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ELECTRICAL TESTINGElectrical Power & Lighting Installation

ELECTRICAL TESTING

Before a completed installation may be connected to the supply a number of tests are required to indicate the general condition of the installation, both with regard to the installation resistance of the conductors and other current-carrying parts and with regard to the conductance of the earthing system. The tests which are to be made are not a complete guarantee of the quality of the installation for all time, and regular testing is necessary in order that it may be maintained in a proper condition throughout its life.

The basic tests for an installation are:-1. Insulation test2. Continuity of ring circuits Test3. Continuity of Circuit Protective Conductor (CPC)4. Polarity test5. Earth-fault loop impedance test6. Operation of Residual Current Devices

INSULATION TESTThe purpose of this test is to ensure that the insulation of the conductors, accessories and equipment is satisfactory. The instrument used for this test is an insulation resistance tester. The minimum insulation resistance required for an installation is 1 MΩ.

There are three insulation resistance tests that carried out:-1. Insulation Resistance between conductors2. Insulation Resistance between conductors and earth3. Insulation resistance of disconnected apparatus.

Before these tests are carried out, it is essential to disconnect any neon indicator lamps and capacitors from the circuit because they are likely to affect test result. In addition, any control devices which contain semiconductors components must also be disconnected as they can be damaged by the test voltage.

Insulation Resistance between conductors This test is made between all the conductors connected to any one pole or phase of the supply, and all conductors connected to any other pole or phase of the supply. The insulation resistance is to be NOT less than 1 MΩ.

All lamps should be removed, all current-using apparatus disconnected and all local switches controlling lamps or apparatus closed. When the removal of lamps and apparatus not practicable, all local switches should be open.

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Figure 1: The connections for the test of a 2 wire installation.

Figure 2: The test for Insulation Resistance between conductors

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For 3-wire and 4-wire installations more than one between phased tests is necessary. A 3-wire direct current installation will require three tests:-

1. between positive line and neutral2. between negative line and neutral3. between positive and negative lines.

A 3-wire 3-phase installation will also require three tests:1. between lines R and Y2. between lines R and B3. between lines Y and B

A 3-phase 4-wire installation will require six tests:1. three separate tests between pairs of lines, R - Y, R - B and Y – B2. three separate tests, R-neutral, B- neutral and Y – neutral.

Figure 3 (a) to (d) : Simplified diagrams of tests between conductors

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Insulation Resistance between conductors and earthFor this test all fuses (or MCBs) should be in place, all switches closed (including the main switch) and all the poles or phase conductors connected together. The minimum acceptable value is 1 Mega ohm (1 MΩ).

Figure 4 shows how the testing insulation resistance of completed installation.

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Figure 5: The test for Insulation Resistance to earth.

Insulation resistance of disconnected apparatusThe tests include insulation test between conductors and between the conductors and earth. The minimum acceptable value is 0.5 Mega ohm (0.5 MΩ).

CONTINUITY TESTElectrical Power

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The purpose of this test is to checks a circuit for a connection from one end to the other. To have continuity indicates that the circuit is not open.

Continuity of ring circuits conductorsThe purpose of this test is to ensure that the circuit conductors of a ring final circuit form a ring without cross connections or incomplete loops. The test can be carried out using the ‘continuity tester (ohmmeter) having a range suitable for readings of very low resistance as 1 milliohm (1mΩ).

The test is made to ensure that the conductors (phase, neutrals and CPC) are electrically continuous and to make sure there has been no inadvertent interconnection between socket-outlets which would constitute a short circuit which in effect creates an apparently continuous ring circuit. In this situation an actual break in a conductor could exist without detection.

The test (as shown in figure 4) is carried out as follows:-

a. Measure the resistance of the conductor (e.g. phase) by separating its end at the consumer unit or distribution board. Note the reading (A)

b. Join the separated ends of the conductor and connect the instrument/meter to the join. The other terminal of the instrument/meter is then connected to the ‘L’ terminal in a socket-outlet which is judged to be midway in the ring circuit. Note the reading (B). this part of the test involves the use of the long test lead.

c. Measure the resistance of the long test lead. Note the reading (C)

d. Check that the readings A / 4 = B – C. If both sides of the equation are virtually equal then the continuity of the phase conductor is satisfactory.

e. Repeat the test for the neutral conductor and the CPC.

Example : Reading A = 0.4 ohm

Reading B = 0.2 ohm

Reading C = 0.1 ohm

Therefore 0.4 / 4 = 0.2 – 0.1

= 0.1 ohm.

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Figure 6: The test for continuity of ring circuit conductors

Continuity of Circuit Protective Conductor (CPC)

The purpose of this test is to ensure the protective conductors are continuous and capable of carrying the currents likely to flow under fault conditions. The test is carried out by using a voltage not exceeding 50V and passing a current not exceeding 25A into the conductor. The instrument used for testing cable conductors may be a low-reading ohmmeter as used for the ring circuit continuity testing,

The test is carried out by using a voltage not exceeding 50V and passing a current not exceeding 25A into the conductor. This simulates an earth-fault current and the level of current will show up any defects in the conductor and its terminations. As shown above in figure 5.

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Figure 7: The test for continuity of the circuit protective conductors

POLARITY TESTThe purpose of this test to ensure that:-

i. the polarity is correct at the meter and consumer unit/distribution boardii. single-pole control and protective devices are connected in the phase conductor only.iii. conductors are correctly connected to socket-outlets and other accessories / equipmentiv. centre-contact bayonet and Edison-type screw lamp-holders have their outer or screwed

contacts connected to the earthed neutral conductorv. all multi-pole devices are correctly installed.

Tests are carried out with all switches closed, lamps and equipment removed. The test instrument can be a continuity tester (Ohmmeter). As shown below in figure 8

Figure 8: The test for correct polarity on Edison screw lampholder

EARTH FAULT LOOP IMPEDANCE

Where protective measures are used which require 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:-

1. at the origin of each installation and at each Distribution Board

2. fixed equipment and socket-outlets

3. 10% (on a random basis) of all luminaries, with a minimum of one luminary, preferably the further one from the consumer units, for each circuit of any building

4. 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.

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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. OPERATION OF RESIDUAL CURRENT DEVICES

These devices must be checked for correct operation under simulated fault conditions, where they have been installed to provide protection from indirect contact and to check the RCD is operational at frequent intervals. The Wiring Regulations require that an RCD be tested so that it trips within the required time.

An RCD with a rated tripping current of 30 mA should be trip within 0.2 second. RCD testers are available which give a choice of tripping current, typically ranging from 5 mA to 500mA and which also display the actual tripping time. The higher the value of tripping current, the less becomes the time of operation. It should be noted that before the test is applied, all loads normally supplied through the RCD should be connected.

CERTIFICATION

The Wiring Regulations require that the following the inspection and testing of an installation, Completion and inspection Certificates should be issued and signed by, or on behalf of, the contractor and given to the client as evidence the procedure has been carried out. These certificates then form part of the documentation relating to the installation and must be available to the person conducting subsequent tests on the installation.

PERIODIC INSPECTION AND TESTING

The Wiring Regulation requires that installation be inspected and tested at regular intervals as follows:-

General installations - 5 years (maximum)

Temporary installations on Construction sites - 3 months (maximum)

Caravan sites - 1 year

Agricultural installations - 3 years (maximum)

No installation is complete until it has been inspected and then tested. In general a visual inspection is carried out to ensure that certain points (e.g. size of conductors, presence of barriers, labeling, etc) comply with the requirements of the regulations. Instrument tests ate then applied to verify the safety of the installations, with regard to such details as effectiveness of earthing and insulation.When carrying out these tests it is important to have all design specifications, plans and schedules available.

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