Residential electrical installation - Distribution boards components

Electrical equipment used in residential premises are commonly certified by third party ensuring conformity with the relevant standards. In this case, equipment shows the certification Mark of the certification body such as VDE, NF, AENOR, IMQ or others. Mark of conformity is a voluntary manufacturer process and implies periodic verifications of the Quality of the products by the third party laboratory. In EU zone CE marking is a mandatory self-declaration for free trade done by the manufacturer/importer.

(see Fig. Q1, Q2 and Q3)

Distribution boards (generally only one in residential premises) usually include the meter(s) and in some cases (notably where the supply utilities impose a TT earthing system and/or tariff conditions which limit the maximum permitted current consumption) an incoming supply differential circuit-breaker which includes an overcurrent trip. This circuit-breaker is freely accessible to the consumer.

Fig. Q1 – Example of typical UK residential distribution board

Fig. Q2 – Example of Italian residential installation (> 125 m², basic level)

Fig. Q3 – Example of French residential installation (3 rooms with electric heater)

On installations which are in a TN earthing system, the supply utilities usually protect the installation simply by means of sealed fuse cut-outs immediately upstream of the meter(s) (see Fig. Q4). The user has no access to these fuses.

Fig. Q4 – Components of a control and distribution board

The incoming supply circuit breaker or main switch

(see Fig. Q5)

The consumer is allowed to operate this circuit breaker or switch if necessary (e.g. to reclose it if the circuit breaker has tripped due to current consumption exceeded the authorized limit, to open it in case of emergency, or for isolation purposes in case of maintenance in the distribution board).

For TT systems, the rated residual current of the incoming circuit-breaker shall be adapted to the maximum earth impedance:

• For example, if the earth impedance is not more than 100 Ohms, the rated residual current of the main RCBO shall not exceed 500 mA: Idn = 50 V / 100 Ohms = 500 mA
• In some countries, the rated residual current of the incoming circuit-breaker is 300mA. In this case, the earth electrode resistance shall be less than R = 50 V / 300 mA = 166 Ohms
• In practice, the earth electrode resistance of a new installation shall be less than 80 Ω (R/2).

Fig. Q5 – Examples of incoming-supply circuit breakers or switch from different countries

• 

  [a] France (CB)

• 

  [b] UK (switch-disconnector)

The control and distribution board (consumer unit)

(see Fig. Q6)

This board comprises:

• A control panel for mounting (where appropriate) the incoming supply circuit breaker and other control auxiliaries, as required
• A distribution panel for housing the MCBs or fuse units, etc.
• Installation accessories like DIN rails for mounting MCBs, fuses bases, connection accessories like comb busbars to simplify interconnection of devices, accessories for fixing conductors, neutral busbar and earthing bar, and so on
• Service cable ducts or conduits, surface mounted or in cable chases embedded in the wall

Note: to facilitate future modifications to the installation, it is recommended to keep all relevant documents (photos, diagrams, characteristics, etc.) in a suitable location close to the distribution board.

The board should be installed at a height such that the operating handles, indicating dials (of meters) etc., are accessible to users. Heights commonly recommended are between 1 metre (or less in some countries) and 1.80 metres from the floor (1.30 metres in situations where handicapped or elderly people are concerned).

Fig. Q6 – Example of control and distribution board (France)

Surge protective devices

The installation of surge protective devices (SPD) at the service position of a LV installation is strongly recommended for installations which include sensitive (e.g electronic) equipment, which is very common nowadays.

These devices must automatically disconnect themselves from the installation in case of failure or be protected by a MCB. See Protection against overvoltages and lightning.

Resistance value of the earth electrode

If, in a TT scheme, the value of 80 Ω for the resistance of the electrode cannot be met, then 30 mA RCDs must be installed to take over the function of the earth leakage protection of the incoming supply circuit breaker

In the case where the resistance to earth exceeds 80 Ω, one or several 30 mA RCDs should be used in place of the earth leakage protection of the incoming supply circuit breaker.