Main Fuse rating 60A/500V - enough?

[Toby]

Excuse my ignorance, but if it is rated at 60A at 500V, then that is 30 kW, therefore at 230V it is therefore 130A isn't it?

[Tim Watts]

No.

it means it is designed for 60A continuous max current of 60A and designed to break a supply of upto 500V, which is related to both the insulation of the fuse body and the ability to quench the arc caused by the fuse blowing under load.

eg if that fuse blew while carrying 10kV, it would do sod all as the arc would just carry on burning across the fuse ends. Hence higher voltage fuses tend to be longer.

Power has no meaning with respect to fuses.

Cheers

Tim

[Toby]

Ahh yes, I see, thanks :-)

[Tim Watts]

There is another element too.

While you have an arc (which is why all but the tiniest of fuses are filled with powder, and massive substation breakers are filled with oil), you have a voltage drop along with a flowing current through the arc. This lasts for a certain amount of time, which is a funtion of the fuse characteristic and the amount of current flowing.

The powder (or oil) helps quench the arc, but it has another role too.

Simple application of E=VIt integrated means that to blow under a fault, there will be some energy imparted in the fuse body which ultimately = heat.

This is why higher voltage and higher current fuses tend to be fatter (100A fuse wire is still pretty thin, no need for a 10mm dia fuse body to contain it at first sight). The more energy given up means the more thermal-mass buffer you need to avoid bad things like the fuse exploding.

There's another parameter that is related to this too - the "rupture capacity". A tiny fuse in a small transformer powered alarm clock can never see much short circuit current.

A dead short on a pair of meter tails with the substation in your back garden means a failt current of several thousand amps burning in the arc in the fuse. Even if the fuse blows in a few tens of milliseconds, that still equates to quite a large number of joules.

:)

[John Rumm]

Agreed. Just to add a general note that the substation in the back garden thing, can be an issue for any circuit breaker really. It is possible to get scenarios where even a normal socket close to the CU in those cases, could have a fault current well in excess of what even the MCBs in the CU can interrupt (usually capped at 6kA, but sometimes 10kA).

[tony sayer]

Matey from UK power networks told me that fuses mainly fail due to olde age rather than overload!...

[John Rumm]

Yup, can well believe that...