simple questions about electric in the home!..

[londonboy123]

hello, i have a few simple basic electrical related questions that i wonder if anyone could help me with...?

1) an mcb trips the circuit if a current over that of the mcb is put down the cable? but why are there 2 fuses/mcbs for the same circuit? for eg/ one in the customer consumer box and the other in the outlet/socket? do they both do the same job? is it for double protection?


2) why does a normal plug socket have a 13amp rating? why not higher or less? if the ring main can take about 30 amps?

3) if a fault occurs and a live wire touches metal, for example a metal pipe, and that pipe is earthed, the fault goes to earth and not through the person who may then touch that pipe. but what signs are there that a fault has occured-will the component stop working correctly?

sorry these are probably VERY simple questions but I would like to try and understand these!! thank you for any help/info.

 

[Steve]

1. the MCB protects the circuit cable and accessories. The plugtop fuse protects the cable going to the appliance, since its normally smaller than that of the circuit.

2. Ring mains are a funny thing. Best not to ask.

3. If a live wire touches an earthed metal object, what happens? Lots and lots of current flows, its a short circuit, and the circuit protective device operates! Therefore, yes, the "component" will stop working!

 

[bongos]

1) The MCB is designed to protect the cable. The fuse in the plug is there to protect whatever is plugged into it. Although a ring will commonly allow 32A you wouldn't want that sort of current to go into a table lamp. Other items plugged into the ring draw more current, such as kettles etc.

2) Not sure why 13A was chosen as the maximum number for plug fuses, sorry! But again, its the fuse protecting whatever is plugged into the socket and the MCB that protects cable between the CU and the plug socket.

3) If you get a Live ---> Earth fault then this is often recognised by an RCD (different to an MCB - look in the wiki section). If the fault causes a large enough current to flow through the MCB then this in turn will trip the MCB protecting that circuit. The signs of this is a tripped MCB or RCD and you will not be able to reset it until the fault has been removed.

 

[Space cat]

A fuse protects the wire that lies beyond it. The first fuse in your house will be the main service fuse. It's sealed in a box and may be 100 amps. (Some older ones are smaller.) This protects the heavy cables that go into the consumer unit (CU).

Inside the CU there are many fuses or breakers of different sizes. These protect the much smaller cables that come out of the CU. Your lights are most likely wired with 1 sq mm cable and protected by a six amp breaker. An electric shower needs a lot more than six amps so it has a bigger breaker and also bigger cable.

The ring main is designed so that lots of things can be plugged into it. Most of these will only take small currents but a few, like electric kettles, will take more. The ring itself is wired with 2.5 sq mm cable on a 32 amp breaker. Now 32 amps is too much for a SINGLE length of 2.5 sq mm cable but this is a ring. Current from the CU can go two different ways to each socket and so the current is shared (though not always equally) between two cables.

Each socket is good for up to 13 amps but the plugs are, or should be, fused to suit the size of flex that comes out of them. That kettle will have a 13 amp fuse and 1.25 sq mm flex. A table lamp doesn't need such heavy flex so you can use 0.5 sq mm flex and a 3 amp fuse - but how often do we find 13 amp fuses in such plugs!

 

[RF Lighting]

2) Not sure why 13A was chosen as the maximum number for plug fuses, sorry!

It goes back to when the 13A plug / socket was introduced.
As a general rule the biggest portable appliance that anyone owned was a 3 bar electric fire. This would use 1kW per bar, which is 3kW total. That conveniently is 13A.

There are not many domestic potrable apliances which require more than 13A even now.