Member - Davoe (WA) replied:

circuit breakers or relays? cb s should have a reset switch, and if they keep tripping out after reseting there is a short. relays have a magnet that conects a power source to its outlet via a secondary power source ( eg spotlights, power from high beam activates magnet which conects power to spotties) relays have a habit of crapping out and you just replace them. Probably havnt helped you but I didnt really understand the Q

MrBitchi replied:

Austravel, posed that question to Narva yesterday and just got this reply:-

"Hi John, the time will depend on the load applied. If the load remains the circuit breaker will not reset in order protect the application. Usually it resets once it cools down which is a matter of mere seconds.

Regards,
Brendan Hughes,
Marketing Services Manager."

Cheers, John.

Member - Sparkie (QLD) replied:

Austravel
Yeh you are right. They are made from a bi-metallic strip which will "pop" open at a set temperature. When they cool enough to reset" usually fairly quickly" it will close again. If you had a short in the system it will do nothing for the protection of the wire untill either the short blows open or the breaker fries itself.

You are better of with a manual reset breaker as it will "pop" open and than you know you need to look at the problem. It could be a pain if it was a fridge but I would rather replace the food than the fridge.

Sparkie(IE not Y) ;-)

Austravel replied:

Thanks all for the replies.

Member - Ross P (NSW) replied:

I have a resettable circuit breaker mounted on the fire wall. During a recent trip in which I experienced overheating problems I noticed that the CB had tripped. I couldn't manually reset it. After checking the wiring and appliances connected to the circuit I determined the CB was at fault. I have fuses in the line as well so as "work around" I by-passed the CB. In the morning I could rest the switch and it has worked fine ever since.
Moral of the story is keep the CBs as far as possible away from heat sources.

drivesafe replied:

Hi Austravel, a little warning about using auto reset Circuit Breakers.
If you decide to use them, you would be well advised to use cable that is at least twice the current rating of the CB.
The problem with auto reset CBs is that if a short circuit occurs, current passing through the CB not only causes the circuit breaker to heat up but the cable also heats up. The circuit breaker goes open circuit and cools down. At this point, the cable also cools down but because the copper in the cable is insulated by plastic which is there to insulate the copper from the outside world, but in this case the insulation works in reverse and keeps the heat in.
As soon as the circuit breaker cools down enough it goes close circuit and both the circuit breaker and the cable heat up again only the cable is still hot from the last short and just gets hotter. Eventually the cable’s heat gets to a temperature where the cable’s insulation ignites.
Using thicker cable has two advantages, because the cable is thicker it takes longer to heat up and the thicker cable allows a higher current to get to the circuit breaker so it trips quicker, this again reduces the heat up time on the cable.
Cheers