Why would spotlights blow fuses?

Submitted: Sunday, Apr 30, 2006 at 17:02
ThreadID: 33380 Views:5788 Replies:10 FollowUps:6
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G'Day All,

I'm having trouble with my spotlights blowing the fuse.

They are Hella and as far as I know 100w. Initially I did watts = volts x amps, and divided 100/12 and got 8.3amps so fitted a 10amp fuse. Which lasted a few days then blew.

I then realised my error, that as its 2 lamps should be 200/12 = 16.6 so fitted a 20amp, which again lasted a short while then blew.

I know its bad practice to just up the fuse, but thought would try 25amp considering a direct short would still blow it, just to see.

True to fashion, it blew aswell.

Now the fuse is blowing but you can sit with lights on full and spotlights and they wont blow. Its only after driving a few days that it seems to blow.

They are wired correctly with a relay and are independently earthed and it is a good tidy job with heat shrink used where needed.

I dont want to up the fuse any higher till I find out the cause of blowing.

Any pointers greatly appreciated.

Ron
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Reply By: Member - Mike DID - Sunday, Apr 30, 2006 at 17:05

Sunday, Apr 30, 2006 at 17:05
Feel the Fuseholders after the lights have been running for a while. If the fuses are making poor contact, they will overheat and eventually blow the fuse.

Mike
AnswerID: 169850

Follow Up By: Mad Dog (Australia) - Sunday, Apr 30, 2006 at 17:11

Sunday, Apr 30, 2006 at 17:11
Didn't work like that for me. I had smoke being released by the compressor feed wire because of poor contact in the holder, fuse didn't blow, just got bloody hot...ouch
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FollowupID: 425198

Follow Up By: Ron173 - Sunday, Apr 30, 2006 at 17:12

Sunday, Apr 30, 2006 at 17:12
Thanks Mike,

It is a blade fuse holder, which is soldered in line, good joints, and heat shrunk, the fuses feel quite tight to remove and replace.

Ron
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FollowupID: 425199

Follow Up By: hl - Monday, May 01, 2006 at 07:19

Monday, May 01, 2006 at 07:19
Hi,

The current surge when you turn on the lights is very much higher than the current they draw when they're on. It is probably best to have a seperate 20 or 25 amp fuse for each light.

Cheers
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FollowupID: 425332

Reply By: Marc - Sunday, Apr 30, 2006 at 17:18

Sunday, Apr 30, 2006 at 17:18
if you have a meter check for a short. Just maybe the wire is rubbing and shorting out.
AnswerID: 169854

Reply By: Member - Mike DID - Sunday, Apr 30, 2006 at 17:19

Sunday, Apr 30, 2006 at 17:19
Sounds like you may have flaky insulation somewhere and the wire is contacting earth from vehicle vibration.

Mike
AnswerID: 169855

Follow Up By: Max - Sydney - Sunday, Apr 30, 2006 at 18:02

Sunday, Apr 30, 2006 at 18:02
Check that the globes are indeed 100 W. That should be somewhere on the bulb (which you should handle with a clean cloth). And check that there is nothing doidgy in the insulation inside the lamps.

:-)
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FollowupID: 425214

Reply By: Markymark - Sunday, Apr 30, 2006 at 19:21

Sunday, Apr 30, 2006 at 19:21
I had this trouble once and eventually found it was shortening out inside the (metal) cover of the spotlight. Made sure it was all insulated and had no worries after that.

Cheers,

Mark.
AnswerID: 169883

Reply By: HJ60-2H - Sunday, Apr 30, 2006 at 19:41

Sunday, Apr 30, 2006 at 19:41
Stick an ammeter on the supply wire an dmeasure how much is being drawn. Give you an idea of where to start.
AnswerID: 169886

Follow Up By: Old Gold - Sunday, Apr 30, 2006 at 20:12

Sunday, Apr 30, 2006 at 20:12
G'Day Ron You could also try running seperate relays. some relays aren't good enough to run two lights
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FollowupID: 425256

Reply By: Jarrod - Sunday, Apr 30, 2006 at 21:41

Sunday, Apr 30, 2006 at 21:41
Is the fuse blown "too smithereens" i.e. vapourised in centre part that you look at or just cracked? if blown to bits, suspect intermittant short.
Also, is fuse "warmish" to touch while spotties on? - if so, poor contact/connection in the fuse holder, increasing resistance. a vicous circle, aka thermal runaway - as temperature increases, so does resistance, this causes further temp. increase, more resistance, hotter, causes more resitance, hotter, bang. blown fuse after a few hours operation.

a fuse should be same temp as all other fuses while things are operating - normal, - not even slightly warm, otherwise bang, thermal runaway.
AnswerID: 169923

Reply By: Gu_Patrol - Sunday, Apr 30, 2006 at 22:37

Sunday, Apr 30, 2006 at 22:37
Mine did the same with 25 amp fuse , I ran 2 seperate relays and only had 10 fuse in each one also I didn't have any power loss because of the 2 single relay's
AnswerID: 169934

Reply By: Member - Jerry C (WA) - Sunday, Apr 30, 2006 at 22:46

Sunday, Apr 30, 2006 at 22:46
Hi Ron,
As a suggestion, I have just looked at the 80 handbook circuits and they have one fuse per side for the head lights and in some models two fuses per side and they are all 15 amp links. So try one 15 amp fuse per spotlight and yes that is basically 100% overload but that is OK, this will also help in fault finding.

Good luck, Jerry
AnswerID: 169936

Reply By: Member - Bradley- Tuesday, May 02, 2006 at 14:50

Tuesday, May 02, 2006 at 14:50
any high resistance in the circuit will cause fuses to go, usually most people use wire that is way too small for the job, and earth them to the bullbar etc.

re-wire them with at least a 10 amp wire for each light ( 15 amp preferred) and use good quality fittings, and return the earth directly to the battery. This will give you a circuit with very little voltage drop and your lights will be nice and bright to boot.

Remember the fuse is there to protect the wiring in case of a short circuit, not the lights, they will only use as much current as they need. the wiring should always be rated higher than the fuse fitted.
AnswerID: 170225

Follow Up By: hl - Thursday, May 04, 2006 at 09:01

Thursday, May 04, 2006 at 09:01
Hmmmm
"Any high resistance will cause fuse to blow"
Are you into qantum physics or did you just make that up?
Cheers
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FollowupID: 426018

Reply By: Sam from Weipa Auto Electrics - Thursday, May 04, 2006 at 09:12

Thursday, May 04, 2006 at 09:12
If there rally 4000's you may have them wired backwards brown is the earth also 30 amp is the size fuse I would use for a spotlight instalation hope this helps
AnswerID: 170596

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