London Electrical Inspection & Testing

Tests Carried Out

Continuity of Protective Conductors and Equipotential Bonding conductors

If any electrical installation can be isolated from the supply it is permissible to disconnect the protective and equipotential conductors from the main earthing terminal in order to verify their continuity. The sequence of operation needed for initial testing can be carried out safely on an existing installation if it is isolated from the supply.

Where an electrical installation cannot be isolated from the supply, the protective equipotential conductors must NOT be disconnected as, under fault conditions, the exposed and extraneous-conductive-parts could be raised to a dangerous level above earth potential. The 'combined' integrity of the conductors shall be established by continuity/earth fault loop impedance tests. The former to establish continuity of conductors and the latter to establish and confirm an appropriate disconnection time in the event of an earth fault.

Polarity

Test shall be made to verify that:-

the polarity is correct at the meter and consumer unit/distribution board
single-pole control and protective devices are connected in the phase conductor only
conductors are correctly connected to socket-outlets and other accessories/equipment
centre-contact bayonet and Edison-type screw lamp-holders have their outer or screwed contacts connected to the earthed neutral conductor
all multi-pole devices are correctly installed

Earth Fault Loop Impedance

Where protective measures are used which require a knowledge of earth fault loop impedance, the relevant impedance shall be measured, or determined by an equally effective method.

Earth fault loop impedance tests are carried out at the locations indicated below:-

at the origin of each installation and at each Distribution Board
fixed equipment and socket-outlets
10 per cent (on a random basis) of all luminaires, with a minimum of one luminaire, preferably the further one from the consumer units, for each circuit of any building
at any location which may be exposed to exceptional damage or deterioration or represent a special hazard at the furthest point of every radial circuit

Where the installation incorporates an R.C.D., the value of earth fault loop impedance obtained in the test should be related to the nominal residual operating device.

Insulation Resistance

Insulation resistance tests should be made on a dead circuit and any electronic equipment which might be damaged by application of the test voltage must be disconnected or isolated. The insulation resistance should be measured between each live conductor and earth and should not be less than 0.5 megohm. Where practicable the tests are applied to the whole of the installation with all fuse links in place and all switches closed. Alternatively, the installation may be tested in parts.

Operation of Devices for Isolation and Switching

Where means are provided in accordance with the requirements of the Regulations for isolation and switching, a 10 per cent sample shall be operated to verify their effectiveness and checked to ensure adequate and correct labeling.

Operation of Residual Current Devices (RCD’s)

Where protection is provided by an R.C.D., the effective operation of each R.C.D. shall be verified by a test simulating an appropriate fault condition independent of any test facility incorporated in the device, followed by operation of the integral test device. The nominal rated tripping current for protection of a socket-outlet for use with equipment outdoors or physically installed outside the general installation equipotential zone must not exceed 30mA (0.03 amps).

In certain situations such as high risk areas, 10mA (0.01 amps) RCD may be appropriate for the particular installation.

Operation of Over-Current Circuit-Breakers

Where protection against over current is provided by circuit-breakers, the manual operating mechanism of each circuit breaker will be operated to verify that the device opens and closes satisfactorily.

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