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