35 replies to this topic

[Dan]

For quick reference there is a chart in the Vehicle Wiring Products catalogue that simply defines the blow caracteristics as instant or not, which is a simple way to define the behaviour of a fuse.

Here however is a link to a page from a fuse manufacturer that gives you more detail. You can see from the charts for the various types of blade fuse that the blow characteristics of these fuses are quite complex. They will hold 110% of their rating for at least 100 hours before blowing. It's only once you get to 200% of rating that the blow time comes down to under 5 seconds, which counts as instant. The exact blow characteristics depend on factors such as the temperature around the fuse box since these are usually open fuses (which are more suceptible to temperature than closed fuses like glass cartridges). For example, in domestic wiring the use of open fuses de-rates cable by 5% to accomodate the temperature fluctuation. It's safe to assume that they will always fail instantly at 200% though and should not be expected to run at anything between 100% and 200% due to heating concerns. You also need to consider a thing called the release current for a fuse. If the current is above a certain level, the fuse may not open far enough when it blows to isolate the circuit, the fuse might be blown wide open but current might still be flowing due to arcing or plasma. So above that level, it won't be capable of 'releasing' the circuit.

Glass fuses on the other hand can be assumed to blow instantly at the rating stamped on them.

Monster Mini,

Due to the differences in characteristics, there are no direct equivalent fuses between blades and cartridges. Without completely re-distributing the circuits and calculating the load in the entire system, the safest thing to do is stick with glass cartridges. A friend of mine caused a fire in his car once by simply de-rating a blade fuse. The fuse was used on a circuit where the current was in the heating range for that fuse. He thougt he was safe because the rating was lower and so thought the fuse would blow earlier. All it did was heat up and set light to the dashboard of his Astra. What blades are you using for each circuit? The wiring is only really rated to run up to the instant rating of the glass fuses but if you swap them for blades of the same instant blow rating you might find that the fuses are heating up in use. Do any of them look melted or schorched?

Incidentally it's not a bad thing that blades are rated in this way so long as you understand them. It's like the difference between b and c type circuit breakers. Blades of a certain rating will allow a wiper motor or air horn compressor for example to draw a high current to start moving and overcome a stall whereas a glass fuse might not.

[monster_mini]

Hello

I've tried to understand all this but still am confused about 1 thing.
If I replace a 30A glass with a 15a blade (which will blow at 30a) and the draw on the circuit is say 20a the only part that can heat up is the blade as the wire was rated for the 30a glass originally

Is this right ?

thanks

[lawrence]

hey monster mini,
what your saying about "If I replace a 30A glass with a 15a blade (which will blow at 30a)" i think is right. And if the draw on the curcuit is only 20 amps then you need a 10 amp blade. but what is the circuit powering?
Im intending to change to blades in my car due to the amount of times my electric fuel pump has failed becuase of rubbish connections
L

[Dan]

No, you are missing the point entirely.

If the load on a circuit is 20 amps you need to use a blade rated to 20 amps, any other blade will heat up. A 20 amp blade though will not instantly fail until it reaches 40 amps so if the circuit is only designed to be protected to a maximum of 30 amps before the wiring starts to heat too then you are in trouble. The fuses aren't really cross-compatible because they behave differently.

It may well be only the blade that heats up to start with but that heat has to go somewhere and since it's connected to conductors the heat will be drawn into the circuit. The wiring will start to melt, the blade casing will melt and drip, eventually there will be a fire.

What fuses have you used on what circuits?

[lawrence]

dam! so you cannot just halve the blade rating becuase the wires that connect to the blade can still melt?
L

[Dan]

Maybe, maybe not. It depends on the circuit and the exact year and model of Mini. Remember that the wiring was not upgraded between 1976 and 1994 really, they just kept adding more circuits and more equipment to the existing circuits. The same applied to the earlier wiring systems, as time went on more and more was added until the whole thing had to be reworked. An early car will have more spare capacity in the wiring than a later car with the same wiring system design.

Also of course, there are no blade fuses with an instant fail rating of 35 amps. There are 7.5 amp (blow at 15 amps like the glass ones) blades but they can be hard to find, I think they're brown.