Residential electrical installation - Protection of people
From Electrical Installation Guide
Contents |
| Where utility power supply systems and consumers’ installations form a TT earthed system, the governing standards impose the use of RCDs to ensure the protection of persons |
On TT earthed systems, the protection of persons is ensured by the following measures:
- Protection against indirect contact hazards by RCDs (see Fig. Q5) of medium sensitivity (300 mA) at the origin of the installation (incorporated in the incoming supply circuit-breaker or, on the incoming feed to the distribution board). This measure is associated with a consumer installed earth electrode to which must be connected the protective earth conductor (PE) from the exposed conductive parts of all class I insulated appliances and equipment, as well as those from the earthing pins of all socket outlets
- When the CB at the origin of an installation has no RCD protection, the protection of persons shall be ensured by class II level of insulation on all circuits upstream of the first RCDs. In the case where the distribution board is metallic, care shall be taken that all live parts are double insulated (supplementary clearances or insulation, use of covers, etc.) and wiring reliably fixed
- Obligatory protection by 30 mA sensitive RCDs of socket outlet circuits, and circuits feeding bathroom, laundry rooms, and so on.
Fig. Q5: Installation with incoming-supply circuit-breaker having instantaneous differential protection
Incoming supply circuit-breaker with instantaneous differential relay
In this case:
- An insulation fault to earth could result in a shutdown of the entire installation
- Where a lightning arrester is installed, its operation (i.e. discharging a voltage surge to earth) could appear to an RCD as an earth fault, with a consequent shutdown of the installation
Recommendation of suitable Merlin Gerin components
- Incoming supply circuit-breaker with 300 mA differential and
- High sensitivity 30 mA RCD (for example differential circuit-breaker 1P + N type Declic Vigi) on the circuits supplying socket outlets
- High sensitivity 30 mA RCD (for example differential load switch type ID’clic) on circuits to bathrooms, shower rooms, laundry rooms, etc. (lighting, heating, socket outlets)
Incoming supply circuit-breaker with type S time delayed differential relay
This type of CB affords protection against fault to earth, but by virtue of a short time delay, provides a measure of discrimination with downstream instantaneous RCDs. Tripping of the incoming supply CB and its consequences (on deep freezers, for example) is thereby made less probable in the event of lightning, or other causes of voltage surges. The discharge of voltage surge current to earth, through the surge arrester, will leave the type S circuit-breaker unaffected.
Recommendation of suitable Merlin Gerin components
(see Fig. Q6)
- Incoming supply circuit-breaker with 300 mA differential type S and
- High sensitivity 30 mA RCD (for example differential circuit-breaker 1P + N type Declic Vigi) on the circuits supplying washing machines and dish-washing machine
- High sensitivity 30 mA RCD (for example differential load switch type ID’clic) on circuits to bathrooms, shower rooms, laundry rooms, etc. (lighting, heating, socket outlets)
Fig. Q6: Installation with incoming-supply circuit-breaker having short time delay differential protection, type S
Incoming supply circuit-breaker without differential protection
In this case the protection of persons must be ensured by:
- Class II level of insulation up to the downstream terminals of the RCDs
- All outgoing circuits from the distribution board must be protected by 30 mA or 300 mA RCDs according to the type of circuit concerned.
Where a voltage surge arrester is installed upstream of the distribution board (to protect sensitive electronic equipment such as microprocessors, video-cassette recorders, TV sets, electronic cash registers, etc.) it is imperative that the device automatically disconnects itself from the installation following a rare (but always possible) failure. Some devices employ replaceable fusing elements; the recommended method however as shown in Figure Q7, is to use a circuit-breaker.
Recommendation of suitable Merlin Gerin components
Figure P7 refers:
1. Incoming-supply circuit-breaker without differential protection
2. Automatic disconnection device (if a lightning arrester is installed)
3. 30 mA RCD (for example differential circuit-breaker 1P + N type Declic Vigi) on each circuit supplying one or more socket-outlets
4. 30 mA RCD (for example differential load swith type ID’clic) on circuits to bathrooms and shower rooms (lighting, heating and socket-outlets) or a 30 mA differential circuit-breaker per circuit
5. 300 mA RCD (for example differential load swith) on all the other circuits
Fig. Q7: Installation with incoming-supply circuit-breaker having no differential protection
