Testing Electrical Installations

Testing electrical installations can be considered from three aspects: first there is the preliminary internal testing that must be carried out when an installation is completed, and before it is made alive; secondly, there are the official tests that are carried out by the electricity board before the installation is connected to the mains; and finally there is the testing necessary to find faults, or to ascertain the condition of an old installation.

Equipment needed

They comprise: (1) A universal test meter. (2) A bell and battery set for tracing wires. (3) A ‘Megger’, combined with a protective conductor continuity tester.

The universal test meter

The universal test meter is a device whereby a single scale can read volts, amperes and ohms, by adjusting the knob with appropriate markings. An instrument of this kind can be invaluable for locating faults. It must be capable of reading up to 500 V a.c. or d.c., and the resistance range should be capable of reading down to less than one ohm.

The resistance range on such meters is operated by means of a small dry battery, inserted into the instrument in a similar fashion to that employed in portable transistorised radio receivers. To allow for variations in battery voltage, an adjusting knob is provided. When the two leads from the instrument are clipped together, obviously the resistance between them is practically zero, and the adjusting knob is moved until the needle reads zero. This adjustment should be carried out before every set of resistance tests is undertaken.

The bell and battery set

For rapid checking of circuit connections, a bell and battery set is extremely useful. Two torch batteries are connected to a bell, and the assembly is connected to two probes or clip leads, so that when the leads are joined together the circuit will be completed and the bell will ring. A long lead will enable circuits in all parts of a domestic installation to be checked out rapidly, or ‘rung out’.

It must be remembered that a ‘ringing out’ test does not prove that the circuits are properly insulated, either between conductors or to earth, but only that there is a continuous conductor path from one point to another.

The ‘Megger’ test meter

The ‘Megger’ is a patented instrument which takes the form of a hand-driven generator that provides a voltage of 500 V at the terminals. It is used for measuring the insulation resistance of the installation.

The ‘universal’ type of instrument, mentioned earlier, is entirely unsuitable for insulation testing, except for rough preliminary checks. Such an instrument relies on a 1.5 V battery, and one can imagine many parts of an electrical installation where bad workmanship or defective fittings or appliances have resulted in two bare wires, phase and neutral, being wrongly situated, and so lying within a hair’s breadth of each other. This gap will still show perfect insulation between them if a testing instrument using only 1.5 V is applied, and the result will be misleading and a dangerous situation on the installation may not be revealed. But the 500 V output of the ‘Megger’ will break down such a gap, and the fault will be revealed.

The ‘Megger’ has a scale that reads in thousands of ohms or in megohms, and it is so arranged that once the turning of the handle has reached a certain speed, no increase in speed will affect the reading.

Some types of ‘Megger’ have a second instrument incorporated in the same case. This instrument measures the impedance of the earth loop at any point in the installation.

With alternating current, as mentioned earlier, there are factors other than pure resistance that affect the amount of current flowing through a circuit. Where coils are concerned, the electromagnetic effect may mean that there is greater opposition to the passage of the current than that due to resistance. The resulting combined opposition to the passage of current is called impedance, a term applicable to a.c. circuits only.

The impedance of the earth loop – that is, of all the conductors used to carry earth current from any one point on the installation to the general mass of earth – includes the resistance of the protective conductors in the installation, the resistance of the earth electrode or other earth connection to the general mass of earth, and the electromagnetic effects mentioned earlier.

In the section on earthing, in a preceding post, we have seen that this earth loop impedance must not be greater than a certain value which depends on the type of protection provided. To check that it is below this impedance, a suitable instrument must be used.

Preliminary testing

These tests are not intended to be the ‘official tests’ of the installation, and are only intended as a guide to the condition of the wiring.

All preliminary testing is carried out before the main fuses have been inserted.

Continuity test

The first test is a simple continuity test to ensure that all connections have been properly made. The bell set can be used for this, and also to check the proper connection of the switches, thermostats, time switches and other devices, to ensure that they are all on the phase side of the circuit. This can be tested by checking continuity from the neutral connection of the circuit being tested, at the consumer unit right through the thermostat, switch, or other interrupting device, making sure that the switch breaks the phase wire and not the neutral wire.

Polarity test

Next there is the question of polarity – socket-outlets having their connections made the right way round. On the ordinary 13 A fused plug socket-outlet, looking at the face of the socket-outlet, the phase connection must be on the right, the neutral on the left and the earth at the top. A simple continuity check from each socket back to the phase fuse will ensure that this is correct.

Protective conductor test

The next step is to check protective conductor continuity. For this purpose, an approximate check that no gross errors have occurred can be made by means of the universal testing instrument, arranged on its ohm-reading scale.

It will be recalled from the section on earthing, that all exposed metal that can possibly become connected to a live circuit must be effectively earthed, in such a way that the resistance of the protective conductor is of low enough value, to carry fault currents without danger. As a guide, the resistance of the protective conductor should not be greater than 0.5 Q, or 1Q if the conductor is copper.

With one lead of the testing instrument on the earth point, and the other arranged as a probe that can be attached to all parts of the fixed appliances, or the portable equipment plugged into the socket-outlets, the protective conductor resistance can be checked with the universal instrument.

However, this cannot be taken as the full, official test. The reason for this again hinges on the fact that the universal instrument only has a 1.5 V battery. Suppose a connection was badly made, so that the wires were simply touching each other, very lightly. This would mean that a 1.5 V battery would show a good connection, but if a heavy current should pass through this circuit, the connection would obviously not be good enough: arcing and ultimate melting of the surfaces in contact would occur, with consequent fire danger.

In fact, the Regulations specify that when testing the protective conductor, alternating current of a magnitude approaching one and a half times the rating of the circuit under test shall be used, with a maximum of 25 A. As mentioned above, only a properly designed instrument can provide a test current of an appropriate value to make sure that the slightest defects in the protective conductor system are observed and can be detected.

Insulation test

This test ensures that the insulation throughout the whole installation has not been damaged in any way, and for low voltage circuits, that is up to 1000V a.c, the test has to be carried out, according to the Regulations, with a direct current voltage not less than twice that which will be normally applied to the installation, although it need not exceed 500 V, for installations rated up to 500 V.

Again, the universal type of instrument would not be suitable, even using its resistance scale. If, for example, the wire running through a conduit had been strained over a sharp edge incorrectly left at the end of a tube, the bare conductor might be within a tenth of a millimetre of the earthed metal, and yet the 1.5 V output of the instrument would not break down this gap and reveal the defect.

The insulation resistance is measured on each circuit by closing all switches on all appliances in circuit, and with the neutral connection disconnected. Under these conditions the circuit will be complete throughout the phase wire, the switch, the appliance, and the neutral wire back to the consumer unit.

The universal tester again may be used to provide a rough check. One lead should be clipped to the official earth point to a water pipe or other convenient earth, and the other lead to the phase wire. The reading on the ohm scale of the instrument should be of the order of 1 or 2MQ. This test is only suitable for finding major errors, such as the live wire firmly touching the protective conductor connection in a fitting or connection box.

But it must be emphasised that the only conclusive and official test is that carried out with the 500 V ‘Megger’.

The bare minimum insulation resistance acceptable under the regulations is 1MQ, applying to the complete installation. This means that when all the phase wires at the consumer unit are connected together and to the testing instrument, all switches closed, all appliances inserted in the circuit, all neutral wires being left disconnected, and the other end of the ‘Megger’ is connected to earth, then there is a minimum of 1MQ between the whole of the installation taken together, and earth. A higher value should always be aimed at, and a good installation might well have an insulation resistance of well over 5M£2.

A second insulation test should be carried out between conductors ; for the purpose of this test all lamps and appliances are removed, or isolated by opening their local switch, and the test instruments’ leads connected to the phase conductor and the neutral conductor, the minimum reading being 1MQ. —6—

If for the purpose of either of the above tests, equipment is removed, then it should be tested in a similar manner separately, and in this case a minimum reading of ViMQ is permissible.

Official testing

The electricity board will test the installation for insulation resistance, for protective conductor continuity, for earth loop impedance back to the earthed neutral on the supply transformer, for correct connection of the final earthing point, and for adequate fusing. They will also check the quality of the wiring, and if an earth leakage circuit-breaker has been installed they will check the operation of this device, and the earth resistance of the electrode used.

They are not concerned with the internal connections of the installation itself and it is not their duty to see that the proper lamps light when the switch is closed, or that appliances such as water heaters and the like, installed by the electrician, operate correctly. They are, however, concerned with the safety aspect of ensuring that all socket-outlets are connected up the right way round, and this point will therefore be checked by the electricity board’s engineers.

They will also check that there are no socket-outlets in bathrooms, and that other safety measures have been observed.

If off-peak storage heating circuits are installed, a check will be made to ensure that these circuits are all terminated directly on the storage heaters, and that no other socket-outlets or other connections exist on these circuits.

Tracing faults

Suppose now that the preliminary tests have revealed a fault on the system.

With the aid of a bell set, and a universal meter, the fault can be traced quite easily by splitting the installation up into sections.

Insulation fault

Let us suppose that the fault is an insulation fault – that means that the whole installation shows that the phase wire is in effect connected to earth, and therefore does not have the required insulation resistance of at least 1MQ with respect to earth.

The circuits comprising the installation can easily be separated out at the consumer unit. Taking one circuit at a time, disconnect the neutral wires and thus both ends of the circuit are clear of any other connection. Then, with all the switches closed and appliances connected, check each circuit under these conditions, until one is found on which there is a fault.

Concentrating on this circuit first the appliances may be disconnected, one by one, until a possible faulty appliance shows up. If no appliance is faulty then proceed as follows: Assume for the moment that there is only one fault in the circuit. Go to the first disconnection point on this circuit, and disconnect both wires.

Return to the consumer unit and once again check both the phase and neutral wires for insulation to earth. Suppose that they are both ‘good’, the fault has thus been removed by this disconnection. It is obviously beyond the point of disconnection.

Transfer the testing instrument to the first joint box, which has already been opened, and proceed further to the next joint box or switch, the next break point on the circuit, and disconnect the wires in this second joint box.

Test again. If the fault is cleared, proceed further down the circuit. If it is not cleared, then obviously it lies in the section between the testing point and the further point at which the circuit has been broken.

Suppose the fault is cleared, then proceed further and go beyond the switch which may perhaps bring you to some fittings or socket-outlets. These must be methodically disconnected, one by one, testing after each disconnection, until the faulty unit is isolated. It must then be inspected to find where the fault lies.

One of the most common causes of faults in domestic installations is that the wiring behind a socket-outlet or a switch has been incorrectly made off, so that either too much bare conductor exists, and is touching the earthed metal or the neutral wire, or, on the other hand, the switch or other fitting may be incorrectly assembled, possibly so that two wires are incorrectly inserted into one terminal.

Suppose the fault has been found to be in a run of cable. If the installation is carried out in conduit, it will not be very difficult to pull out the cable in that section and inspect it, and the fault will probably soon be found. It may well be due to the insulation being damaged as the cable was drawn into the conduit.

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If the installation is carried out in sheathed wiring embedded in plaster, then it will be necessary to break out the section containing the fault and replace with with new wire. It is unwise to attempt to locate the fault and to patch it up. New cable must be used.

Up to now it has been assumed that the fault is one of bad insulation. There are, however, two other kinds of faults.

Short circuit

First, there is a short circuit. Both the neutral and the phase wires may remain well insulated from earth, but are short-circuited to each other.

In the majority of cases this is due to incorrect connections in fuses, switches or fittings. It is unlikely that the wires within a conduit, or inside the sheath on a sheathed cable, have come into contact with each other without at the same time going down to earth, although this type of fault could not necessarily always be ruled out. A short circuit could occur if severe mechanical damage has occurred to sheathed cable, perhaps under a floor, where for example, a workman from another trade, such as a plumber fitting water pipes, or a gas fitter, has inadvertently severely manhandled the cable.

The short circuit can be isolated by the same methodical sectionalising test methods as those outlined above.

Open circuit

Another type of fault is the open circuit. Here again, in this case where there is no circuit between two points, circuit for the fan is connected to another, which is not part of the off-peak system, since the fan may be needed to increase the output when the off-peak power is not available.

Insufficient protection for sheathed wiring It is frequently found that when sheathed wiring is out of sight, it is also out of the electrician’s mind, in the sense that he has not provided any protection at all. It is just as important that sheathed wiring should be protected from mechanical damage in such places as lofts and cellars, which are infrequently used, as it is in cases where the wiring is obviously visible.

Conduit installation troubles

Insufficient attention is often given to the filing away of sharp edges on the ends of conduit piping, and the lack of the provision of bushes. This means that all cables drawn through this particular piping may well be damaged, and liable to subsequent failure.

Insufficient attention has often been paid to the protection of conduits, ducts, and trunking systems against the entry of water.

Incorrect cable for heating appliances

Heat-resistant insulated cable should always be used for connections to immersion heaters, thermal storage heaters and indeed any appliance that gets hot. It has often been found that ordinary PVC insulated cable is employed.

Short circuit

First, there is a short circuit. Both the neutral and the phase wires may remain well insulated from earth, but are short-circuited to each other.

In the majority of cases this is due to incorrect connections in fuses, switches or fittings. It is unlikely that the wires within a conduit, or inside the sheath on a sheathed cable, have come into contact with each other without at the same time going down to earth, although this type of fault could not necessarily always be ruled out. A short circuit could occur if severe mechanical damage has occurred to sheathed cable, perhaps under a floor, where for example, a workman from another trade, such as a plumber fitting water pipes, or a gas fitter, has inadvertently severely manhandled the cable.

The short circuit can be isolated by the same methodical sectionalising test methods as those outlined above.

Open circuit

Another type of fault is the open circuit. Here again, in this case where there is no circuit between two points, the fault is most likely to lie in a switch or other fitting. There have been cases where open circuits have arisen when fittings have been incorrectly made off, and the electrician, roughly baring the ends of the wire, has nicked through all the strands, and then when the tension comes on the ‘tail’ which has been inserted into a switch or socket-outlet or some other fitting as the unit is screwed together, the weakened wires break away, leaving an open circuit.

Cables that have been badly kinked before installation could give rise to open circuits, as the kinking may have broken the copper conductors.

The open circuit may not be so easy to find, under certain circumstances, as the other types of fault, but with the aid of a long test lead, it is not difficult to parallel each installed wire by an external lead across the gap between, say, two junction boxes, or the consumer unit and a fitting, and if a complete circuit is obtained in this way, shorten the test lead until the section or fitting in which there is an open circuit is found.

Typical faults in installations

Incorrect system of fusing

Where the normal supplies of phase and neutral are concerned, double-pole fusing has frequently been found. This is most often found in a final circuit where there is a circuit distribution board.

Proximity of different phases

Small business installations are frequently fed by a 3-phase system, in which a voltage of 415 V exists between wires connected to different phases. In general all the conductors in one room should be connected to the same phase. There is an exception that allows points between which a voltage exceeding 240 V is present to exist in the same room if they are 2 m or more apart, but as mentioned earlier where storage heaters are installed, very often it has been found that conductors of two different phases are used near together, since the storage heater is supplied independently from one circuit which may be connected to one phase, and the local control circuit for the fan is connected to another, which is not part of the off-peak system, since the fan may be needed to increase the output when the off-peak power is not available.

Insufficient protection for sheathed wiring It is frequently found that when sheathed wiring is out of sight, it is also out of the electrician’s mind, in the sense that he has not provided any protection at all. It is just as important that sheathed wiring should be protected from mechanical damage in such places as lofts and cellars, which are infrequently used, as it is in cases where the wiring is obviously visible.

Conduit installation troubles

Insufficient attention is often given to the filing away of sharp edges on the ends of conduit piping, and the lack of the provision of bushes. This means that all cables drawn through this particular piping may well be damaged, and liable to subsequent failure.

Insufficient attention has often been paid to the protection of conduits, ducts, and trunking systems against the entry of water.

Incorrect cable for heating appliances

Heat-resistant insulated cable should always be used for connections to immersion heaters, thermal storage heaters and indeed any appliance that gets hot. It has often been found that ordinary PVC insulated cable is employed.

Omission of any identification at the consumer unit If the circuits are not identified and the type of fuses that are appropriate for each circuit clearly stated, it will be much more difficult to put right any trouble of any kind that may arise on the installation, now or in the future.

Failure to use flameproof or intrinsically safe equipment In places where flammable liquids are stored, or where, for example, the gas bottles used for welding, or for domestic gas purposes in conection with caravans and the like are located, properly designed flameproof equipment and wiring are necessary.

Insufficient attention to the bonding of all metalwork to earth Every piece of metal in any way associated with an electrical appliance should be bonded to earth, and it is often found that this is omitted.

Omission of protective shield or skirt to lampholder of bathroom, in lighting pendant or fittings All landholders in bathrooms must be insulated and fitted with a protective skirt.

General note on testing old installations

When rectifying a fault on old wiring, do not start by assuming that the supply is the usual single-phase and neutral 240 V a.c. system. Several other systems are still in use in remote parts of the country.

The first test, therefore, should be to find out the system in use. The nameplate on the meter, plus the information given on any lamp that may be in use, will provide a useful starting point. The meter will show if the system is a.c. or d.c., and the frequency employed, and probably the voltage as well, and in any case the lamp will show the voltage.

But the universal test instrument should be applied to check the voltage from each supply wire, separately, to earth.

It should always be assumed that the system is wired up wrongly: never take it for granted that single-pole fusing is used or that single-pole switches are placed in the phase conductor, or that the neutral wire is in fact at or near earth potential, or that any earth connection at all does in fact exist.

Test records

Keep a record of all tests, with the date and leave a copy with the installation. It will greatly assist anyone working on the installation at a later date.

Testing an installation after fire or water damage If a fire has occurred in some part of a house, or if, for example, flooding has taken place, or if a tank in a loft has overflowed, a’ check should be initiated immediately before any current is used.

The main switches on the consumer unit should be opened at once, and the ‘Megger’ should be applied to each circuit to make two insulation tests. First, when the fuses are removed, access can be obtained to the phase end of each circuit and the neutral connections can be taken out one by one, and with all appliances disconnected but the switches on the circuits closed, an insulation test can be carried out between phase and neutral on each circuit. Then a similar insulation test is carried out between each conductor and earth.

With plastic wiring, heat may have caused considerable damage to the wiring and major renewal operations will have to be commenced. On the other hand, water damage will not have affected the insulation itself, but may have given rise to pockets of water in socket-outlet boxes, switches and other fittings. This means that each of these must be opened and dried carefully, and for this purpose an ordinary hair dryer fed from some circuit in another part of the house which is in good order, may conveniently be used. It is probable that at least one circuit may have escaped major damage.

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