battery and earth cable getting warm

[California Dreamin]

You are correct as the general rule of 'fusing' a circuit is that the fuse does not rate higher than the wire feeding it....makes sense really.

http://www.autoelectricsupplies.co.uk/" onclick="window.open(this.href);return false;

Martin

[colinthefox]

This will sound like an idiotic question, but i'm trying to obtain cable and connectors to replace the wiring from the junction/relay box to the glow plugs.

I have ordered a new relay and strip fuse, of which is rated 50amps. I assume this means that i need cable rated to at least that, but all the cable rated to and above this is much larger than whats on there, so i'm now confused.

The cable currently on there is original and has 54 strands (don.t know strand size) and measures 3mm across the conductive part and 5mm o/d. Does anyone have any advice or knowledge of where is best to get the right cable? I can get some cable 65/0.3 which is more strands and bigger than the original but says is only rated to 40 something amps.

I really do suck at electrics!

Thanks all.

No it's not an idiotic question. I'll try not to get too technical..........here comes the science bit.

The cables you buy are rated for continuous use at a particular ambient temperature and carrying a particular amount of current. That is the amount of current it will take indefinitely without getting to a temperature which would melt the insulation. The cable supplying the glow plugs only takes a known high current for at the most 30seconds so it doesn't heat up enough to melt the insulation. So an engineer at VW worked out that a smaller cable would be safe and appropriate. What's good enough for VW will be OK for you too. You can select a cable of the same physical size as the original knowing that it will be safe.

So what's the 50 amp fuse for then? That will be protecting the cable against short circuits, not against overloads. (the circuit should never be overloaded because it only ever supplies the four glow plugs)A short circuit will blow the fuse in a very short time, again before the cable has time to heat up.

[Oldiebut goodie]

But...... VW diesel handbook states initial current may be up to 140A so don't use a quick blow fuse! (The usual short circuit will be dropping a spanner on the glow plug busbar when the relay is on. )

[bigherb]

Just to add what Colin has said it is also the length of the cable run that determines the capacity, the shorter it is the more amps it can carry. In this case it is only a short length and as Colin says also a short duty cycle which is why VW used 4.0mm cable.

[colinthefox]

Just to add what Colin has said it is also the length of the cable run that determines the capacity, the shorter it is the more amps it can carry. In this case it is only a short length and as Colin says also a short duty cycle which is why VW used 4.0mm cable.

Sorry to be pedantic. Clarification needed here............The current carrying capacity is the same whatever the length of the cable.

But the longer the length, the more voltage you will lose along the length, which may mean in laymans terms, that there is not enough volts left at the far end to run what you want to run. The way to reduce voltage drop is to use a fatter cable. Nothing to do with current carrying capacity.

[87diesel]

Well thanks all for your input on this, I've ordered the cable and connectors, so hopefully i'll be reporting that it's all up and running at the weekend.

[87diesel]

hi all,

Got the new glow plug wiring, relay and correct fuse in today. Initialized glow plugs 3 times and checked voltages before, during and after. All seems ok so fired it up for a short while before turning it off to see if it was still drawing from the battery...... It seems not, and glowplug fuse hasn't blown, which i assume is the reason that some oiek had put a piece of wire in there.

Will carry my multimeter in the van for a while to keep eye though i think, first test will be to the shops tomorrow.

Thanks again to those who helped out along the way