like me continue to buy them. Maybe it's time for me to look in the mirror.EDIT:When I bought my current nest I checked all of the outlets and found that 3 (of the many outlets) had a reversed line and neutral.

author: Cubdriver
author_rank: Unknown
timestamp: July 19, 2020, 08:41:31 am
content: Quote from: Ian.M on July 15, 2020, 02:28:17 pm(But not much...)-Pat

author: Zero999
author_rank: Super Contributor
timestamp: July 19, 2020, 05:15:38 pm
content: Quote from: duckduck on July 17, 2020, 10:44:34 pmWhy not just return it and the same for the original poster?These cunts shouldn't be allowed to sell defective products like these!