Overthinking Fuses?

[Gixer]

I was reading a thread on another forum (sorry, but not boat related) and they were talking about continuous current and blow current ratings for fuses. This has caused me concern as I wasn't aware of this before, I always assumed the rating was at which they blow..

Questions are,
How do you know if the fuses you have are rated for continuous current or blow current?
Are glass and blade rated differently?
Is it really an issue when you're talking about low current, for example. If you have a 12v circuit with 1.5mm cable running 10 LED lights each rated at 0.11amp's. You already have 2 amp glass fuses so would like to use them but I guess they would have a blow rate at 4amps? So should i be using a 1amp fuse and would this be different if I was using a blade type fuse?

Thanks

 

[RichardS]

I was reading a thread on another forum (sorry, but not boat related) and they were talking about continuous current and blow current ratings for fuses. This has caused me concern as I wasn't aware of this before, I always assumed the rating was at which they blow..

Questions are,
How do you know if the fuses you have are rated for continuous current or blow current?
Are glass and blade rated differently?
Is it really an issue when you're talking about low current, for example. If you have a 12v circuit with 1.5mm cable running 10 LED lights each rated at 0.11amp's. You already have 2 amp glass fuses so would like to use them but I guess they would have a blow rate at 4amps? So should i be using a 1amp fuse and would this be different if I was using a blade type fuse?

Thanks

I don't understand. If you have 10 x 0.11A, why would you use a 1A fuse?

Presumably the current capacity of the cable is greater than even 4A?

Richard

 

[pvb]

Fuses are there to protect the cable. Fuses should be chosen to match the current carrying capacity of the cable, not the current imposed by the loads.

 

[st599]

You should be able to find a graph like this for the type and size of fuse you're using. The fuse is there to protect the cable or wire, to prevent it overheating. You need to calculate the current carrying capacity of the cable run (taking in to account proximity to combustible materials, airflow around it, number of cables in the run etc. then choose a fuse to prevent that being exceeded.

 

[PaulRainbow]

I was reading a thread on another forum (sorry, but not boat related) and they were talking about continuous current and blow current ratings for fuses. This has caused me concern as I wasn't aware of this before, I always assumed the rating was at which they blow..

Questions are,
How do you know if the fuses you have are rated for continuous current or blow current?
Are glass and blade rated differently?
Is it really an issue when you're talking about low current, for example. If you have a 12v circuit with 1.5mm cable running 10 LED lights each rated at 0.11amp's. You already have 2 amp glass fuses so would like to use them but I guess they would have a blow rate at 4amps? So should i be using a 1amp fuse and would this be different if I was using a blade type fuse?

Thanks

1.5mm cable is rated at 21a, so you can fuse at anything between the rated current of the equipment and 21a. There is no point looking up graphs, working out cable runs, worrying about glass fuses, blade fuses, fuse wire etc. There is no need to fit 20 types/ratings of fuse, you can just fit 10a fuses into everything that is rated at less than 10a (even if you use 2.5mm cable for voltage drop). If it's over 10a, fit 15a or 20a, this way you only have to carry three spare fuses.

There are a few exceptions to the above. Electric motors usually need fusing for the motor current, rather than the cable. The cable is usually rated much higher than the motors run current to allow for inrush and voltage drop over the cable run. For instance, a windlass might be wired with 400a-500a cable, but if you fitted a 500a fuse and overloaded the motor you could damage the motor, so you would use the type of protection the windlass manufacturer specifies (usually a thermal breaker) and follow their specification for its rating.

 

[Gixer]

I don't understand. If you have 10 x 0.11A, why would you use a 1A fuse?

Presumably the current capacity of the cable is greater than even 4A?

Richard

Thanks Richard,

Does the 1amp glass fuse blow at 1amp or 2amps? Its an academic question I guess as the cable is rated for 21amps.

EDIT - sorry wrote this before then posted after the other responses

 

[PaulRainbow]

Oh, glass fuses are rubbish, replace them all with blade fuses. And equipment that comes with an inline fuse, cut it out and wire it to a blade fuse. One type of fuse and only a few rating means you don't need a suitcase full of spare fuses and blade fuses are very reliable.

 

[Moodysailor]

Agreed with the above re: fuse protection for the cable, with the comment that the maximum fuse size should be rated for the cable - it is perfectly OK to install a smaller fuse in order to protect a consumer as well as the cable.

It is not "good practice" to have a cable sized too close to the current rating of the consumer as cable length, connections and degradation will reduce the current available at the consumer. So, in an ideal situation, the cable will be sufficiently over-sized, and the fuse will be specified for the consumer - which by a matter of default will be well within the cable rating.

A very general rue of thumb for cable current rating in a 12V system is 10amp for 1mm2 single cable. This is reduced when cables are bundled together (i.e current rating of 2.5mm2 tinned copper single cable is just over 20amps @12V, but 17.5 amps@12V for twin-core flat cable the same size). But this general rule of thumb can be used for initial assumptions when considering what size cable to use.

As a real-life example, on our boat which is 24V, I use 2.5mm cable for most of the runs. This gives minimal voltage and current drop and as as most of our circuit breakers are between 6 and 15 amps, there is a lot of redundant capacity within the wiring.

 

[RupertW]

People often say, ”Fuse for the cable not the load” as if it was the only rule. It should better be stated, “Fuse for the cable or lower“. For example house domestic cables are fused at 13Amps but can be fused lower to protect the load if you want - there is nothing inherently wrong in a 3amp fuse if there are parts of the load that shouldn’t take 13Amps.

 

[Gixer]

Thanks guys, that graph was really helpful in my understanding.

I replaced and 'upped' all the size and spec of my cabling last year but didn't changing the fusing and wasn't aware of the continuous current and blow current differences. Learning about that now caused me a bit of concern but my set up looks to be well within the tolerances.

Paul - I was thinking of switching to blade but my main panel is still glass type which would mean carrying two types of fuses.

Thanks again guys

 

[Moodysailor]

There is no need to fit 20 types/ratings of fuse, you can just fit 10a fuses into everything that is rated at less than 10a (even if you use 2.5mm cable for voltage drop). If it's over 10a, fit 15a or 20a, this way you only have to carry three spare fuses.

+1 for this. Too many times people overthink basic loads. As @PaulRainbow said, be careful with motors, other switched loads, follow the KISS theory.