Gt6 Mk1 repeatedly blowing fuse - it's back

[Clive]

1 hour ago, johny said:

well we're talking about the power needed to operate the fuse/trip so you could use P=VI but the voltage would have to be that dropped across the fuse/trip (mV?) not the circuit supply voltage.

Power of a component is usually given at a set voltage. So your 3kw kettle would be 3kw IF you had a 240v supply. But you don't. (and I think there are plans to drop supply voltages further to reduce electricity coinsumption)

Re fuses, they don't "blow" at a small amount over their rating. I don't have the figures to hand, but would suggest they can handle 5x their rating for a second or more. But a dead short (the usual issue) will provide very high currents and the fuse will melt almost instantly.

[johny]

yes its a bit more complicated with fuse/trips as they are in series with the circuit load so the circuit supply might be 12v but, for example, 0.05v will be across said fuse/trip and 11.95v across the load (headlights, horn etc).

As you say, fuses dont blow at exactly their rated value as time is needed for the heat produced by 0.05v x fault current to melt the link and obviously ambient temperature will also have an effect....

[NonMember]

13 minutes ago, clive said:

Power of a component is usually given at a set voltage.

No, Clive, that's not actually true. The power rating of a resistor is the power it will cope with, and usually quoted for the whole family. The voltage or current you need to get that power depends on the resistance, but the power is quoted for the type of resistor, not the value, so they explicitly ARE NOT quoting "at a set voltage". The power capability of a switch mode power supply is usually quoted for a nominal input voltage, but generally doesn't vary very much with the mains supply.

Johny was referring specifically to PPTC resettable fuses, which are very definitely "power" sensitive - they trip due to heat, which is provided by the power - but tend to be quoted by current because they are used in series with the real load and it's not the real load's power they're sensitive to. The same, actually, is true of old-fashioned fuses - if they didn't have any resistance at all then they wouldn't dissipate any power, wouldn't get hot, and wouldn't blow. The entire reason they blow faster at currents massively over their rating is that those very high currents, passing through the fuse's (small) resistance, dissipate a lot of power, which heats the fuse wire past its melting point very quickly.

[DerekS]

Colin, if you're having problems getting hold of fuses, I can easily send you a few in the post, not a problem.

Derek.

[Colin Lindsay]

They can be hard to find, Derek, no outlets local to me but I buy them online; I was buying them ten at a time recently at around £1 each:

https://www.ebay.co.uk/itm/5x17A-CONTINUOUS-35A-BLOW-CAR-AUTO-AUTOMOTIVE-GLASS-RADIO-CHARGER-FUSE-6-3x-31mm/291022081317?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

However I've just found Paddocks selling them at 35p each so will stock up on a few - thanks for your very kind offer but I don't want to use up your stock! Shouldn't have any more bother (I hope!!) now that I've identified the problem (I hope!!). You can see in the photo how the bullet connector has come back out of the connector tube plus damage that has been caused to the outer sheath by the use of ordinary pliers, not bullet pliers, and how close it was to the engine earth strap where it shorted out. You learn from your mistakes... all sorted now and rewiring on-going.

 

 

 

[Badwolf]

Colin - Can you not rewire some of your fuses with good old fashioned fuse wire. It must still be available somewhere. Doug/Johny can advise if the rating is different in any way. Must be cheaper while you are still 'testing'

[DerekS]

Colin, I'm pleased you're getting sorted, the offer is still open, you wouldn't be using my stock up! 

Derek.

[johny]

43 minutes ago, Badwolf said:

Colin - Can you not rewire some of your fuses with good old fashioned fuse wire. It must still be available somewhere. Doug/Johny can advise if the rating is different in any way. Must be cheaper while you are still 'testing'

yes fuse wire cards still available on ebay and although probably not very accurate will do the job. Just be aware that being 'open' the heat produced when blowing is exposed...

[Colin Lindsay]

46 minutes ago, derekskill said:

Colin, I'm pleased you're getting sorted, the offer is still open, you wouldn't be using my stock up! 

Derek.

In that case I'll PM you!

[Clive]

8 hours ago, NonMember said:

No, Clive, that's not actually true. The power rating of a resistor is the power it will cope with, and usually quoted for the whole family. The voltage or current you need to get that power depends on the resistance, but the power is quoted for the type of resistor, not the value, so they explicitly ARE NOT quoting "at a set voltage". The power capability of a switch mode power supply is usually quoted for a nominal input voltage, but generally doesn't vary very much with the mains supply.

Johny was referring specifically to PPTC resettable fuses, which are very definitely "power" sensitive - they trip due to heat, which is provided by the power - but tend to be quoted by current because they are used in series with the real load and it's not the real load's power they're sensitive to. The same, actually, is true of old-fashioned fuses - if they didn't have any resistance at all then they wouldn't dissipate any power, wouldn't get hot, and wouldn't blow. The entire reason they blow faster at currents massively over their rating is that those very high currents, passing through the fuse's (small) resistance, dissipate a lot of power, which heats the fuse wire past its melting point very quickly.

Apologies, by component I meant something like a lamp, (or indeed kettle, horn, dishwasher etc)

The power is a function of current, controlled by resistance. I squared R and all that. Of course, resistance can vary with temperature, which is again a function of current. 

I don't do electronics, just electrics. Simple stuff.....

[Peter Truman]

Colin those bullets sleeves look newish so they should hold the bullets tight.

I’ve had problems with the old original 50 year old bullet sleeves/connectors the spring metal internals have relaxed and tarnished and eventually break a lug off so the bullet is loose inside.

I brought a swag of new bullet sleeves single and double connectors and have been slowly replacing them on the car so they are tight. Reconditioning also includes using a small Dremmel rotating wire brush to clean up the bullets before reconnecting Pushing them home.

Replacement started with problematic connections under the bonnet especially at the front lights and in the boot tail lights, but continues on I will probably have to buy around another dozen or so single and half that Double connectors To finish, Think I’ve already used around 20.

[Pete Lewis]

and to add there are various sizes of bullets just to make things more of a challenge , 

i have some nice brass at 4.7mm    old solid push fit at 5.1mm   and the old split ones at 5.5mm  

the brass ones just fall through unless you over fold the wire to fill it out 

Pete

[Prince Brainier of Meccano]

Get a headlamp bulb and wire the 2 filaments to come on together.  Wire this across the fuse contacts, without the fuse in. Then wiggle away.  The light will come on when you have the fault.  Horn draws a lot of current so it will always turn the light on.  This gives you a chance to test for the fault without changing the fuse.

Without this your whole car just becomes a fuse tester.  When the fuse pops; you know it was a good one.

I used this years ago in a house with random fuse failure.  Put a kettle across the fuse and that allowed time to find the short.  It was a cable over an uninsulated heating pipe, under a loose floorboard.  Only went when the heating was really warm and you trod on the floor board. That then pushed the earthed pipe through the soft insulation.  Tread on the floor board: Make tea in the garage!

[Badwolf]

Interesting. Many years ago my assistant told me of a situation at her home. Occasionally she smelled a slight whiff of gas on the landing of her house but not strong enough to worry. Eventually a new carpet was fitted on the landing and the fitter found a nail going through a gas pipe. Been like that since the floor board was put down when the house was built.

[Chris A]

30 minutes ago, Badwolf said:

Interesting. Many years ago my assistant told me of a situation at her home. Occasionally she smelled a slight whiff of gas on the landing of her house but not strong enough to worry. Eventually a new carpet was fitted on the landing and the fitter found a nail going through a gas pipe. Been like that since the floor board was put down when the house was built.

Similar case with me way back years ago. Brand new house, bought off plan, moved in a few days before Xmas. Gas heating, big problem leak in concealed pipe in kitchen wall. clearly plasterboard nail in it. Result new visible pipe installed after their Xmas hols. Evidently they had been  so keen to hand over the house they didn't test the gas system.