Recommended Initial Frequencies of Inspection of Electrical Installations.

Portable Appliance Testing (P.A.T.)

A portable appliance is classed as any piece of equipment which can be connected to the common supply by means of a plug and socket arrangement. It must be maintained to be safe at all times. In effect any electrical equipment that has a mains plug on it.

While performing the tests we normally repair or relace damaged plugs and fuses. Our test equipement is always in calibration and is suitable for IT work.

Once the testing is complete you will have :

• An asset list of all P.A.T. equipement
• All the test data for any local authority requirement
• A list of all Failures
  
• This data is available (members area) securely, both viewable and printable for our customers who have an account on this website.
• You will automatically receive a reminder when retesting is due. Timescale changes with individual clients.
  

Electrical Installation Testing

All aspects of the electrical installation are inspected and tested for strength, integrity and compliance with the IEE Regulations.

Any faults or defects found will be recorded, and a suggested method for correcting the fault will be given to you. Upon completion of the tests a complete schedule of the tests will include the following.

• Periodic inspection certificate
• Detailed circuit assessment sheets
• Faults discovered while surveying
  
• Survey report specific faults, locations & suggested solutions
• Laminated circuit schedules, as required
• Computer generated drawings
  
• Dangerous conditions report

Electrical Testing for Safety

8.6.1 - Testing earth electrodes

The earth electrode, where used, is the means of making contact with the general mass of earth. Thus it must be tested to ensure that good contact is made. A major consideration here is to ensure that the electrode resistance is not so high that the voltage from earthed metalwork to earth exceeds 50 V. Where an RCD is used, this means that the result of multiplying the RCD operating current (in amperes) by the electrode resistance (in ohms) does not exceed 50 (volts). for normal dry locations, or 25 (volts) for construction sites and agricultural premises.

There are several methods for measurement of the earth electrode resistance. In all cases, the electrode must be disconnected from the earthing system of the installation before the tests commence.

Fig 8.13 - Measurement of earth electrode resistance with a dedicated tester

1. - Using a dedicated earth resistance tester

The instrument is connected as shown in {Fig 8.13} with terminals C1 and P1 being connected to the electrode under test (X). To ensure that the resistance of the test leads does not affect the result, separate leads should be used for these connections. If the test lead resistance is negligible, terminals Ci and P1 may be bridged at the instrument and connected to the earth electrode with a single lead.

Terminals C2 and P2 are connected to temporary spikes which are driven into the ground, making a straight line with the electrode under test. It is important that the test spikes are far enough from each other and from the electrode under test. If their resistance areas overlap, the readings will differ for the reason indicated in {Fig 8.14}. Usually the distance from X to Y will be about 25 m, but this depends on the resistivity of the ground. To ensure that resistance areas do not overlap, second and third tests are made with the electrode Z 10% of the X to Y distance nearer to, and then 10% further from, X. If the three readings are substantially in agreement, this is the resistance of the electrode under test. If not, test electrodes Y and Z must be moved further from X and the tests repeated.

The tester provides an alternating output to prevent electrolytic effects. If the resistance to earth of the temporary spikes Y and Z is too high, a reduction is likely if they are driven deeper or if they are watered.

Fig 8.14 - Effect of overlapping resistance areas
a) resistance areas not overlapping
b) resistance areas overlapping

2. - Using a transformer, ammeter and voltmeter

The system is connected as shown in {Fig 8.15}. Current, which can be adjusted by variation of the resistor R, is passed through the electrode under test (X) to the general mass of earth and hence to the test electrode Y. The voltmeter connected from X to Z measures the volt drop from X to the general mass of earth. The electrode resistance is calculated from:

As in the case of the dedicated tester, the test electrode Z must again be moved and extra readings taken to ensure that resistance areas do not overlap. It is important that the voltmeter used has high resistance (at least 200 Ohms /V) or its low resistance in parallel with that of the electrode under test will give a false result.

Fig 8.15 - Measurement of earth electrode resistance
with a transformer, ammeter and voltmeter

3. - Using an earth fault loop impedance tester

The tester is connected between the phase at the origin of the installation and the earth electrode under test as shown in {Fig 8.16}. The test is then carried out, the result being taken as the electrode resistance although the resistance of the protective system from the origin of the installation to the furthest paint of the installation must be added to it before its use to verify that the 50 V level is not exceeded. If an RCD with a low operating current is used, the protective system resistance is likely to be negligible by comparison with the permissible electrode resistance.

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

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.

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:

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.

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

Domestic Electrical Safety Testing to BS7671 (16th Editition)

There is little point in setting up Regulations to control the way in which electrical installations are designed and installed if it is not verified that they have been followed. For example, the protection of installation users against the danger of fatal electric shock due to indirect contact is usually the low impedance of the earth-fault loop; unless this impedance is correctly measured. this safety cannot be confirmed. in this case the test cannot be carried out during installation, because part of the loop is made up of the supply system which is not connected until work is complete.

In the event of an open circuit in a protective conductor, the whole of the earthed system could become live during the earth-fault loop test. The correct sequence of testing would prevent such a danger, but the tester must always be aware of the hazards applying to himself and to others due to his activities. Testing routines must take account of the dangers and be arranged to prevent them. Prominent notices should be displayed to indicate that no attempt should be made to use the installation whilst testing is in progress.

The precautions to be taken by the tester should include the following:

1. - make sure that all safety precautions are observed

2. - have a clear understanding of the installation, how it is designed and how it has been installed

3. - make sure that the instruments to be used for the tests are to the necessary standards (BS 4743 and BS 5458) and have been recently recalibrated to ensure their accuracy

4. - check that the test leads to be used are in good order, with no cracked or broken insulation or connectors, and are fused where necessary to comply with the Health and Safety Executive Guidance Note GS38

5. - be aware of the dangers associated with the use of high voltages for insulation testing. For example, cables or capacitors connected in a circuit which has been insulation tested may have become charged to a high potential and may hold it for a significant time.

8.2.1 - Notices and other identification

The installation tester, as well as the user, must have no difficulty in identifying circuits, fuses, circuit breakers, etc. Re must make sure that the installation is properly equipped with labels and notices, which should include:

I. - Labels for all fuses and circuit breakers to indicate their ratings and the circuits protected

2. - Indication of the purpose of main switches and isolators

3. - A diagram or chart at the mains position showing the number of points and the size and type of cables for each circuit, the method of providing protection from direct contact and details of any circuit in which there is equipment such as passive infra-red detectors or electronic fluorescent starters vulnerable to the high voltage used for insulation testing.

4. - Warning of the presence of voltages exceeding 250 V on an equipment or enclosure where such a voltage would not normally be expected.

5. - Warning that voltage exceeding 250 V is present between separate pieces of equipment which are within arm's reach

6. - A notice situated at the main intake position to draw attention to the need for periodic testing

7. - A warning of the danger of disconnecting earth wires at the point of connection of:

a). - the earthing conductor to the earth electrode

b). - the main earth terminal, where separate from main switchgear

c). - bonding conductors to extraneous conductive parts The notice should read Safety electrical connection - do not remove

8. - A notice to indicate the need for periodic testing of an RCD.

9. - A notice for caravans so as to draw attention to the connection and disconnection procedure as indicated in

10. - Warning of the need for operation of two isolation devices to make a piece of equipment safe to work on where this applies

11. - A schedule at each distribution board listing the items to be disconnected (such as semiconductors) so that they will not be damaged by testing.

12. - A drawing which shows clearly the exact position of all runs of buried cables.