Fail Of The Week: Hair Dryer As Light Switch

Home electrical, it’s really not that hard. But when you’re dealing with the puzzles left for your by someone else things can get really weird. [Daniel’s] sister and her husband ran into this recently. The video demonstration of their fail includes a lot of premature laughter, but it’s worth hanging in there… you’ll quickly see why she can’t contain her amusement.

The project at hand is a replacing a bathroom fan with a simple light fixture. Once the swap was made the light switch works just as anticipated. But a second switch which used to control a different light now behaves strangely. It doesn’t activate the original light, but instead switches the new fixture. Even stranger is that the original switch apparently now acts as a bizarre dimmer when the second switch is on. That’s odd, but the coup d’état of the fail is when they plug in a hair dryer and switching it on illuminates the light but doesn’t activate the hair dryer.

As with all Fail of the Week segments, the goal here is not to criticize but to commiserate. What do you think is causing this? We can’t wait to see what you come up with. Posting simple diagrams is encouraged (you can use HTML img tags in the comments). Ladies and gentlemen, start your conjecture.

[Thanks to Hackaday alum Todd Harrison for sending in the tip]


2013-09-05-Hackaday-Fail-tips-tileFail of the Week is a Hackaday column which runs every now and again. Help keep the fun rolling by writing about your past failures and sending us a link to the story — or sending in links to fail write ups you find in your Internet travels.

69 thoughts on “Fail Of The Week: Hair Dryer As Light Switch

  1. Hmm – Looks like Lighting circuits and power circuits have been mixed. This (at least in Australia) is a no-no.

    I would be going back up to the light and removing the power circuit from it.

    1. I believe in the USA, when they install a new socket or whatever, they just run it from the nearest bit of cable they can find. They also tend to do a lot more of that sort of thing, I suppose cos their houses are made of wood, so it’s easier to make holes in.

  2. Somewhere a neutral is open and/or a neutral and hot are swapped.

    I had a loose connections in a fluorescent fixture feed back through the ground in my shop. I only figured that out after “stabbing” myself with the pins on some through-hole components I was soldering. Kept thinking I was squeezing the board too tight and poking myself. Then I realized the tingling/”poking” sensation was matched not with my grip, but when the soldering iron touched the same trace as I was holding.

    A quick check of the plug with a tester showed a wiring fault. The shop plug was on the same circuit as the lights, which were poorly wired.

    I’ve also accidentally wired a switch in parallel with a light. When I turned on the breaker the light came on (strange – I thought I left the switch off). Flip the switch and the light went off. Guess I must just have the switch in up side down. Flip the switch again and the light stays off. WTH? Go check the breaker and it’s tripped! D’Oh!

      1. I don’t remember the details – but no 3-way switch was involved. It was at best a loose connection that left a neutral or ground floating. I remember taking a volt meter to the outlet I had the soldering iron plugged into and either the neutral or ground was showing ~70VAC on a 110VAC system. The other thing I remember was the fault was in one of the lights.

  3. I bet they dion’t have the kind of circuit breakers that detect arcs. There’s a partial short in there somewhere, I’m betting, and one clue is that a light draws somewhere under one amp while a drier typically draws 10-15.

    We had a light switch that for a while didn’t do anything but blow out a breaker for something in another room…

    1. Nope doesn’t look like any kind of partial short I’ve seen. I’m more inclined to believe that the neutral return from the light is wired to an active of one or more of the switches or plugs.

  4. They’ve managed to wire the hair dryer outlet in series with the light. When the hair dryer is on, it looks like a dead short compared to the resistance of the lamp, so the lamp lights and the hair dryer seems to be “off”. In reality there’s probably 90W in the 100W bulb (assuming 100W bulb) and 10W in the hair dryer, but since the hair dryer needs to consume 1200W, it won’t do anything with only 10W.

    1. I agree. The hair dryer would be about 12 ohms relative to the light that would be 144 ohms. I think the outlet is in parallel with the second switch and the first switch is in series with that parallel combination.

          1. An open neutral makes the loads on opposite phases behave as if they are in series. Here’s why.

            Two-phase AC is fed into a house by the secondary of a transformer (the power company drives the primary). The center tap of the secondary is grounded via a ground rod (an eight-foot metal rod driven into the earth) where it comes into the house. In the breaker box, all the neutrals from all the circuits are connected together and wired to that ground. The two hot phases come into the breaker box from opposite ends of the transformer’s secondary.

            A 240V circuit just takes a hot from either side of the secondary. A 120V circuit feeds off one of the hots and the center tap/neutral. If you have an open in the neutral between the house and the center tap of that transformer, then the loads on opposite phases of the hots are essentially operating in series with 240V divided between them according to the load impedances. Switching on a 120V hair dryer makes that voltage divider change.

            When I worked for my Dad (an electrician), we went to a trailer that had an open between the house and that center tap+ground rod. Depending on the load between the phases, the voltage on the trailer’s “ground” (which was not grounded) would change, and was 0V ONLY when the two were perfectly balanced. When he grabbed the doorknob (which was electrically connected to the floating ground), he got a nice jolt.

            Very dangerous.

          2. I dunno why the USA bothers with the multiple-phase thing, seems like it’s asking for trouble. In Europe it’s 230V everywhere, single phase. Neutral is neutral, live is live.

            They should stick to one or the other. High-power appliances could run on 110V just as easily, you’d need some thicker cable, but nothing outrageous. Whose idea was it to make washing machines etc run on 220? Have houses always been wired like that? That, and seemingly having no standards for what can be wired to where, is asking for problems like this. Without the combined brains of HaD, where would they be?

        1. Typically they tap the heating element somewhere near one end to create a voltage divider that runs a low voltage motor. With a lamp in series, the voltage over the heater would drop, and the voltage for the motor would drop as well.

        2. most modern hair dryers use a low voltage motor(like 12V) and use the big heating resistor as resistance divider to get the 12V for the motor. There really is a tap contact on the big chunk of wire that forms the heating element. So from the mains perspective you have a 1200W or whatever it is load. I think the low voltage motor is for cost reasons, it would be more expensive to make a motor with the same characteristics operating at 120/220V.
          The ones that have cool air have like a 100W heating element always in series with the motor plus another 1100W element. You will not really feel the 100W of heat, even though it is still there when you use the cool air feature.
          The only hair dryers using maine voltage motors are the very old ones. Whoever thought about doing it like today is a brilliant person.

          Speaking of what was happening here, as a kid i got a very basic electronics kit, and what you could do with it was almost like here: a motor, 2 lights and 2 switches. you could switch between different configurations. I remember that one of the switches was SPST and one was SPDT.

    1. That way OCD lies, with complete madness just a short hop away.

      In countries that HAVE standards for housing, a bathroom is required to have an extractor fan if it doesn’t have a window. For the obvious reason of sucking damp air out. Are they removing the only fan? Why?

    1. Presumably the second light is also in series with the first light but in parallel with its switch. Turn on second light’s switch, main light comes on more dimly because of the resistance of the other light. Turn on main light switch and it gets brighter.

      Their wiring is obviously seriously fucked in any case.

      1. I will agree that the wiring is most likely fucked up, but there is an approved wiring method that places two hot lines on each hot of an outlet. You are running 120VAC from each outlet, and 240VAC from one hot line of one plug to the hot of the other; but they share the same neutral. It will balance, or so the book I read it from said. And, you DO have to remove the little tab that connects both of the hot terminals. I do not remember the name of the book, but it referenced all the applicable sections of the (?) 2004 NEC (National Electrical Code; US) for everything it described.

        I am not an electrician, but I have done a good bit of my own electrical wiring. And as such, I know that other people don’t seem to bother with proper wiring. So long as the bulb (incandescent) lights up, “the wirings good”. Which leads to such odd things as the new LED bulbs I screwed into the ceiling fan in the kitchen have a faint glow, even when all the switches are turned off.

        1. CFLs flicker when the X-10 lamp module it is controlled by is turned off. Because a lamp module needs to know if the lamp connected to it has been switched on/off which serves as a signal to the module to turn the lamp on.

  5. When they replaced the fixture the crows nest of wires likely ran to that second switch. Main power runs to that box, splits off to outlet and a loop to the switch (and back again) for the light. Another split off the main incoming goes to that second switch and off to some other light. Don’t realize that and reconnect them incorrectly and strange things happen. That’s likely how the outlet and light wound up in series.

    I’ve removed old ceiling lights and was shocked (pardon the pun) at what wiring mess was behind it. Sometimes 3 or 4 completely unrelated wires were run there because they needed a junction box and were too lazy to add another.

    1. “because they needed a junction box and were too lazy to add another.”

      Blind, i.e. unexposed, junction boxes are illegal in the U.S. If they needed a junction box, it would have to be accessable, that might mean leaving a blank faceplate in a wall, floor, or ceiling. Some customers would call that “ugly” so, the person doing the wiring (I hesitate to call them an “electrician”) used an existing junction box.

      Instead of using a blank faceplate, they could put in a switch plate with a switch that is not connected, leaving future occupants to wonder, “I wonder what this switch does?” B^)

    2. I think you’ve got it right on the money. The old electrical code allowed a separate switched leg to run from the light to the switch in the wall. Power could be routed directly to the light fixture and that works ok until someone changes things and gets confused between all of the wires in the box..

      The modern code no longer allows this. Power must be run to the switch box _before_ it is routed to the light fixture. One reason for this is so that things like timers and other accessories can use that power to perform their normal functions. I suspect it also avoids this particular problem.

      1. Dam close. Just reverse the incoming Hot and Neutral in the ceiling because the light switch switches Hot and uses a red and black. The red wire going to the light switch (previously fan switch) has been swapped with the incoming red hot.

        Also your colors should be Hot – Red (Brown), Neutral – Black (Blue) and not shown, Earth – Green (Green/Yellow).

        1. US colors are Hot-Black, Switch-Red, Neutral-White, Ground-Green. Although other colors (blue,red, black) for hot and switched are often interchanged and sometimes depend on phase. Although to be fair to [ehud42], white neutral wouldn’t really show up on the background

          Also, often (in the US), if the power hits the light before the switch, a switch feed will be dropped to the switch with the normally white neutral wire (in a Romex bundle) being used as the switch lead and often marked with a piece of colored tape on both ends. This can confuse non-electricians (ie homeowners) trying to rewire their houses.

    1. +1

      This seems funny, but the video is an indication of some serious problems in the house (not just the bath, which could potentially be even more lethal.)

      It’s funny until someone gets dead, or appliances start exploding.

    2. I live in an older house that had REALLY bad wiring. I was upgrading to 200A service, so the first prep step was to run new romax and add 3-prong outlets (took 2 years to rewire entire house). Eventually discovered two circuits that were tied together. I traced a run back to the old box, pulled the fuse and still got zapped. Found I had to pull 2 fuses to de-energize the circuit.

      They also tied two 60A boxes together to make “120A service”, but didn’t upgrade the run from the meter to the breaker box. When I removed the old meter base I noted how the wire insulation had melted. Not sure how I survived.

      1. He had one either on the dryer or in the wall. Towards the end mentions pushing the switch and it turning off the light. Just doesn’t call it a GFI, calls it a breaker.

        He needs to invest the $4 in a test light that plugs in and tells about open neutral, grounds and swapped hot/neut. Although it wouldn’t help with the fixture itself.

  6. I had a neighbor once, who was having electrical problems and asked me to come look. I came in his house, and then he turned off his clothes dryer. As soon as he did, ALL the power to the living room went out. Then he hit the start button on the drier, and it and all the power came back on. It was hilarious!

    It turned out that a wire in the wall had broken. Splicing that wire back together made everything normal again. I never found out exactly what that wire went to, as we rent.

  7. I think the problem comes from the ventilator, which usually continue to work, for some time, after you switch off the light.
    That means, he has an always hot wire feeding the lamp and he switch the neutral which is very dangerous.
    Take the fuse out, verify that there is no voltage on the wires, use an ohmmeter and draw the schema.

  8. the house we moved into, has 110V lights around the garden, one of the switches was worn out so i replaced, and wired it with the usual wiring. turned on the lights and the whole house buzzed, turns out the cheeky devils had run 3 wires to each lamp, but two effective circuits. and used the earth wire as the switched line. with local earths..

    wired up something unexpected, label it pls:) especially if it runs underground or under concrete.

    1. I once owned a house which had 14/2 (no ground) Romex running to the furnace, with its own dedicated 15A subpanel, connected to the properly-old 6-circuit fuse box.

      This, in itself, wasn’t that big of a deal: After all, the house had old wiring from the “what’s a ground” period of wiring, and the furnace and associated wire were fairly new. There was no ground rod to speak of, which isn’t at all unusual for the period.

      There also was a green wire, itself extended with an open mid-span solder joint, connecting to a cold copper water pipe with a clamp, which I presumed to be some half-ass attempt at grounding the furnace.

      And everything worked fine.

      …until I was in the process of gutting the plumbing, cut the water main…and sparks jumped between the pipes. Eek! The cold pipe was used as neutral for the furnace, using the crufty green wire.

      The circuit layout, it turns out, was actually like this:

      Hot at fuse box -> dedicated disconnect -> furnace -> bodged-together green wire -> water pipe -> earth (!!) -> ground wire at the pole by the street via earth (again: !!!!) some 100 feet away.

      I fixed it (obviously). The house was getting new panel and proper grounding anyway. But it was alarming, to say the least, and amazing that it worked (for years and years, from the looks of it).

      And then there’s the story about the $28 hotel in rural Georgia with most of the wall sconce lighting run with 28AWG zip-cord speaker wire tied to the rear of whatever outlet was nearest. (I guess I got my money’s worth.)

  9. Stayed at a holiday home once. Noticed the lights dimmed a bit when the microwave etc was turned on. Then we found the hottub pump didn’t work. Electrician came and sorted out the wiring to the pump, and the problem with the dimming lights went away. I insisted there was something else wrong and reluctantly he performed a loop test. Found the neutral was open where it connects at the street, so the ground circuit was carrying all the neutral current. The ground stake was corroded and all that power was going out through the pump casing, until the wiring had melted. With the high resistance of the corroded ground the voltage would drop when the current draw went up.

  10. At one company I had worked at the metal benches had consumer plastic housing power strips mounted to the benches by drilling holes through the housings using bolts! Nothing was wrong until I unplugged a power cord from the strip and touched the neutral blade to one of these mounting bolts. There was an arc and a loud bang from the back of the building. The company owner had wired up the power drops himself and reversed the line and neutral. On top of all that he had left some loose hardware inside the main panel which got knocked loose and shorted bus bars.

  11. I purchased a house with a detached shop. Previous owner ran power in a conduit under ground to the building. The shop had a 220 outlet and a few 110 shop lights. I didn’t give it much thought. Then one day I see an overhead single conductor, unprotected, running from the building via the trees into an exterior light fixture into the house. This raised the red flag of wtf and so I cut the power, cut said single overhead conductor to prevent eventual electrocution while trimming said trees. Thinking it might have been something left over from something else etc.

    Can anyone guess what happened when I turned on the shop lights the next time?

    Yep they ALL exploded at once. Turns out said previous owner decided to get creative and ran 2 conductors not 3 for a 220 service. Realizing that wouldn’t work and that he could not fit the needed natural in the undersized conduit he just plopped up a natural into the house via an overhead run. I cut the natural removing the 110 path from the circuit and allowed 220 through the shop wires, POP.

  12. Years ago, my Girlfriend and I visited her mother who lived in a tornado magnet, er, trailer house. Years before GF’s brother built and wired an entry shed on the side of the trailer. Her mom was so proud of the work he’d done. (As far as carpentry, it was a good job) The bathroom exhaust fan (inside the trailer) wasn’t working so I pulled out my toolbox as well as my volt meter to check it out. I was getting nowhere until I discovered that Neutral was HOT! I tried to explain to GF and her mother that the trailer was mis-wired. They just couldn’t believe the young man could’ve made a mistake in the wiring. (Sigh!)

  13. Wow… This seems mad. In the UK, we’ve got laws about wiring, and we only have one phase to residential properties, so you can’t do really stupid stuff like this. Not meaning to troll, but Americans often post stuff about how 110 is so much safer than 230, but seeing this, I think they’re wrong! One phase of 230 seems much safer.

  14. first, unwire everything and verify what’s wired in that room. find your source hot and neutral and work from there.
    second, it looks like a neutral is missing somewhere….so you’re getting 2 hots to the lamp when both switches are on.

    Perhaps they had 2 separate phases wired to the fan system. They bussed them together when they rewired the fan.

  15. Reminds me of the light on the extractor hood in my kitchen. If I fire up the hand blender on a certain few sockets while the hood is open I get blinking lights in the hood. No other apparent way of switching the light on. the whole kitchen is wired in an odd fashion, immersion heater, 8 odd plugs and I assume the extractor hood all on one loop. Only discovered it when my wife put on the kettle, immersion and washing machine at the same time and blew a fuse. will have to take a serious look at it soon. Especially given the interesting wiring methods employed in my garage.

    1. Also consider the wire guage is (too) small, such as 14 AWG instead of 12. Also consider running an additional wire from the breaker/fuse box for half of the outlets. Also consider the breaker may be getting old/weak and need replacing.

      1. I suspect I’ll put in a new consumer unit entirely. There’s only 4 fuses for the whole house, and they’re all fuse wire jobs. Some nice little breakers would make life a bit easier :-)

  16. Wiring issues can definitely cause a person to go bald/grey. WAY back in the day, I had a 286 computer that was blowing parallel-port cards. After it blew a second one, I left it in for a while till I could get a new one. Well, I turned on the light/vent fan in the bathroom and heard the dot-matrix printer in the other room reset (the board wasn’t TOTALLY fried but it definitely wouldn’t work the printer correctly.) Turned it off and back on and the printer resets again. Opened up the vent fan and unplugged the motor. Never had another issue with the parallel-port card on the computer once I got a 3rd new one.

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