It can be quite common for people to 'daisy chain' four gang extensions together meaning that a load of appliances are sourced from one outlet.

Now I know we shouldn't be doing this, and our maintenance team is commencing a program to remove these and, where necessary, provide additional sockets.

Could someone explain to me, in laymans terms, why this is dangerous and what specifically causes the fire risk.

I know I will be asked and electricity isn't by strong point. It's the work of the devil - just like magnets!!

Ian

Ian,

A normal ring main is designed to take a predetermined current evenly spread via all the socket outlets connected on it upto 30A. (quite a large safety factor on domestic circuits on the basis of what appliances are likely to be used and the fact that all sockets are not likely to be in use at the same time!)
By plugging in extension leads you are drawing the current for those extra sockets through one socket outlet on the ringmain.
Excess current being drawn produces heat at various points particularly at any metal to metal interface such as in the sockets as well as in the copper of the wire. Hopefully micro circuit breakers will trip when this becomes excessive for the ring but this main not occur before the current drawn is greater than the normal 'rating' of the socket.
Put together with the housekeeping issues usually associated with how extension leads are generally run I think you will be able to see that they are an item whose use needs to be monitored etc.

This is a simplified explanation and you may like to look on the HSE and the IET (theiet) websites for more detailed info.

Regards

An extension to this thread (pun intended) is look out for extension cable reels used to extend the length of a line of those 4 or more gang socket thingys. If the cable reel is not fully out there is localised heating within the reel, (induction coil affect I think). Another potential cause of fire.

OK, whilst all the advice so far has been good, to put another explanation on it from a more electrical standpoint, more my background.

All of the daisy chained extensions all run back to a single plug. This has a 13A fuse fitted as a maximum, unless seriously doctored, or bodged! It can be done even with modern moulded plugs, trust me and don’t ask!

Thus, the fuse should blow at a rating of less than that of the fixed circuit protective device (the one protecting the wiring to the sockets in the wall), the circuit may not be a ring, it may be a radial, you can't tell by just looking at the outside!

A 13A fuse of the type in a plug is designed to disconnect the supply in fault conditions in 0.4 sec to comply with the current wiring regulations.
To do this a current of around 95A must flow.

This is much less than the 159A required to trip off a miniature circuit breaker (mcb) of 32A which would be typically the protective device on a ring circuit.
A ring circuit does not have to be designed to carry equally distributed loads however, the maximum loading is obviously limited.

A typical twin outlet “13A” socket is continuously rated at around 20A for both outlets, not the 26A you would assume.

A fault on something plugged into a socket outlet, thus an extension lead SHOULD blow the fuse in the plug first.

Now it gets a bit "techie".
When you add extensions one after the other, each has a plug/socket connection, this is not good, and has increased "resistance" in Ohms, I believe it is mentioned in the electrical bit of the NGC, was in mine anyway!
Also you have more length of wire to the appliance at the end of the daisy chain, this has the same effect of increased resistance to the flow of electric current.
The more Ohms you have the less current can flow to blow the fuse(trip the mcb) in the required 0.4 sec.

Next, the more Ohms you have in the daisy chained extension leads, then in the event of a fault, say at the old fashioned metal kettle plugged into the end of the last daisy chained extension lead, any exposed metal, the body of the kettle to which the fault connects can have a voltage on it proportional to the resistance (Ohms) of the wiring attached, thus you can get a shock from the exposed metal if you offer the electricity a better path to earth. Say wet hands whilst filling the kettle from the tap whilst it is still plugged in and your other hand is on the metal handle of the tap?

So you must keep the wiring related resistance sufficiently low so that this cant happen, and to be safe and make sure the fuse blows.

Coiled extension leads can suffer from inductive heating, they also suffer from the heating effect of the current flowing through the wires heating the wires up, think kettle element, this heat cannot escape from the cable as it is tightly coiled one layer on top of the other, as the lead heats up the resistance gets higher, and it heats up more and as it heats up the resistance… Well you get the idea, and eventually it melts and there is a bang, hopefully a small one as the circuit breaker trips or the fuse blows before it catches fire or electrocutes someone, though this is not guaranteed!

Hope this helps and it not too technical.

Paul