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#187565 07/02/09 01:24 AM
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WESTUPLACE #187566 07/02/09 02:37 AM
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two words: "Selective co-ordination."

renosteinke #187570 07/02/09 04:31 AM
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From the comments:
Quote
OMG! Something similar happened to me in high school. When I was working in the nurse's office, I got bored and put a staple in a light switch. My thumb got burned and I was left with an impression of a staple on it. The lights went out in the office and the entire first floor of the school for about a half hour. No one ever figured out what caused the lights to go out.


It's spreading. turkey

Trumpy #187577 07/02/09 09:27 AM
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A testimonial to tamper resistant receptacles!

Yes, selective coordination would help.



John
HotLine1 #187578 07/02/09 10:43 AM
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No breakers like these can be properly coordinated to address a dead short, unless you spend the big bucks for one that has no instantaneous setting.
All of the standard molded case breakers have an instantaneous set at 10-12 times the rated current. That breaker will pop, even if it's the main.


Ghost307
ghost307 #187582 07/02/09 01:21 PM
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My physics teacher in high school loved experiments and loved to get the kids to play with them. One of which was a board with some light bulbs and terminals mounted on it, all bare exposed metal. To which he handed us a couple cut-off lamp cords with the wires split and stripped, so we could hold the wires by the insulation and touch the leads to the terminals.

I have no idea what he was thinking when he turned this stuff over to a bunch of 17 year old kids with no concept of the dangers involved, but I managed to create quite a shower of sparks while putting the classroom in the dark. It's an experiment none in my class will forget, at least.


SteveFehr #187605 07/03/09 02:21 PM
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If there's a main breaker rather than a main fuse I always thought the main is supposed to be slow enough never to trip in case of a short unless the branch circuit breaker fails (that's why type E main breakers in Germany are 15 times the cost of regular B or C branch circuit breakers).

Texas_Ranger #187655 07/06/09 01:55 PM
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That's the sort of thing that's covered in a coordination study. It's hard to coordinate a panelboard with non-adjustable breakers, though. A lot of the cost of main breakers is the interrupt current rating; other breakers in the panel can be series rated, but the main actually has to be able to interrupt the full fault current.

SteveFehr #187675 07/07/09 06:30 AM
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Originally Posted by SteveFehr
That's the sort of thing that's covered in a coordination study. It's hard to coordinate a panelboard with non-adjustable breakers, though.

A co-ordination study is part of the design of ANY electrical installation.
Sure, you could use the main breaker, to protect the whole installation, from the last light back to the main, but the cable sizes are not the same.

Most electricians I know, rate the protection to the size of the cable, this gets smaller as the circuits head away from the main breaker and at sub-boards.
After all, you are protecting the cables on "that part of the circuit, forward".
It is a cascading thing.

I disagree that it is hard to effect co-ordination with Non-adjustable CB's, most of what we have here are just that and they are set to protect given sizes of cable, not the loads likely to be carried by them, especially where that could affect the integrity of them cables.

Trumpy #187748 07/08/09 02:16 PM
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It's not just overload, but also fault clearning. If you have a 100A panel that's heavily loaded, and suffer a high-current fault on a 40A or 50A branch circuit breaker that sees only itermittant load, it's going to trip the main first almost every time. The proper solution would be to adjust the trip curves so that a branch circuit will always clear the fault prior to the main opening, but if the main isn't adjustible... there's nothing you can do, you're stuck with whatever it's set at. If it's set at the same delay as the branch circuits, you're SOL for any fault that exceeds that threshold.

With fuses, you could at least select a slower fuse, but proper coordination is simply not possible with small panels like it is with larger adjustible trip switchboards. It's actually quite a big problem in certain segments that require coordination of these small panels.


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