Blown 13 Amp Fuse

[sammmo]

This may seem like a strange question.. at the time a 13 amp fuse blows what current is actually flowing?

[sammmo]

sorry thats a normal 13 amp HH fuse

[douglas9401]

It will depend on the draw of the appliance that it's protecting.

[jim360]

Any value over 13 amps. It's not that at 12.9999 amps the fuse is fine, and at 13 .00001 amps the fuse will break immediately. There's a transition period where the fuse will start to blow, but will take some time to do so.

For 13 A fuses the guidelines are, according to wikipedia, that the fuse should last for :

1–400 s at 30 A,

0.1–20 s at 50 A

0.01–0.2 s at 100 A.

[The Builder]

As with any fuse, Sammo, it'll depend on the impedance of the "fault loop" as to what the fault current may be.

Also, under a steady load, a 13A fuse may carry several amps above its rating.

[sammmo]

An electrician asked me this question.. he said its in excess of 10,000 amps he admits it was only for milliseconds.. but does anyone know exactly how many amps?can it be worked out?

[jim360]

Not exactly in practice -- there is a range associated with the material of which the fuse is made, the characteristics of the current flowing through it, and so on.

Basically a 13 amp fuse has guaranteed tolerance up to about 13 amps, and then a level of tolerance up to maybe 15 or 16, and will start to break for currents somewhere above that value -- the time taken depending on the design and the particular value of current.

[mibn2cweus]

The current that will flow in a circuit depends upon the combined impedance (resistance to current flow) of the circuit including that of the load, the supply and that of the fuse as well, all play a role in determining the final sum of the current flow.

[chakka35]

douglas9401's post repeats a common error. The fuse is not there to protect the appliance from the misbehaviour of the outside world but to protect the outside world (i.e. the circuitry feeding the appliance) from misbehaviour of the appliance, e.g a short-circuit within it.

[sammmo]

this electrician is adamant that if it was measured at the exact time the fuse blows it would be in excess of 10,000 amps i said 15 amps and another electrician said 13.1 amps..whoever is the farthest away buys the cakes tomorrow..

[The Builder]

For domestic electrical systems, Sammo............. what you're referring to is the PFC (Prospective Fault Current)

Using Ohm's Law........ R = E/I

Therefore I (in this case the PFC) = E/R

R, in this case is the value of the EXTERNAL resistance between you and the Power Station or Transformer .......... called Ze

Ze will depend on your system........
TNCS system 0.35 ohms PFC = 657 amps
TNS system 0.8 ohms PFC = 287 amps


I (PFC) = 230/Ze

In practice, Ze can be much lower. I have a sub-station at the end of my garden, so my Ze is actually 0.1 ohms giving me a PFC of 2300 Amps.

In effect, PFCs are around 600 - 800 Amps. It'll vary around the country of course, but power supplies are unlikely to have the capacity to give a PFC of much more than that.

[BRIGHT SPARK]

As Jim.....95a..0.1sec
76a...0.2sec
65a...0.4sec
55a...1sec
If the impedance of the circuit is 0.1ohm between phase and earth on a TNCS system then the fault current will be around the 2300 mark resulting in a fuse fail disconnection time of a few micro seconds on a circuit of negligible impedence.The actual fail current of a healthy new fillament would be around 25 a.

[douglas9401]

But not your fuses, ooodles. :)

[sammmo]

thanks builder thanks bright sparks, I am not buying the cakes.. and being a mechanical maintenance engineer having an argument with 2 electrical engineers and being nearest to the correct answer has just put icing on my cake..