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I bought a small ceiling spotlight fixture that I meant to convert into an outlet-powered desk lamp. It has three wires (brown, blue, yellow-green) coming from it, e.g. it has a grounding wire because the fixture is of a metallic material. I'm in Finland, the lamp is from Germany.

Because I plan to use it for a smart LED bulb that is commanded by software (electricity to device always on), I thought that I probably wouldn't need to get an on-off switch to the wire; Also, I found it very difficult to find desk lamp switches with three instead of two wire terminals. So I bought a regular mains cable that has exactly the same color coded three wires in it. I connected them to the lamp's wires by fastening them to the kind of screw terminal that you usually use for ceiling fixture installations. A very simple installation, and technically not one that I hadn't done many times before. But because I had tampered with the wire myself and I was about to switch this fixture on for the first time ever, I hesitated to do so by having to be so near; So instead of plugging the device directly to the mains outlet, between them I put a remote controlled plug adapter so I could switch it on from a remote control. I use them all the time, to switch on/off some of the hard to reach devices around the house. But when I switched it on, this new lamp blew the fuse in that part of the house! At the moment when that happened, the fixture base lied on heavy wooden material, no contact with metals. The bulb that was connected to the fixture has not been damaged, it lights up when tested with another fixture. It's a GU10.

I re-checked my connections and detached the fixture's socket to investigate. I found no visible flaws or imperfections. I got a multimeter but the instructions are of absolutely no help to someone who doesn't already know everything. But I think that I have now ruled out that the main circuit doesn't short: By setting the meter dial to Ω2000➜├ and touching each prong with the measuring pins, the meter reads "1". I have not measured ground.

As a complete amateur I can only come to think of a few things:

  1. Should I have just skipped the remote controlled plug adapter and plugged the device directly to the outlet? On the other hand, I don't understand how the adapter could've been the problem: I use them all the time, also on devices with a grounded wire, and the adapter has metallic contacts for receiving the ground signal from the plug.
  2. Or is it the actual mains outlet's fault? I live in an old house with outlets so old that they don't have the ground contacts, outlets with ground contacts are only found in the kitchen and the bathroom, likely a more recent addition. But then again, I'm using devices with grounded plugs here all the time and I've heard that it shouldn't be a problem.
  3. Or did I just happen to have too many devices on at the same time and that was the last straw? It does seem a bit odd though, after all, we're talking about switching on a single LED bulb – hardly a heavy strain.

I haven't attempted to plug the device in since. Did I create something dangerous, is some part of the system malfunctioning, or was this a one-off to ignore and simply retry? What should I test or attempt to do next?

  • I don't see anything which jumps out from your description. Maybe if you add a couple of photos of your device & wiring ... ? – brhans May 17 '17 at 14:53
  • If I understand your measurement correctly you have 1 ohm across the fixture or a short. – Ed Beal May 17 '17 at 15:36
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You don't need a 3-wire switch. You only need 2 wires, the switch interrupts the "hot" wire, the other two wires run through. Bonus points if you get a 2-pole switch which also interrupts neutral. That solves the problem if someone plugs the plug in upside-down, many European plug types make it possible to reverse polarity to the lamp by accident. This is why we New Worlders revised NEMA 1 and 5 to add polarizatin.

Get comfortable with your ohmmeter

Ohms are a simple thing: Ohms = Volts / Amps. If you have a 230V heater that draws 2.3 amps, it will be 230/2.3 = 100 ohms.

Holding the probes apart should be infinity ohms. Holdong them together should be 0 ohms (it will read a little higher).

Use your ohmmeter on other stuff that you expect to work, don't try to learn it on something you are troubleshooting.

Is your main "fuse" an RCD?

One big thing over there is having the main house overcurrent protection (OCD or ”breaker") be also a ground-fault or residual current detector. Ideally this device should have an indicator that tells whether it tripped for overcurrent, or residual current.

A residual-current aka grounda-fault trip is rather unlikely if there are only 2 wires actually reaching the device and it's not connected to any viable grounding path. By definition a ground fault requires a third current path.

There is also a new thing called arc-fault protection. This puts a tiny computer in the circuit breaker which listens to the power line for the telltale sound of arcing (you know the sound if you've ever fidgeted with a headphone jack or speaker hookup). It is especially valuable in older homes (or newer, cheaply made homes) where wiring might fail by arcing and start a fire. A sloppy wiring job could result in this trip.

Divide and conquer

What you do is disassemble it , and add one thing at a time until the overcurrent device trips again. Start with nothing plugged in. Then plug in the remote switch, but no lamp in it. Then disassemble the lamp until it's just the line cord (no connection block). Then add the connection block (nothing on the lamp side). Then add the lamp wiring. Then assemble the lamp. Then add the bulb.

Very likely at some step in there, blammo, and there's the problem. My bet is the junction block isn't wired internally like you think it is.

  • (Comment character limit is very limited, sorry this will be in parts.) I can’t really make much deductions about the main fuse; All I can say is that the board looks old, the entire apartment’s main switch is the only I/O switch and the fuses are in their porcelain holders. Nothing digital or of the current millennium. All of the outlet fuses have a label saying they’re grounded, but the actual old outlet covers do not reveal ground pin contacts that today would be a standard for all. Except for kitchen+bathroom. – user158589 May 19 '17 at 10:28
  • I just took apart a switch that I found from a lamp that I’m not using, but there absolutely is no room for accommodating all three wires so I’ll have to go shopping. There would be just enough room for the blue and ground wires to pass through it side by side, but the problem is that there are no terminals, so I’d have to join the plug and the lamp wires manually, and for that, there is no room inside this particular switch. – user158589 May 19 '17 at 10:31
  • That’s the reason why I originally looked for a 3-wire switch; If I was merely adding a switch to a continuous wire that doesn’t have one, 2-wire switch would be fine because the N and G wires are already insulated from one another thus can run side by side. In my case, is the brown wire the ‘hot’ wire and the blue one the neutral wire whose purpose is to provide a return path for the current? – user158589 May 19 '17 at 10:33
  • Actually looking up eBay I effortlessly came across a switch that has terminals for three wires, definitely looks like something that I could do. But the available switch components have different amperage; Now that the device is merely a single LED bulb, is the amperage rating of the switch crucial? The information I get from my device is volts and watts, so I’m not sure what it implies for the switch amperage. – user158589 May 19 '17 at 10:33
  • And also of course thank you very much for the tips; I will look up some simple tutorials on the meter. Was probably iFixit that had a surprisingly easy to follow, illustrated, step by step guide. I prefer those over long and heavy articles when trying to grasp core concepts for the first time; Reading heavy articles gets easier after that. I think I should get a switch anyway and then start over and 'conquer'. – user158589 May 19 '17 at 10:37

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