The electrical tests
8.4.1 - Protective conductor continuity
All protective and bonding conductors must be tested to ensure that they are electrically safe and correctly connected. Provided that the supply is not yet connected, it is permissible to disconnect the protective and equipotential conductors from the main earthing terminal to carry out testing. Where the mains supply is connected, as will be the case for periodic testing, the protective and equipotential conductors must not be disconnected because if a fault occurs these conductors may rise to a high potential above earth. In this case, an earth-fault loop tester can be used to verify the integrity of the protective system.
Where earth-fault loop impedance measurement of the installation is carried out, this will remove the need for protective conductor tests because that conductor forms part of the loop. However, the loop test cannot be carried out until the supply is connected, so testing of the protective system is necessary before supply connection, because connection of the supply to an installation with a faulty protective system could lead to danger.
There are three methods for measurement of the resistance of the protective conductor.
1. - Using the neutral conductor as a return lead
A temporary link is made at the distribution board between neutral and protective conductor systems. Don't forget to remove the link after testing. The low resistance tester is then connected to the earth and neutral of the point from which the measurement is taken (see {Fig 8.2}). This gives the combined resistance of the protective and neutral conductors back to the distribution board. Then
| Rp = R x | An |
| An + Ap |
| where |
| Rp - is the resistance of the protective conductor |
| R - is the resistance reading taken |
| An - is the cross-sectional area of the neutral conductor |
| Ap - is the cross-sectional area of the protective conductor. |
Note that the instrument reading taken in this case is the value of the resistance R1 + R2 calculated from This method is only valid if both conductors have the same length and both are copper; in most cases where steel conduit or trunking is not used as the protective conductor, the test will give correct results.
Fig 8.2 - Protective conductor continuity test using the
neutral conductor as the return lead
2. - Using a long return lead
This time a long lead is used which will stretch from the main earthing terminal to every point of the installation.
First, connect the two ends of this lead to the instrument to measure its resistance. Make a note of the value, and then connect one end of the lead to the main earthing terminal and the other end to one of the meter terminals.
Second, take the meter with its long lead still connected to the point from which continuity measurement is required, and connect the second meter terminal to the protective conductor at that point.
The reading then taken will be the combined resistance of the long lead and the protective conductor, so the protective conductor value can be found by subtracting the lead resistance from the reading.
| Rp = R - RL |
| where |
| Rp - is the resistance of the protective conductor |
| R - is the resistance reading taken |
| RL is the resistance of the long lead |
Some modem electronic resistance meters have a facility for storing the lead resistance at the touch of a button, and for subtracting it at a further touch.
3. - Where ferrous material forms all or part of the protective conductor
There are some cases where the protective conductor is made up wholly or in part by conduit, trunking, steel wire armour, and the like. The resistance of such materials will always be likely to rise with age due to loose joints and the effects of corrosion. Three tests may be carried out, those listed being of increasing severity as far as the current-carrying capacity of the protective conductor is concerned. They are:
1 - A standard ohmmeter test as indicated in 1 or 2 above. This is a low current test which may not show up poor contact effects in the conductor. Following this test, the conductor should be inspected along its length to note if there are any obvious points where problems could occur.
2 - If it is felt by the inspector that there may be reasons to question the soundness of the protective conductor, a phase-earth loop impedance test should be carried out with the conductor in question forming part of the loop.
3 - If it is still felt that the protective conductor resistance is suspect, the high current test using 1.5 times the circuit design current (with a maximum of 25 A) may be used (see {Fig 8.3}. The protective circuit resistance together with that of the wander lead can be calculated from:
| voltmeter reading (V) |
| ammeter reading (A) |
Fig 8.3 - High-Current ac test of a protective conductor
Subtracting wander lead resistance from the calculated value will give the resistance of the protective system.
The resistance between any extraneous conductive part and the main earthing terminal should be 0.05 Ohms or less; all supplementary bonds are also required to have the same resistance.
8.4.2 - Ring final circuit continuity
The ring final circuit, feeding 13 A sockets, is extremely widely used, both in domestic and in commercial or industrial situations. It is very important that each of the three rings associated with each circuit (phase, neutral and protective conductors) should be continuous and not broken. If this happens, current will not be properly shared by the circuit conductors. {Fig 8.4} shows how this will happen. {Fig 8.4(a)} shows a ring circuit feeding ten socket outlets, each of which is assumed to supply a load taking a current of 3 A. In simple terms, current is then shared between the conductors, so that each could have a minimum current carrying capacity of 15 A. {Fig 8.4(b)} shows the same ring circuit with the same loads, but broken between the ninth and tenth sockets. It can be seen that now one cable will carry only 3 A whilst the other (perhaps with a current rating of 20 A) will carry 27 A. The effect will occur in any broken ring, whether simply one live conductor or both are broken.
Fig 8.4 - illustrating the danger of a break in a ring final circuit
a) unbroken ring with correct current sharing
b) broken ring with incorrect current sharing
It is similarly important that there should be no 'bridge' connection across the circuit. This would happen if, for example, two spurs from different points of the ring were connected together as shown in {Fig 8.5}, and again could result in incorrect load sharing between the ring conductors.
The tests of the ring final circuit will establish that neither a broken nor a bridged ring has occurred. The following suggested test is based on the Guidance Note on Inspection and Testing issued by the IEE.
Fig 8.5 - A 'bridged' ring final circuit
Test 1
This test confirms that complete rings exist and that there are no breaks. To complete the test, the two ends of the ring cable are disconnected at the distribution board. The phase conductor of one side of the ring and the neutral from the other (P1 and N2J are connected together, and a low resistance ohmmeter used to measure the resistance between the remaining phase and the neutral (P2 and Ni). {Figure 8.6} shows that this confirms the continuity of the live conductors. To check the continuity of the circuit protective conductor, connect the phase and CPC of different sides together (P1 and E2) and measure the resistance between phase and CPC of the other side (P2 and El). The result of this test will be a measurement of the resistance of live and protective conductors round the ring, and if divided by four gives (Ri + R2) which will conform with the values calculated from
Fig 8.6 - Test to confirm the continuity of a ring final circuit
Test 2
This test will confirm the absence of bridges in the ring circuit, see {Fig 8.7}. First, the phase conductor of one side of the ring is connected to the neutral of the other (P1 and N2) and the remaining phase and neutral are also connected together (P2 -and Ni). The resistance is then measured between phase and neutral contacts of each socket on the ring. If the results of these measurements are all substantially the same (within 0.05 Ohms), the absence of a bridge is confirmed. If the readings are different, this will indicate the presence of a bridge or may be due to incorrect connection of the ends of the ring. If they are connected P1 to NI and P2 to N2 then readings will increase or reduce as successive measurements round the ring are taken, as is the case where a bridge exists. Whilst this misconnection is easily avoided when using sheathed cables, a mistake can be made very easily if the system consists of single-core cables in conduit. It may be of interest to note that the resistance reading between phase and neutral outlets at each socket should be one quarter of the phase/neutral reading of Test 1.
Measurements are also taken at each socket on the ring between the phase and the protective conductor with the temporary connection made at the origin of the ring between P1 to E2 and between P2 to El. Substantially similar results will indicate the absence of bridges.
Fig 8.7 - Test to confirm the absence of bridges in a ring final circuit
8.5.1 - Testing insulation resistance
A low resistance between phase and neutral conductors, or from live conductors to earth, will result in a leakage current. This current could cause deterioration of the insulation, as well as involving a waste of energy which would increase the running costs of the installation. Thus, the resistance between poles or to earth must never be less than half of one meg ohm (0.5 M Ohms) for the usual supply voltages. In addition to the leakage current due to insulation resistance, there is a further current leakage in the reactance of the insulation, because it acts as the dielectric of a capacitor. This current dissipates no energy and is not harmful, but we wish to measure the resistance of the insulation, so a direct voltage is used to prevent reactance from being included in the measurement. Insulation will sometimes have high resistance when low potential differences apply across it, but will break down and offer low resistance when a higher voltage is applied. For this reason, the high levels of test voltage shown in {Table 8.8} are necessary.
Before commencing the test it is important that:
1. - electronic equipment which could be damaged by the application of the high test voltage should be disconnected. Included in this category are electronic fluorescent starter switches, touch switches, dimmer switches, power controllers, delay timers, switches associated with passive infrared detectors (PIRs), RCDs with electronic operation etc. An alternative to disconnection is to ensure that phase and neutral are connected together before an insulation test is made between them and earth.
2. - capacitors and indicator or pilot lamps must be disconnected or an inaccurate test reading will result.
Table 8.8 - Required test voltages and minimum resistance |
||
| Nominal circuit voltage |
Test voltage (V) |
Minimum insulation resistance (M Ohms) |
| Extra-low voltage circuits supplied from a safety transformer | 250 |
0.25 |
| Up to 500 V except for above | 500 |
0.5 |
| Above 500 V up to 1000 V | 1000 |
1.0 |
| The insulation resistance tester must be capable of maintaining the required voltage when providing a steady state of current of 1mA. | ||
Where any equipment is disconnected for testing purposes, it must be subjected to its own insulation test, using a voltage which is not likely to result in damage. The result must conform with that specified in the British Standard concerned, or be at least 0.5 M Ohms if there is no Standard.
The test to earth {Fig 8.10} must be carried out on the complete installation with the main switch off, with phase and neutral connected together, with lamps and other equipment disconnected, but with fuses in, circuit breakers closed and all circuit switches closed. Where two-way switching is wired, only one of the two strapper wires will be tested. To test the other, both two-way switches should be
Fig 8.11 - Insulation tests between poles
operated and the system retested. If desired, the installation can be tested as a whole, when a value of at least 0.5 M Ohms should be achieved, see {Fig 8.10}. In the case of a very large installation where there are many earth paths in parallel, the reading would be expected to be lower. If this happens, the installation should be subdivided and retested, when each part must meet the minimum requirement.
The tests to earth {Fig 8.10} and between poles {Fig 8.11} must be carried Out as indicated, with a minimum acceptable value for each test of 0.5 M Ohms. However, where a reading of less than 2 M Ohms is recorded for an individual circuit, (the minimum value required by the Health and Safety Executive), there is the possibility of defective insulation, and remedial work may be necessary. A test result of 2 M Ohms may sometimes be unsatisfactory. If such a reading is the result of a re-test, it is necessary to consult the data from previous tests to identify deterioration. A visual inspection of cables to determine their condition is necessary during periodic tests; perished insulation may not always give low insulation readings
As indicated above, tests on SELV and PELV circuits are carried out at 250 V. However tests between these circuits and the live conductors of other circuits must be made at 500 V. Tests to earth for PELV circuits are at 250 V, whilst FELV circuits are tested as LV circuits at 500 V. Readings of less than 5 M will require further investigation.
8.5.2 - Tests of non-conducting floors and walls
Where protection against indirect contact is provided by a non-conducting location, the following requirements apply.
1. - there must be no protective conductors
2. - if socket outlets are used they must not have an earthing contact
3. - it should be impossible for any person to touch two exposed conductive parts at the same time
4. - floors and walls must be insulating.
To test this last item and so to make sure that the floors and walls are non-conducting, their insulation has to be tested.
The requirements are shown in {Fig 8.12}, the electrodes used for making contact
Fig 8.12 - Insulation test of floors and walls for non-conducting location
with floors and walls being a special type which are pressed onto the surface with a force of not less than 750 N (77 kg or 169 lb) for floors or 250 N (26 kg or 56 lb) for walls. The resulting insulation resistance of not less than three points on each surface, one of which must be between 1 m and 1.2 m from an extraneous conductive part (if there is one), measured at 500 V, must not be less than 0.5 MΩ. Attention is drawn to the natural reduction in the insulation resistance of a surface as humidity increases. Where insulation is applied to an extraneous conductive part to provide a non-conducting location, this insulation must be tested with an alternating p.d. of 2 kV. In normal use, the leakage current should not exceed 5 mA.
8.5.3 - Tests of barriers and enclosures
Throughout the Regulations reference is made to the use of barriers and enclosures to prevent contact with live parts (direct contact). If manufactured equipment's comply with the British Standards concerned, they will not need further testing, but where barriers and enclosures have been provided during erection of the installation, they must be tested. The two most common tests are for:
1. - IP2X - no contact can be made with a probe 12 mm in diameter and 80 mm long
- in other words, a human finger
2. - IP4X - no contact can be made with a rod of diameter 1 mm.
8.5.4 - Tests for electrical separation of circuits
This section is concerned with tests necessary to ensure the safety of Separated Extra-Low Voltage (SELV), Protective Extra-Low Voltage (PELV) and Functional Extra-Low Voltage (FELV) circuits. In general, the requirement is a thorough inspection to make sure that the source of low voltage (most usually a safety isolating transformer) complies in all respects with the British Standard concerned, followed by an insulation test between the extra-low voltage and low voltage systems. The test is unusual in that a 500 V dc supply (from an insulation resistance tester) must be applied between the systems for one minute, after which the insulation resistance must not be less than 5 MΩ for SELV or PELV systems, or 0.5 MΩ for FELV systems. A further test at 3750 V dc for one minute is passed if no flashover occurs. This test in particular can be dangerous, and special care should be taken.
For SELV and FELV circuits, additional inspection must ensure that the low voltage requirements (not exceeding 50 V ac or 120 V dc) are met. If the voltage exceeds 25 V ac or 70 V dc (60 V ripple-free), barriers and enclosures must be tested to IP2X and a 500 V dc insulation test applied for one minute between the live conductors and metal foil wrapped round the insulation should produce a result of at least 0.5 MΩ
When insulation testing on electrically isolated circuits or on equipment which might be damaged by the test voltage, phase and neutral must be connected together and the test applied between them and earth.
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To achieve compliance with the legal requirements of the Electricity at Work Regulations 1989 requires proof that an electrical system is safe, which involves amongst other things, proper inspection and testing of a system by competent people and the creation and maintenance of records.
Electricity at Work Regulations 1989 is law in the United Kingdom.
No person shall be engaged in any work activity where technical knowledge or experience is necessary to prevent danger or, where appropriate, Injury, unless he possesses such knowledge or experience, or is under such degree of supervision as may be appropriate having regard to the nature of the work.