What are fuses for?

YorkshireBoy

djohn2002uk,

There's been some good solid advice and facts given by the electricians on this thread. I just felt that your "30A is the most current that will flow" statement warranted some extra clarification.

Re the heat damage, I think it's fair to say that most domestic fires are caused by appliance faults causing overcurrent/overload situations, rather than a direct short circuit which would, in virtually all cases, cause the protective device to operate. Where cable faults have caused fires, this is usually down to a failure to maintain the ring/connect spurs correctly, running cables through thermal barriers/insulation, or simply loose connections in junction boxes or wiring accessories.

Re your lightning example, I suspect the cable armourings were providing a path to earth for the lightning current.

Talking of high voltage, as an 11,000V senior authorised person, responsible for the operation, switching and maintenance of an 8MVA network with a 107MVA fault level at the supply intake, I've seen the aftermath of many an incident - both in practice and in training videos. The sheer power of the fault levels involved is astounding, and can make busbars 'dance' - and that is an amazing sight to witness when played in slow-mo in the training videos.

Mr_Grumpy_7

A couple of comments, well alright, three comments:

Many appliances in the IT world use 'kettle leads', an IEC connector at one end and a 13A plug at the other. The fuse in the plug is there to protect the power lead and the IEC connector which is normally rated at 10A. The appliance is protected by its own fuse. What I'm saying is that where a 'kettle lead' is used to connect the appliance to the mains, the fuse in the plug cannot be relied upon.

Another comment regarding heat damage to wiring in over current situations. Many years ago a friend was using an old fashioned AVO meter to locate an electrical fault on a car. Somehow he effectively connected the meter across the battery with it set on DC Amps, in effect a short circuit. The AVO meter uses a mechanical cut-out based on the needle reaching full scale deflection with sufficient force, nominally 10 x full scale deflection, if I remember right. In the time it took the needle to reach FSD, a tenth of a second at the very most, the test leads and probes which were rated at something like 20A melted. The AVO meter was fine.

One last comment. I once attended an electrical safety course aimed at the IT industry. One of the discussions related to working on live power distribution units. The PDUs in question are the size of 2 or 3 walldrobes, supplying 50-100 32Amp circuits. Apparently, were you to short the mains with a spanner, the spanner would not just melt, it would vapourize, and would then attach itself to the engineers hands and face. I've seen the pictures, but cannot quite comprehend the energy required to do such a thing. I suppose, when you look at a blown fuse with blackened glass, it's the same thing but on a massive scale. What this does demonstrate is that whatever the potential short circuit current, and in some installations this number can be huge, it can do unimaginable harm.

rygon

The vaporising bit Ive seen picture of that as well Mr Grumpy. I work at a power station and when thing go wrong they go wrong in style. When doing the appreciation training we had pics of 6.6kV switchgear melted. As you said it does just spark it explodes and vapourises. If you are unlucky enough to get caught in that If the burns dont get you the metal inside your body will kill you in a few days (due to liver failure).

mikey-mike_2

The ohms law example above is fine as far as it goes but the resistance of all the cables and inline equipment from your fault all the way back to the local transformer must be added as well. When you add in all this extra resistance the fault current will drop from 16,000 to probably a couple of hundred amps.

This total resistance is normally called the earth loop and is one of the things electricians check during testing.

The wiring regs stipulate that the earth loop must be low enough to ensure that under short circuit conditions fuses protecting fixed equipment will blow within 5 sec (half a second for stuff that is plugged in)