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Residential electrical installation - Distribution boards components

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(see Fig. Q1)

The quality of electrical equipment used in residential premises is commonly ensured by a mark of conformity situated on the front of each item

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. Q1Presentation of realizable functions on a consumer unit

On installations which are TN earthed, the supply utilities usually protect the installation simply by means of sealed fuse cut-outs immediately upstream of the meter(s) (see Fig. Q2). The consumer has no access to these fuses.

Fig. Q2Components of a control and distribution board

The incoming supply circuit-breaker

(see Fig. Q3)

The consumer is allowed to operate this CB if necessary (e.g. to reclose it if the CB has tripped due to current consumption exceeded the authorized limit; to open it in case of emergency or for isolation purposes).

The rated residual current of the incoming circuit-breaker in the earth leakage protection shall be 300 mA.

If the installation is TT, the earth electrode resistance shall be less than

R=\frac{50\, V}{300\, mA}=166\, \Omega.

In practice, the earth electrode resistance of a new installation shall be less than 80 Ω   \left ( \frac{R}{2} \right ).

Fig. Q3Incoming-supply circuit-breaker

The control and distribution board (consumer unit)

(see Fig. Q4)

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 1, 2 or 3 rows (of 24 Acti 9 units) or similar MCBs or fuse units, etc.
  • Installation accessories for fixing conductors, and rails for mounting MCBs, fuses bases, etc, 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 between 1 metre and 1.80 metres from the floor (1.30 metres in situations where handicapped or elderly people are concerned).

Fig. Q4Control and distribution board

Lightning arresters

The installation of lightning arresters at the service position of a LV installation is strongly recommended for installations which include sensitive (e.g electronic) equipment.

These devices must automatically disconnect themselves from the installation in case of failure or be protected by a MCB. In the case of residential installations, the use of a 300 mA differential incoming supply circuit-breaker type S (i.e slightly time-delayed) will provide effective earth leakage protection, while, at the same time, will not trip unnecessarily each time a lightning arrester discharges the current (of an overvoltage-surge) to earth.

Resistance value of the earth electrode

If, in a TT scheme, the value of 80 Ω for the resistance of the electrode can not 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.