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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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Testing an Electric Blanket
Nelson wrote:
Not really electronics, I know, but I'm hoping some one can throw me a pointer. I'd like to test for both safety and shorts/opens/too much resistance. The only thing I could find on the web is safety testing by electric utilities. Any DIY help would be appreciated. Put a GFCI receptacle in where you plug this blanket in. -- Paul Hovnanian ------------------------------------------------------------------ There was a man who entered a local paper's pun contest. He sent in ten different puns, in the hope that at least one of them would win. Unfortunately, no pun in ten did. |
#2
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Testing an Electric Blanket
Paul Hovnanian P.E. wrote:
Nelson wrote: Not really electronics, I know, but I'm hoping some one can throw me a pointer. I'd like to test for both safety and shorts/opens/too much resistance. The only thing I could find on the web is safety testing by electric utilities. Any DIY help would be appreciated. Put a GFCI receptacle in where you plug this blanket in. I'm not sure a GFI would do any good. All electric blankets I have ever seen are two-wire devices, and there is no ground in or around the blanket. No ground = no ground fault = happy GFI. I did see a blanket fail once, out in the middle of the blanket (i.e., not near the connector). I THINK the failure mechanism was a heating wire that had been flexed enough that it was ready to break - but hadn't quite broken yet. The fracturing wire got thinner, which increased the resistance at that point, which overheated the wire at the incipient break, which caused a hot spot that was hot enough to scorch the blanket material (the material does not support flame, by the way). This was NOT a short circuit. The heating wires are well separated, and anchored in place quite well. Bill |
#3
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Testing an Electric Blanket
"Bill Jeffrey" wrote in message ... Paul Hovnanian P.E. wrote: Nelson wrote: Not really electronics, I know, but I'm hoping some one can throw me a pointer. I'd like to test for both safety and shorts/opens/too much resistance. The only thing I could find on the web is safety testing by electric utilities. Any DIY help would be appreciated. Put a GFCI receptacle in where you plug this blanket in. I'm not sure a GFI would do any good. Of course it would, provided the victim had a path to ground, ie through the bed itself.. All electric blankets I have ever seen are two-wire devices, and there is no ground in or around the blanket. No ground = no ground fault = happy GFI. I can't speak for the rest of the world, but AIUI here in the UK such safety devices work by detecting an inconsistency between live current and neutral current, which would indicate some current is flowing to earth, either through a human being or other fault condition. It works just as well for two wire as three wire devices, and in fact the device doesn't care or know one way or the other. Dave |
#4
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Testing an Electric Blanket
I can't speak for the rest of the world, but AIUI here in the UK such
safety devices work by detecting an inconsistency between live current and neutral current, which would indicate some current is flowing to earth, either through a human being or other fault condition. It works just as well for two wire as three wire devices, and in fact the device doesn't care or know one way or the other. Right, but still not much of a help with an electric blanket, since there is unlikely to be another path to ground. The hazards a (1) a fire, or (2) person in bed making electrical contact with the heating element at 2 points that are at substantially different voltages. |
#5
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Testing an Electric Blanket
"Bill Jeffrey" wrote in message ... Paul Hovnanian P.E. wrote: Nelson wrote: Not really electronics, I know, but I'm hoping some one can throw me a pointer. I'd like to test for both safety and shorts/opens/too much resistance. The only thing I could find on the web is safety testing by electric utilities. Any DIY help would be appreciated. Put a GFCI receptacle in where you plug this blanket in. I'm not sure a GFI would do any good. All electric blankets I have ever seen are two-wire devices, and there is no ground in or around the blanket. No ground = no ground fault = happy GFI. I did see a blanket fail once, out in the middle of the blanket (i.e., not near the connector). I THINK the failure mechanism was a heating wire that had been flexed enough that it was ready to break - but hadn't quite broken yet. The fracturing wire got thinner, which increased the resistance at that point, which overheated the wire at the incipient break, which caused a hot spot that was hot enough to scorch the blanket material (the material does not support flame, by the way). This was NOT a short circuit. The heating wires are well separated, and anchored in place quite well. Bill Ground fault current interrupters do not sense ground current directly - they sense any difference between the current in the line and return wires, if the line current exceeds the return current then it must be going somewhere it shouldn't!!! |
#6
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Testing an Electric Blanket
"mc" wrote in message ... I can't speak for the rest of the world, but AIUI here in the UK such safety devices work by detecting an inconsistency between live current and neutral current, which would indicate some current is flowing to earth, either through a human being or other fault condition. It works just as well for two wire as three wire devices, and in fact the device doesn't care or know one way or the other. Right, but still not much of a help with an electric blanket, since there is unlikely to be another path to ground. The hazards a (1) a fire, or (2) person in bed making electrical contact with the heating element at 2 points that are at substantially different voltages. If there's no other path to ground - getting electrocuted might be difficult! |
#7
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Testing an Electric Blanket
ian field wrote:
"Bill Jeffrey" wrote in message ... Paul Hovnanian P.E. wrote: Nelson wrote: Not really electronics, I know, but I'm hoping some one can throw me a pointer. I'd like to test for both safety and shorts/opens/too much resistance. The only thing I could find on the web is safety testing by electric utilities. Any DIY help would be appreciated. Put a GFCI receptacle in where you plug this blanket in. I'm not sure a GFI would do any good. All electric blankets I have ever seen are two-wire devices, and there is no ground in or around the blanket. No ground = no ground fault = happy GFI. I did see a blanket fail once, out in the middle of the blanket (i.e., not near the connector). I THINK the failure mechanism was a heating wire that had been flexed enough that it was ready to break - but hadn't quite broken yet. The fracturing wire got thinner, which increased the resistance at that point, which overheated the wire at the incipient break, which caused a hot spot that was hot enough to scorch the blanket material (the material does not support flame, by the way). This was NOT a short circuit. The heating wires are well separated, and anchored in place quite well. Bill Ground fault current interrupters do not sense ground current directly - they sense any difference between the current in the line and return wires, if the line current exceeds the return current then it must be going somewhere it shouldn't!!! Ian - You are right - thanks for the lecture. I phrased my objection badly, but I was responding to a previous post that suggested a GFI could protect against line-to-line shorts, or shorts across part of the heating element. I stand by my statement that if there is no ground anywhere, there cannot be a ground fault. So the question becomes, as another poster noted, where will you find a ground connection if you are snug in your bed, and if the power cord (a two-wire cord) doesn't bring a ground into the picture? I suppose that if you set up your bed in a bathtub, you might do it. But as I look back, I have never had a ground connection within casual reach of any bed I have ever slept in. OK, so now all of you lecturers can tell me about beds set up against steam radiators, copper water pipes run along the wall, radio antennas tacked to the wall, antique light switches, etc. I'm not sure that does the OP any good. On the other hand, if he uses and trusts a GFI, based on your comments, he could be in big trouble in case of a real short. Bill |
#8
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Testing an Electric Blanket
Bill Jeffrey wrote:
ian field wrote: "Bill Jeffrey" wrote in message ... Paul Hovnanian P.E. wrote: Nelson wrote: Not really electronics, I know, but I'm hoping some one can throw me a pointer. I'd like to test for both safety and shorts/opens/too much resistance. The only thing I could find on the web is safety testing by electric utilities. Any DIY help would be appreciated. Put a GFCI receptacle in where you plug this blanket in. I'm not sure a GFI would do any good. All electric blankets I have ever seen are two-wire devices, and there is no ground in or around the blanket. No ground = no ground fault = happy GFI. I did see a blanket fail once, out in the middle of the blanket (i.e., not near the connector). I THINK the failure mechanism was a heating wire that had been flexed enough that it was ready to break - but hadn't quite broken yet. The fracturing wire got thinner, which increased the resistance at that point, which overheated the wire at the incipient break, which caused a hot spot that was hot enough to scorch the blanket material (the material does not support flame, by the way). This was NOT a short circuit. The heating wires are well separated, and anchored in place quite well. Bill Ground fault current interrupters do not sense ground current directly - they sense any difference between the current in the line and return wires, if the line current exceeds the return current then it must be going somewhere it shouldn't!!! Ian - You are right - thanks for the lecture. I phrased my objection badly, but I was responding to a previous post that suggested a GFI could protect against line-to-line shorts, or shorts across part of the heating element. I stand by my statement that if there is no ground anywhere, there cannot be a ground fault. So the question becomes, as another poster noted, where will you find a ground connection if you are snug in your bed, and if the power cord (a two-wire cord) doesn't bring a ground into the picture? I suppose that if you set up your bed in a bathtub, you might do it. But as I look back, I have never had a ground connection within casual reach of any bed I have ever slept in. OK, so now all of you lecturers can tell me about beds set up against steam radiators, copper water pipes run along the wall, radio antennas tacked to the wall, antique light switches, etc. I'm not sure that does the OP any good. On the other hand, if he uses and trusts a GFI, based on your comments, he could be in big trouble in case of a real short. Bill In that case you need the new arc detect circuit breaker that kills power when something sparks. The US electrical code requires them for new construction because a lot of electrical fires start in bedrooms due to damaged cords and appliances. I am in the process of making a new control cable for my hospital bed. If it was the power cord that was damaged, I could have had a nasty fire. -- Service to my country? Been there, Done that, and I've got my DD214 to prove it. Member of DAV #85. Michael A. Terrell Central Florida |
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