Tim Benstead from ELECSA outlines the main electrical safety considerations for the installation of an electric central heating boiler.
Electric central heating boilers are sometimes installed in domestic and similar premises as an alternative to a gas- or oil-fired boiler. These electric boilers commonly have a rating of between 4 kW and 12 kW, at 230 V single-phase a.c., which is a large load in the context of a domestic installation.
As with all electrical equipment, an electric central heating boiler should be selected and installed in accordance with the manufacturer’s instructions, as well as to allow compliance with BS 7671 (Regulation 510.3 refers).
The adequacy of the main incoming electricity supply to the premises should be checked before the boiler is purchased or any installation is work carried out.
“As for any circuit, the wiring of the boiler final circuit must be suitable for the external influences to which it will be subjected, such as ambient temperature, external heat sources, presence of water, and impact.”
The contractor or other designer should check that the capacity of the supply is sufficient for the overall electrical maximum demand, taking account of the boiler load and all the other loads at the premises (indent (v) of Regulation 313.1 refers).
If the supply capacity is found to be inadequate, this will need to be upgraded by the distributor if the boiler is to be installed.
Guidance on the assessment of maximum demand is given in IET Guidance Note 1 – Selection and Erection.
Depending on the characteristics and usage of the central heating system and hot water system, the electric central heating boiler may operate continuously for long periods. In the absence of better information, it may therefore be prudent not to allow any diversity on the electrical load of the boiler.
Type and size of final circuit wiring
Whilst the most common type of wiring used in domestic installations is twin and earth cable, other wiring systems are not precluded and may be necessary.
As for any circuit, the wiring of the boiler final circuit must be suitable for the external influences to which it will be subjected, such as ambient temperature, external heat sources, presence of water, and impact (Section 522 refers), and the conductors must be sized in accordance with Chapter 52 and Appendix 4 of BS 7671.
Type and rating of circuit overcurrent protective device
The type and rated current of the overcurrent protective device for the circuit should be chosen taking account of the boiler manufacturer’s instructions.
Where the protective device is a circuit-breaker, this may have to be of Type C to BS EN 60898, to reduce the likelihood of unwanted tripping when the central heating load is switched on.
Table 1 gives the minimum rated current for the protective device for a load of 4 kW, 9 kW or 12 kW, at 230 V single-phase.
|Load (kW)||Current demand (A)||Minimum rating of overcurrent device (A)|
Table 1 – Overcurrent protective device current rating (minimum) at 230 V single-phase
Additional protection by an RCD
If the circuit cable is to be concealed in a wall or partition, the probable requirement for additional protection to be provided for the cable by an RCD should not be overlooked (Regulations 522.6.101 and 522.6.102 refer). Such protection is also required if the central heating boiler is installed in a location containing a bath or shower (Regulation 701.411.3.3 refers).
Circuit arrangement (location of protective device)
There are two main options for supplying the circuit that supplies the boiler:
- from an overcurrent protective device in the consumer unit or distribution board (see below).
- from a switched fused disconnector or circuit-breaker fed from the supply side
of the consumer unit (see below).
Option two may be appropriate in circumstances such as where the boiler is being connected to an existing installation in which the consumer unit does not have a suitable spare outgoing way or sufficient spare loading capacity.
Where Option two is used, the switched fused disconnector or circuit-breaker must meet the requirements of Regulations 537.1.4 and 537.2 for the main switch of an installation. Also, a suitable warning notice should be fixed in such a position that a person seeking to operate either the switched fused disconnector/circuit-breaker or the main switch of the consumer unit is warned of the need to operate both these devices to achieve isolation of the installation (Regulation 514.11.1 refers).
The circuit conductors on the supply side of the switched fused disconnector or circuit-breaker in Option 2 are effectively ‘meter tails’. Therefore, the electricity distributor will generally stipulate requirements for their minimum cross-sectional area, maximum length and type of cable, which must be complied with.
In any event, these conductors must be sized and installed in accordance with Section 434 of
BS 7671 for protection against fault current. This means that they must either be sized so that they are protected by the fuse in the distributor’s cut out (this will be achieved by meeting the distributor’s requirements mentioned above), or, as required by Regulation 434.2.1, the conductors must:
- not exceed three metres in length, and
- be installed in such a manner as to reduce the risk of fault to a minimum, and
- be installed in such a manner as to reduce to a minimum the risk of fire or danger to persons.
Switching off for mechanical maintenance
A device for switching off for mechanical maintenance for the boiler is generally required by Regulation Group 537.3. This will require a means of locking the device in the ‘off’ position unless it is continuously under the control of the person performing the maintenance (Regulation 5126.96.36.199 refers).
If the device will also be relied on to provide isolation (such as to allow electrical work to be carried out on the boiler), it must also meet the requirements of Regulation Group 537.2 for isolation.