Domestic central heating systems must be fused at 3 amps
Most heating systems have a fuse which is located in a double pole switch (dipole switch) which electrically isolates the central heating system and boiler. The switch is often found near the boiler or near the programmer. It may also be found near the circulating pump and motorised valve(s) in an airing cupboard. The switch is called a double pole switch because it breaks both the Live (Line) and Neutral contacts. The regulations require that both poles must be isolated when disconnecting the electrical power.
Occasionally the power for the heating system may be supplied via a plug and socket, in which case the fuse will be in the plug. If the supply is via a plug and socket, the socket must be unswitched. This is to force you to pull out the plug to disconnect the power supply. Pulling the plug out breaks both the Live and the Neutral contacts. The switch on a switched plug socket breaks only the Live contact, which is why switched sockets are not allowed as isolation for domestic heating systems.
There should only be one isolation point for the whole of the heating system. If there is more than one, each would only isolate part of the heating system. A user who turned off only one of the switches might assume that all of the system was safely isolated, though parts would still remain live.
Why does the heating fuse rating matter?
In a fault situation, fuses prevent the wiring and electrical components from overheating and causing damage or a fire. If an electrical short circuit occurs, far too much current will pass and this would rapidly overheat the wires and components. The fuse is designed to melt and break the circuit before any other electrical components or wires overheat. Fuses are contained within ceramic cartridges which are designed to safely contain the melting fuse wire, without overheating.
Why are the wrong fuses often fitted?
When double pole switches are sold, they usually come with a 13 amp fuse fitted. For domestic central heating systems this must always be changed for a 3 amp fuse. If you’ve had your kitchen refitted, with new sockets and switches, check the fuse rating in the double pole switch for the heating system.
Using the wrong fuse really can be dangerous. If a fuse blows, never replace it with a higher rated fuse, even temporarily.
When we service gas boilers we check the fuse rating of the system. From time to time we still come across fuses which have too high a rating and this can make the central heating system dangerous.