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So what have I done wrong (this time)


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It has been a day of doing the little jobs on the 110 today, fitted the new bull bar and the new HID spots fixed some leaks and dried out the carpets and generally checked the fluids etc, I noticed that the 4 spot lights on my roll hoop were not working, upon checking the fuse had not just gone but melted the fuse holder, I replaced the fuse holder with a 40amp waterproof holder and 20amp fuse.

I went out tonight to check the alignment of the new HID spots and thought I would just check the roof spots, no joy quick check with the multimeter pointed to the relay, relay replaced with a new 35amp, lights working again however I noticed the fuse holder was getting very hot but had not blown.

The lights have been installed for around 4 years now and I have never had a problem with them before so don't think there is an issue with the original install.

Now I left school 20 years ago but can just about remember physics lessons and dividing Watts by Volts should give me the Amps therefore 200w (4x50) /12v 16.66 amps hence the 20amp fuse/35amp relay.

So questions:

  • Have I got my maths wrong?
  • If my maths is correct what is pulling so much juice?
  • Why is the wire and fuse holder getting so hot (not able to touch)?
  • Why is the fuse not blowing?

Cheers, Jason.

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The fuse holder can ONLY get hot if it is introducing a resistance. In theory if it was zero Ohms resistance across the fuse it would not introduce any resistance-current-watts heat) so i would look at re-doing the fuse connections as a starting point. Over time the current through the fuse can introduce increased resistance due to oxidation of the connections and fuse blade.

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Are they deffo 50W bulbs? not 80w bulbs? which would give more amps?. Just thinking out loud. You can't really measure 16A on a standard DVM, you could take all but 1 bulb out and try on the 10A range of a DVM. Should give around 4A.

Remember the fuse is only there to protect the wiring, so if you have 40A cable 30-35Afuse

Other wise I'd go for

low volts to the relay, less volts = more current. Check feed and earths

Cheapo relay and or fuse holder. could only be rated peak/intermittent amps.

Have you got a pic or part no of the fuse holder, It should have some meaty cable to avoid heating issues

something like te 30A version of this as a minimum,

http://www.vehicle-wiring-products.eu/VWP-onlinestore/fuses/photo/fh430.jpg

loose short to earth after the fuse in cage screen area?

Fuse holder should be blade or maxi fuse for 16A

Relay 40A I'd go branded, VWP Lucas etc. not halfords.

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Jason, your maths is correct and your relay and fuse should be fine. What size cable are you running to the lights? Are you using proper terminals or those horrid pre insulated things?

Hi Paul,

Thanks for the conformation of the maths!! The feed for the spot lights comes off the back of the alternator via a 40amp cable it then goes to an 40amp waterproof in-line fuse holder soldered to the feed wire and then covered with heat shrink the out wire is soldered in the same way then soldered to a waterproof relay holder and all heat shrunk sealed. The relay is switched from an LR aux light switch in my >03 dash with the live feed from that taken from the original loom feed.

The live feed from the relay comes out on the same 40amp wire I then connected this (again soldered and heat sealed) to two 35amp wires each feeding two lights split and soldered just before the first light in each to make a top loom connected in the back of the lights via spade connectors. The negative feed is two 35amp wires spade connected rings through an earth bonded bolt, run up the side of the hoop with the live feeds in a long length of heat shrink they are then again split 1 feeding the first two the second feeding the second two again they are soldered where split and covered in heat shrink, again connected directly to the bulbs in the back of the lights via spade connectors.

Jason.

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The fuse holder can ONLY get hot if it is introducing a resistance. In theory if it was zero Ohms resistance across the fuse it would not introduce any resistance-current-watts heat) so i would look at re-doing the fuse connections as a starting point. Over time the current through the fuse can introduce increased resistance due to oxidation of the connections and fuse blade.

Thanks Steve, the new fuse holder is brand new it is a Halfords 40amp jobie before everyone starts going on about Halfords standards etc the previous fuse holder was installed for four years before it melted so I think there is something else going on and not just the fuse holder, clearly something is causing the resistance, could the lights have started to break down corrode and be causing excessive load on the circuits? If this is the case why has the fuse not blown?

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Are they deffo 50W bulbs? not 80w bulbs? which would give more amps?. Just thinking out loud. You can't really measure 16A on a standard DVM, you could take all but 1 bulb out and try on the 10A range of a DVM. Should give around 4A.

Remember the fuse is only there to protect the wiring, so if you have 40A cable 30-35Afuse

Other wise I'd go for

low volts to the relay, less volts = more current. Check feed and earths

Cheapo relay and or fuse holder. could only be rated peak/intermittent amps.

Have you got a pic or part no of the fuse holder, It should have some meaty cable to avoid heating issues

something like te 30A version of this as a minimum,

http://www.vehicle-w...photo/fh430.jpg

loose short to earth after the fuse in cage screen area?

Fuse holder should be blade or maxi fuse for 16A

Relay 40A I'd go branded, VWP Lucas etc. not halfords.

Yes they are 50w bulbs The Halfords fuse holder is installed temporarily although correctly and will be replaced when I get a new one, the new relay is a Lucas one, when I checked the feed I was getting just over 12.6v without the engine on I checked the earth bolt which is where I got the negative feed for the reading from and it is showing very little resistance between the relay and the bolt.

Jason.

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The Halfords fuse holder should be fine, I've used a similar one for glow plugs before now, but if it's getting hot( you've melted one already) then there is a resistance which means it's not gripping the fuse blade securely.

What brand of fuses have you used?

Personally for that kind of current, long term, I'd use 2 relays and 2 fuses, that way you limit the heat at the fuse and it doesn't go dark when the fuse blows.

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The Halfords fuse holder should be fine, I've used a similar one for glow plugs before now, but if it's getting hot( you've melted one already) then there is a resistance which means it's not gripping the fuse blade securely.

What brand of fuses have you used?

Personally for that kind of current, long term, I'd use 2 relays and 2 fuses, that way you limit the heat at the fuse and it doesn't go dark when the fuse blows.

The previous fuse holder came with the waterproof relay kit so is different from the replacement, the fuse blade on the new one is tight I checked when installing it as careful to make sure fuse was seated, the fuse I am using came from a batch of Lucas fuses which I brought for my trailer, the others have been fine so I don't suspect them as the previous fuse again had been in there from installation.

I did think about using two relays hence the two feeds in the loom, but given my calculations I didn't think I needed to and just split the feed from the single relay, as I say I have used the lights for nearly 4 years now without a problem so am struggling to understand why it is causing problems now as nothing has changed, as above the only extra resistance I could see getting in the circuits would be in the lamps if they are starting to break down which I will check in the morning however if this was the case my understanding is the lamps would be duller they would not draw more current, or am I wrong?

Jason.

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'Melting' of the fuse holder is not necessarily an instantaneous event. It could have been running hot for a while but you only just realised it because the damage to the fuse holder was building up over time and you have not noticed it before.

Which is not to say that overheating fuse holders are a good thing, but it may explain why this has 'suddenly' happened (i.e, it hasn't).

Any small resistance (lower than you will be able to measure with the ohms range on a multimeter) will cause heating at the current your lights are drawing. Instead of trying to measure resistance, put your multimeter onto 'volts' and look for the voltage drop across the components. Most of the voltage drop should be across the bulbs, but you will find small drops across the relay, wires, fuse, fuse holder etc. More volts equals more resistance, and if the drop is (relatively) high across a short distance there will be enough power to cause significant heating.

If the connections where the fuse holder are joined to the wiring are close to the fuse holder, I would check these carefully. The heat generated in the joints could be conducted up the fuse holder leads and will add to any heat generated in the fuse holder and fuse.

Nick.

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'Melting' of the fuse holder is not necessarily an instantaneous event. It could have been running hot for a while but you only just realised it because the damage to the fuse holder was building up over time and you have not noticed it before.

Which is not to say that overheating fuse holders are a good thing, but it may explain why this has 'suddenly' happened (i.e, it hasn't).

Any small resistance (lower than you will be able to measure with the ohms range on a multimeter) will cause heating at the current your lights are drawing. Instead of trying to measure resistance, put your multimeter onto 'volts' and look for the voltage drop across the components. Most of the voltage drop should be across the bulbs, but you will find small drops across the relay, wires, fuse, fuse holder etc. More volts equals more resistance, and if the drop is (relatively) high across a short distance there will be enough power to cause significant heating.

If the connections where the fuse holder are joined to the wiring are close to the fuse holder, I would check these carefully. The heat generated in the joints could be conducted up the fuse holder leads and will add to any heat generated in the fuse holder and fuse.

Nick.

Thanks Nick,

That was going to be my course of action today, checking all of the joints etc, however as most of them are heat shrink sealed then it will be a PITA hence the post, was hoping someone would say its because of this!!

Jason.

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Are your new HID spots sharing any wiring with these faulty spots?

No the HID spots are on their own feed and switched via the high beam and an isolator switch on the dash, after sleeping on it I wondered if as above the fuse had been melting for a while and I just didn't notice it, as the feed comes from the back of the alternator and the relay is in the engine bay could this wire have been breaking down with the heat? causing extra resistance around the fuse?

But if it was that why did the relay blow and the fuse didn't? could it just have been weakened by the first melting fuse?

Jason.

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That Halfords fuse holder, whilst branded halfords looks identical to a Durite one I bought recently, they are a good bit of kit :)

Your original fuse holder, IMO, is most likely to have melted from corrosion or poor fuse quality causing a high resistance(as mentioned above by RRB and Nick), which over time has been enough to generate the heat needed to reshape the holder. Remember P= I2R so as the resistance of the connection increases, the amount of power (heat) disappated increases. >20A is already quite a bit, so connections are very important. Also, as the heat increases, the speed that the terminal/connection corrodes/breaks down witll speedup, so the failure is an exponential event, if that makes sense :)

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Guys,

First of all thanks for all of your replies, they really did help focus my mind in the right direction, and the lights are now working without any problems or overheating...

R.C.A.

The original fault appears to have been caused by water ingress into the original relay (photo attached) which was only apparent when I started the RCA process and opened up the relay housing to investigate why it had failed.

This appears to have put to much load onto the original cabling causing the fuse housing to melt and blow, when I changed the fuse holder I just de-soldered the original one and added the new one, this then lead to the faulty relay giving up. As you are aware I then changed this relay but the new fuse started to get hot again.

As recommended above I measured the voltage drop rather than the resistance across the various parts of the circuits and found that the wire (including fuse holder) to the input of the relay was far more than it should have been (about 1/2 a volt) I therefore removed this wire completely and replaced the entire length soldering in the replacement fuse holder, this has now been refitted and all appears to be working correctly and as it should be.

I can only deduce that the increased load from the faulty relay melted the fuse holder but also caused the original wire to also break down and increase the resistance causing the replacement fuse to get hot.

I guess the moral of the story is to not just replace the obviously faulty bits but check the other bits including the wiring as this could have also been damaged even if there is no visible damage.

Jason.

post-15513-0-35588900-1299676895_thumb.jpg

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Good to see you've resolved it.

Forgive my ignorance, Jason, but what's an RCA ?

Mo

Cheers Mo,

RCA = Root Cause Analysis, it's a methodology for finding the route cause of faults/fault finding we mainly use it at work in the post incident mop up to find why things have happened and how we can prevent things from happening again. It tends to be not just that x part failed buy why did it fail, is there a more fundamental fault with that part or the design/use of that part. As I seemed to be chasing the fault I decided that I needed to do some more formal fault finding, didn't like the thought of an electrical fire and in this case it worked.

Jason.

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