6.5.3 - Off-peak appliance circuits
All Electricity Supply Companies offer
extremely economic rates for energy taken at off-peak times,
usually for seven hours each night (economy 7). The supply
meter is usually arranged so that off-peak inexpensive energy
can only be obtained from a special pair of terminals, whilst
a second pair provide energy throughout the 24 hour period,
charging for it in terms of the times at which it is taken.
In most cases, energy used at the cheap rate must be stored
for use at other times. There are two major methods of storing
energy, in both cases involving its conversion to heat.
- storage heaters, which are used for space heating.
Circuits feeding them should always be wired radially, with
only one flexible outlet for each. This will help to avoid
problems in the event of the storage heater being changed
for one of a different power rating.
2. - immersion heaters, the energy
being stored as hot water in a lagged tank for use during
the day. Since the amount of hot water used is variable,
it is usually necessary to have a method of increasing the
water temperature should that heated at the cheap rate be
used up. This involves the use of a second immersion heater,
or a single heater with a double element. Since convection,
and hence water heating, takes place mainly above the active
heater, an immersion heater placed low in the tank and fed
by the off-peak supply will heat the whole tank, whilst
a second heater, placed higher in the tank and connected
to the normal supply, will be switched on when necessary
to top up the temperature of the hot water stored (Fig 6.12(a)).
Sometimes a top mounted dual heater is used for the same
purpose as shown in (Fig 6.12(b)). The normal and off-peak
heaters must be supplied through totally separate circuits.
3 kW heaters must be connected permanently to a double pole
switch and not fed via a plug and socket.
Fig 6.12 - Arrangement
of immersion heaters for off-peak supplies