Rewired lamp blew out circuit - why?


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Old 10-16-09, 06:11 AM
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Rewired lamp blew out circuit - why?

I rewired an old table lamp with new socket and a new cord that came with the plug already attached. When I turned it on it sparked at the plug and blew out a third of our lights. The electrician speculated I must have wired the ground and hot wires backwards to the polarized plug, but when I took it apart everything was correct. Any idea how this could have happened? I was careful not to let any wires touch each other. I don't have extensive experience in rewiring lamps, but thought I understood the basics.
Second question: I've rewired the lamp again but now am afraid to test it! Is there any kind of device you can buy that allows you to test a lamp without involving all the house lighting? Many thanks for any and all advice!
 
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Old 10-16-09, 06:57 AM
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I also suspect what the other electrician did. Although you may not have wired it wrong, there may have been a stray bit of wire that was causing a short.

To test your lamp, buy a electrical meter. A cheap $10 one would be fine as long at it has an Ohm function. (which most do) Without a bulb in the lamp, set it to Ohms and touch each lead to the plug. (one on each blade) You should not get any continuity. IE: the needle should not move or the digital display should not read any number.
 
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Old 10-16-09, 07:14 AM
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In general I agree a short from mis-wiring but if the electrician really said
I must have wired the ground and hot wires backwards to the polarized plug,
Then he too is a bit confused. A lamp cord usually has no ground and if he meant swapping neutral and hot connecting to the bulb socket that would cause a safety issue but not a breaker to trip.

Do you have a switch separate from the bulb socket on the lamp? If so did you connect the neutral and hot to the switch? Only the hot goes to the switch. The neutral continues on from plug to lamp socket unbroken. It does not attach to the switch. The neutral wire is usually the one with a slight raised rib but may be marked another way. It goes to the shell of the lamp holder.
 

Last edited by ray2047; 10-16-09 at 07:31 AM.
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Old 10-16-09, 08:27 AM
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Rewired lamp blew out circuit - why?

Ray, This was the simplest rewire - the two wires from the cord going directly to a socket that included the switch. The second cord/plug I purchased (to redo the lamp after the short) had instructions that confirmed what the electrician said...the wire with the ribbed coating should go to the silver screw and the other to the gold - just what I'd done. I was careful not to leave any stray wires. My husband suggested that either the socket or the cord/plug might have been defective. Possible?

Tolyn Ironhand, I may never figure out what went wrong here but the ability to test a lamp first (without involving our landlady and her electrician!) gives me back the confidence to fix lamps myself. Thank you so much - I will get an electrical meter with Ohms today!
 
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Old 10-16-09, 08:35 AM
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If you are going to fix many lamps you might also want to build a test socket with a 1a fast blow fuse for when you plug them in for the first time.
My husband suggested that either the socket or the cord/plug might have been defective. Possible?
Oh yes. In my years of handyman work bought a lot of defective items. With cheap imports, usually all you can get, we are the quality control testers not the manufacturer.
 
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Old 10-16-09, 08:48 AM
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Originally Posted by ray2047 View Post
If you are going to fix many lamps you might also want to build a test socket with a 1a fast blow fuse for when you plug them in for the first time. Oh yes. In my years of handyman work bought a lot of defective items. With cheap imports, usually all you can get, we are the quality control testers not the manufacturer.
Can you suggest a book or website that shows how to build a test socket with 1a fast blow fuse?
 
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Old 10-16-09, 12:21 PM
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Originally Posted by Calli View Post
Can you suggest a book or website that shows how to build a test socket with 1a fast blow fuse?
Here is a crude pictorial that gives you an idea of what I mean. A switch box and octagon box would be mounted on a board. The keyless lamp holder mounted to the octagon box and the receptacle and switch plate in the switch box. use plastic boxes and#16 lamp cord for hook up. This is for very brief testing only. The fuse is called "Edison base". First run the lamp cord to the lamp holder box. Cut only the hot conductor. Leave the neutral wire uncut. Connect the side of the hot wire from the receptacle to the silver screw of the lamp holder and. connect the side of the hot wire from the plug to the brass screw of the lamp holder. Neutral to silver screw of receptacle. Hot wire from lamp holder goes to brass screw of receptacle. Use with caution. Never change a fuse when plugged in.

Note: Picture does not show actual wiring. Just the path of the conductors.

[
 

Last edited by ray2047; 10-16-09 at 12:43 PM.
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Old 10-16-09, 12:44 PM
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Ray has a good idea although it might be a bit hard to find standard "Edison base" fuses in sizes under 15 amperes and if you can find them they would likely be a bit on the expensive side, especially if you were to blow them on any regular basis.

I would suggest instead a low-amperage circuit breaker. These are available in fairly small sizes on the surplus market at reasonable cost, less than $10. and they are resettable when they trip. You would have to figure out a different means of mounting the circuit breaker.
 
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Old 10-16-09, 12:57 PM
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Ray has a good idea although it might be a bit hard to find standard "Edison base" fuses in sizes under 15 amperes and if you can find them they would likely be a bit on the expensive side, especially if you were to blow them on any regular basis.
I too doubted it but I found them on line but yes expensive, around $9. A circuit breaker would be a better idea but maybe a bit more difficult to construct. Also a cartridge fuse and holder would work.
 
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Old 10-16-09, 04:30 PM
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Originally Posted by ray2047 View Post
Here is a crude pictorial that gives you an idea of what I mean. A switch box and octagon box would be mounted on a board. The keyless lamp holder mounted to the octagon box and the receptacle and switch plate in the switch box. use plastic boxes and#16 lamp cord for hook up. This is for very brief testing only. The fuse is called "Edison base". First run the lamp cord to the lamp holder box. Cut only the hot conductor. Leave the neutral wire uncut. Connect the side of the hot wire from the receptacle to the silver screw of the lamp holder and. connect the side of the hot wire from the plug to the brass screw of the lamp holder. Neutral to silver screw of receptacle. Hot wire from lamp holder goes to brass screw of receptacle. Use with caution. Never change a fuse when plugged in.

Note: Picture does not show actual wiring. Just the path of the conductors.

[
Ray - How kind of you to provide this diagram of the device! We will try and make one!
 
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Old 10-16-09, 06:25 PM
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For safety reasons just be sure the brass screw of the receptacle connects to the silver screw of the lamp holder. While you should never change the fuse with the tester plugged in if you forget this will mean the shell isn't hot. Also don't use a bulb larger then 60 watts in the lamp you are testing, That way you will be well within the limits of the 1 amp fuse.
 
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Old 10-16-09, 08:06 PM
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Not trying to down play Ray's idea but testing with a meter would not involve blowing any fuses or working with any energized circuits. Pick your own test but I would use a meter.
 
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Old 10-16-09, 08:28 PM
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Originally Posted by Tolyn Ironhand View Post
Not trying to down play Ray's idea but testing with a meter would not involve blowing any fuses or working with any energized circuits. Pick your own test but I would use a meter.
So would I. So let me amend this to say use this if you want but only after testing.

Sometimes my age shows. Grew up making my own test instruments. Use to test for voltage using two 120v bulbs in series.

Bottom line Tolyn's way is better and cheaper.
 

Last edited by Tolyn Ironhand; 10-16-09 at 08:30 PM. Reason: Fixed my name ;)
 

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