Fuses and Circuit Breakers - What short surge will they accept before blowing /
Submitted: Wednesday, Nov 26, 2008 at 19:24
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Member - Norm C (QLD)
I know that most fuses and circuit breakers will tolerate a short current surge
well above their rated level. But I don't know how much above and for how long.
Reason is, I want to put a circuit breaker or fuse in line with the starter cable on by boat. To balance the boat, I have put the battery on it's side under the front thwart (bench
seat). The cables are about 4 meters long and they run under the plywood floor. With this length and with the cables being out of sight, I want to fuse at the battery.
I've tried to measure the current drawn with a clamp meter. When starting, it never measures above 50 Amps, but I'm not sure that the meter is able to measure any higher short surge that might occur as I first press the start button.
I'm looking at using either a 60 Amp or 100 Amp manual reset circuit breaker (anything bigger is too big, too expensive and too difficult to mount). The 60 Amp one I have found on the ABR website is the most convenient in design. Just for argument purposes, lets assume there is a very short surge of 200 Amps when I first press the button. Will the circuit breakers I have mentioned trip?
The motor is new and currently I only need to press the start button for about a second or two for it to start.
Reply By: Member - Redbakk (WA) - Wednesday, Nov 26, 2008 at 19:50
Wednesday, Nov 26, 2008 at 19:50
Norm...what you are trying to do is probably one of those suck and see situations. Most cranking connections are solid and are not protected by fusing of any sort...while accessory and auxcillary device connections are.
As a general rule at 125% most fuse/circuit breakers will hold for some time (up to an hour) however as current climbs the time gets shorter so that at say 10x the rated current capacity of the fuse/CB the trip time is very quick....6 amp x 10 =60 amps.
Fuses blow due to heat...they melt but circuit breakers trip because they have a bi metal strip or disc that operates mechanically...some CB's even use an electro-magnet to achieve high current trips.
AnswerID:
336998
Follow Up By: Member - Redbakk (WA) - Wednesday, Nov 26, 2008 at 19:52
Wednesday, Nov 26, 2008 at 19:52
I want to add that wire size and distance also effects fuse/CB trip times as
well.
FollowupID:
604710
Reply By: Affordable Batteries & Radiators - Wednesday, Nov 26, 2008 at 20:23
Wednesday, Nov 26, 2008 at 20:23
Hi Norm
This may help.
Regards
Derek.
AnswerID:
337004
Follow Up By: Member - Norm C (QLD) - Wednesday, Nov 26, 2008 at 20:35
Wednesday, Nov 26, 2008 at 20:35
Thanks Derek,
So your 60 Amp breaker would handle 300 Amps for about a second and 120 Amps for about 7 to 10 seconds? That sounds like it should do the job for me.
I'll contact you separately and order one. Worst case, if it can not handle the job, I can use it as a spare for the fuse I currently have in line for my electric motor.
By the way, it is the IEB 100 AH AGM I got from you that I now have under the front thwart in the boat. Still going strong after about a year of use.
Norm C
FollowupID:
604714
Follow Up By: Member - Redbakk (WA) - Wednesday, Nov 26, 2008 at 20:46
Wednesday, Nov 26, 2008 at 20:46
Thanks for putting up the graph Derek...went looking for one but yours is best.
FollowupID:
604717
Follow Up By: Member No 1- Thursday, Nov 27, 2008 at 07:43
Thursday, Nov 27, 2008 at 07:43
Derek
howabout spitting that graph up to members downloads stuff as a PDF file?
FollowupID:
604749
Follow Up By: Affordable Batteries & Radiators - Thursday, Nov 27, 2008 at 08:28
Thursday, Nov 27, 2008 at 08:28
Done No 1
;-)
Members FileSwap -
Power Wiring and Electrics -
Breaker Trip Times
Regards
Derek
FollowupID:
604753
Reply By: Member - Serg (VIC) - Thursday, Nov 27, 2008 at 09:47
Thursday, Nov 27, 2008 at 09:47
Base on my personal experience I would steer away from any sort of high-current circuit breakers – way too unreliable and too much voltage drop. Fuses of 60A not that expensive as
well as their holders. If you feel that 60A not good enough, just put 2 in parallel. Also there are “fast” fuses and “slow” fuses, but I do not think that they do this sort of thing for high current like 60 A for automotive application.
Cheers
Serg
AnswerID:
337051
Reply By: Member - Phil G (SA) - Thursday, Nov 27, 2008 at 15:58
Thursday, Nov 27, 2008 at 15:58
Gday Norm,
My preference is for fusible links - same as what is hanging off your 79series starting battery.
I little bit of info here.
Fuses in holders usually corrode when mounted next to batteries and have to admit to a bad run with using a circuit breaker in high temperatures.
I get fusible link wire from Motor Traders in
Adelaide, but most auto electricians carry it. I expect you'd need 12Ga or 10Ga. Your other option is to use a replacement 79series fusible link from Toyota and then you're also carrying a spare!
AnswerID:
337098