In this article we will discuss about the tests that are carried out while the house electrical wiring is in progress and also after the same is completed.
Tests Carried out While the House Electrical Wiring is in Progress:
(a) Continuity Test of Wiring:
When there is some progress in electrical wiring, electrical continuity among different lengths of wires and cables is tested; otherwise lamps connected in the circuit afterwards may not burn. This test is specially important where many wires or cables are drawn through the same conduit or casing.
A dry cell is connected in series with an electric bell. Two pieces of long insulated wires, one from the free terminal of the cell and the other from that of the electric bell, are taken for testing. If necessary, more than one cell may be used. The testing leads must be sufficiently long to reach the farthest end of the electrical wiring. The connections of the dry cell and the electric bell are shown in Fig. 184.
Now, if the bell rings when the two long testing cables touch the two end-leads of the wiring system, it is proved that there is good electrical continuity between different lengths of wires of the same circuit. The advantage of using an electric bell is that, even in a large building the sound of the bell may be heard from any part of the building so that one can understand that the electrical continuity of the circuit has been maintained.
If a dry cell and an electric bell are not handy, continuity test may be carried out with the help of an insulation testing megger. Two long single-core cables are at first connected to two terminals of a megger. The other ends of these cables are then joined to end-leads of the circuit.
The handle of the megger is now turned, and if the pointer indicates zero, the continuity of the electrical wiring has been maintained. If, however, the pointer indicates infinity or any high value of resistance, one should realize that there is no proper connection between different lengths of wires of the circuit.
ADVERTISEMENTS:
The continuity test of electrical wiring of a house is often carried out at the terminals of the main switch. This helps to conduct the test with comparatively shorter lengths of cables. Besides, the circuit of each individual load point can be tested separately.
At first put the main switch off. Connect up properly the outgoing terminals of the main switch with the terminals of the bell and the battery or with the terminals of an insulation testing megger. Now, the load or the loads are connected at their respective points and the circuit under test is switched on.
If the bell rings, or, on turning the handle of the megger, if the pointer of the instrument indicates zero, it is to be understood that the continuity of the circuit has been maintained. The bell will not ring if the conductor of the circuit is open or snapped somewhere. If, however, the megger is used, its pointer will indicate ‘infinity’ or some high resistance under this condition.
ADVERTISEMENTS:
(b) Continuity Test of Conduit or Lead Sheathing:
Electrical continuity of metal conduits or lead sheaths is very essential. The metallic armor of the armored cable should also be tested for continuity. Earth continuity in a house electrical wiring must be carried out in such a way that the total resistance of conduits or lead sheaths from the farthest point of the load circuit up to the earth electrode or up to the earth terminal provided by the supplier should not, under any circumstance, exceed one ohm. There are various methods by which this resistance can be measured. Only one of them is described below. The result obtained from this measurement is sufficient for normal working.
Let the left portion of Fig. 185 is the farthest point of the conduit line. The same method is applicable to lead-covered wiring also. The farthest end of the earth lead is rubbed bright by means of an emery paper (or sand paper) and a clamp is fixed at this end. One end of the testing cable is connected with this clamp and the other end of the cable is connected with the positive terminal of a low-range ammeter. The negative terminal of the ammeter is joined with the negative terminal of a 4-volt battery in series.
Now, another testing cable is connected between positive terminal of the battery and the farthest end of the conduit line or the metal sheath which is also rubbed bright by means of an emery paper. The testing lead is connected to the conduit line with the help of a clamp (Fig. 185). At this stage the ammeter will indicate a flow of current through the closed circuit thus formed. The total resistance between conduit line or metal sheath and the earth lead will be clear from the amount of current indicated by the ammeter.
Let the two cables of size 1/0.064″ each have been used as testing leads and let their total lengths be 200 ft. From the table it can be found that the resistance of this size of cable for 1,000 yds is 7.463 ohms.
Therefore, the resistance for a length of 200 ft (i.e. 200/3 yds) is:
(7,463/1,000) x (200/3) = 0.4974 ohm.
Now, if the combined resistance of the conduit or lead sheath as well as the earth wire is not to exceed 1 ohm, the total resistance of this circuit including the testing leads shall not exceed 1.4974 ohms. As a result, current drawn from a 4-volt battery for this circuit indicated by the ammeter cannot be less than
ADVERTISEMENTS:
4.0/1.4974 = 2.666 amperes
But if the ammeter indicates less, one should understand that the resistance of the conduit line or metal sheath to earth is greater than 1 ohm. (The resistance of the ammeter has not been considered here as it is normally negligible. Of course for greater accuracy this resistance should also be taken into account).
If the current is not less than 2.666 amperes, the combined resistance may be accepted for working purpose. But if the current is less, there is certainly some defect somewhere in a joint of the conduit or in a bonding system of the lead-covered cables. In that case all the screwed joints of the conduits should be unscrewed, cleaned properly and re-screwed or bonding in lead-covered wiring should be examined and defects removed.
(c) Testing of Polarity of Single-Pole Switches:
Indian Electricity Rule No. 32 states that where supply line to a house include an earthed neutral conductor, an indication of a permanent nature shall be provided by the owner of the earthed neutral conductor. It also states that no cut-out, link or switch other than a linked switch arranged to operate simultaneously on the earthed neutral conductor and the live conductor, shall be inserted or remain inserted in any earthed neutral conductor.
It has to be specially observed that every single-pole switch is inserted in that wire which is not earthed and socket outlet for wall-plug etc. are connected to proper live wire.
This test is necessary to check up whether the house electrical wiring has been carried out in compliance with the above-mentioned rule. Single-pole switch or cut-out is to be inserted in the live line all throughout the wiring; it should not at all be inserted in the earthed neutral wire.
In an old house where this rule has not been observed (or it is not known if the rule has been observed), the owner shall be responsible for violation of this rule, if any. In case of new electrical wiring supply connection will not be given if this rule is violated.
Now, the method of testing is described in detail below:
If the electrical wiring is done according to procedure of using red-coloured insulation for live wire all throughout the wiring, there will be no difficulty in identifying the live wire. It is then as easy to know where to insert the switch during wiring as to check up at any time the polarity of the switch after wiring.
But where this procedure has not been followed during wiring, the test may be carried out as follows:
(1) Polarity Test of a Single-Pole Switch by means of a Test Lamp:
(i) Test carried out with switch ‘off’:
Let the switch of a lamp is under test. A single piece of long insulated wire is connected to earth as shown in Fig. 186. The wire should be of such length as would reach up to switch terminal from the earth lead.
Wire may be earthed as described below:
If there be a cold water pipe in the premises (in cold countries there may be hot water pipe, but that is not meant here), a suitable part of its outer surface should be stripped of paint, dirt etc. by rubbing with a knife or emery paper and made shiny before the earth wire is wound over and properly fixed with if with the help of a pair of pliers.
In such case it is a good practice to prepare a clamp and to fix the earth wire with the pipe by means of this clamp. If the earth be wet, often the end of the earth wire may be stripped of insulation and set into the earth by means of a large screw driver (used as an iron rod). Of course, how far this will be useful depends on how wet the earth is.
Connect one terminal of the test lamp with the earth wire near the switch under test. The voltage rating of the test lamp should be the same as the circuit pressure. Now, put the switch off and open its top cover. The left-hand terminal of the switch is next touched by the lead of the insulated wire connected with the other terminal of the test lamp.
If the switch is properly inserted in the live line as shown in the upper diagram of Fig. 186, only the test lamp will glow with full brightness. If the switch is wrongly put in the neutral wire, no lamp will glow when the lead coming from the test lamp is touched to left-hand terminal of the switch. This is shown in the lower diagram of Fig. 186.
Now, the right-hand terminal of the switch may be touched by the lead coming from the test lamp. If the switch is properly connected, no lamp will burn. But if the connection is wrong, the test lamp and the lamp at the point will be in series and will burn dimly.
If the switch is wrongly connected, it should be disconnected from the line and inserted properly in the live wire.
(ii) Test carried out with switch ‘ON’:
If one terminal of the test lamp is connected to earth and its other terminal is joined with either terminal of the switch under test, it will burn with full brightness when the switch is properly inserted in the live wire. But if the switch is wrongly connected, the test lamp will not burn at all.
Of the two methods of testing described above, it has been found that testing with switch ‘ON’ is the easier and more reliable method.
At the time of electrical wiring often the connections of the switch may be reversed through mistake, i.e. the live wire may be connected to the right-hand terminal instead of the left-hand terminal of the switch. As a result, the test lamp will not burn at all when connected between earth wire and the left-hand terminal of the switch which is put off but properly inserted in the live line.
But with the same mistake in wiring when the switch is put ‘ON’, both the switch contacts remain in touch with each other, and if the switch is inserted in the live line, the lamp is sure to burn with full brightness [Fig. 187(a)].
Besides, when the switch is inserted in the neutral wire and kept in ‘OFF’ position and the wattage of the lamp or fan connected at the point is much higher in comparison to that of the test-lamp, if the test lamp is connected between earth wire and the right-hand contact of the switch under test, it will burn brightly instead of glowing dimly. This may lead a person to reach a wrong decision. But with the switch ‘ON’ the test lamp cannot burn at all under such circumstance [Fig. 187(b)].
In view of the above the polarity of a single-pole switch is usually tested with the switch kept in ON-position when the supply is available.
(2) Polarity Test of a Single-Pole Switch by means of an Insulation Testing Megger:
If supply is given to a house, the polarity test of a single-pole switch can be carried on by means of a test lamp. But immediately after completion of wiring when the supply is not available, test lamps are of no use. In that case such a test can be performed by means of an insulation testing megger.
The procedure of this test with a megger is described below:
In the main switch or in the distribution fuse board the wire connected to fuse cut-out is certainly the live wire. At first connect this live wire very securely with the earth wire near the main switch or the D.B. No load (i.e. lamp or fan) should remain connected to the point, the controlling switch of which is to be tested. Open the cover of this switch and put it ‘ON’.
Now, connect up two pieces of single-core insulated cables to two terminals of the megger and peal off some insulation near the open ends of the cables so that there is open conductors at their tips. The open tip of one cable is well-connected to the earth wire and the tip of other cable is used to touch any contact of the switch.
Now, turn the handle of the megger. If the switch is inserted in the live line, the megger will indicate ‘zero’ reading. On the other hand, if the pointer of the megger shows ‘infinity’ or any high value of resistance, it may be concluded that the switch has not been inserted in the live wire; it has been placed in the neutral or some other wire, i.e. the polarity of the switch is not correct.
The connections of the megger with the earth wire and the switch contact for this test is shown in Fig. 188.
(d) Insulation Resistance Test between Conductors:
Put all the switches in the ‘ON’-position, but take off lamps and disconnect fans from the points. Open the lid of the main switch (which is also placed at the OFF-position) and connect up its upper terminals, i.e. outgoing terminals with the two terminals L & E of the megger (Fig. 189). Now, if the handle of the megger is turned, its pointer will be stable in a position on the scale showing the insulation resistance between conductors of the wiring.
If in a particular circuit or in the entire house there are 20 load points, the insulation resistance indicated in this test should not be less than,
50/20 = 2.5 megohms.
It should be borne in mind that at the time of testing, all the switches (except the main switch) must be placed at the ON-position and all the loads must remain disconnected from the points. If a person performs this test with loads connected at their respective points and controlling switches at the OFF-positions, the result is sure to be wrong.
The reason for this will be clear if the path of leakage current is traced on a copy of the connection diagram. With proper procedure the current should leak through the insulation of the entire wiring system. This is possible only with former system of connection and not with the latter system.
According to Indian Electricity Rule [Annexure VI, Sub Rule 10(e)] where insulation resistance test is to be carried out of a wiring of permanent nature before the light fittings, lamps etc. are connected to the points, the minimum insulation resistance between conductors shall be one-half the insulation resistance to earth (i.e. between a conductor and earth).
In such a test it is not necessary to attempt any improvement of the insulation resistance of any circuit to be greater than 1 megohm.
According to Indian Standard Code of Practice 8.1 A.2 of No. 732-1963 where supply pressure does not exceed 650 volts, the insulation resistance in megohm should be 50 divided by total number of load points (outlets). However, the insulation resistance of the entire electric installation need not be greater than 1 megohm.
The purpose of this test is to check up the insulation resistance of the wiring system without fittings. If the result of this test is satisfactory before fixing the fittings and unsatisfactory after the fittings are connected, it is obvious that the fault lies with the fittings. Much trouble is spared by this procedure.
In case the insulation resistance of a permanent wiring, when tested by a 500-volt megger, is found to be less than 0.5 megohm, for further testing, the entire wiring system is divided into a number of sub-circuits each of which is to be tested separately. The satisfactory result is obtained when the insulation resistance in each case is found to be at least 0.5 megohm. The total insulation resistance will then be as specified above.
Tests Carried out after the Electrical Wiring is Completed:
(a) Insulation Resistance Test to Earth after completion of Wiring:
This test is performed as the last item after all the work on wiring is completed. It is only when the result of this test is satisfactory that the circuit is connected to the supply line. Fig. 190 shows the connection diagram for such a test.
When all the work is over including fixing of lamps, fans, fuses and other fittings, keeping the main switch at OFF-position all other switches should be placed at ON-position. A piece of wire well-earthed through connection with a cold water pipe or a piece of metal rod buried in the earth or supplier’s earth terminal should be connected to earth terminal (marked ‘E’) of the megger.
Another piece of wire connected to line terminal (marked ‘L’) of the megger is drawn up to main switch and the two outgoing terminals of the switch are touched by the open end of this wire, one by one. Each time the handle of the megger is turned fast and the reading on the scale indicated by the pointer of the instrument is noted.
From these readings it may be ascertained whether test result is satisfactory or not. In this connection one should remember that the earth connection should be perfect. Before the insulation test is carried out, perfection of earth connection is tested with the help of a megger.
The terminal ‘E’ of the megger is connected to earth and the terminal ‘L’ is connected to unpainted (or paint removed from a small portion) surface of the metal conduit or lead cover of the metal sheathed cable. On turning the handle of the megger if the pointer shows ‘Zero’ reading on the scale, earth connection is perfect; but if the pointer does not show ‘Zero’ reading, the very connection with earth is defective.
To Find out if the Test Result is Satisfactory:
After the completion of a new wiring or wiring of additional load points and before connecting permanently this new wiring with the supply line, the insulation resistance of the wires to earth is to be tested. The test result is deemed to be satisfactory when it is not less than (50/total number of points of the sub-circuit) in megohms. However, the insulation resistance to earth for the entire installation of the house need not be greater than 1 megohm.
At the time of performing this test all fuse-links should remain fixed at their respective positions, all switches (including main switch, if possible) should be ‘ON’ and all lamps, fans and other fittings excepting concentric earthed wiring should be fixed at their respective points.
The insulation resistance of the entire house wiring or of the additional wiring, when tested by an apparatus producing twice the circuit voltage or by a 500-volt d.c. instrument, must not show less than 0.5 megohm. If it is less than 0.5 megohm, the entire new wiring or the new additional wiring must be divided into a number of small sub-circuits each of which should be tested separately. The insulation resistance in each case shall then be at least 0.5 megohm.
At the time of performing this test heater, stove, fan etc. may be disconnected from the circuit, if so desired. But in that case the insulation resistance to earth (i.e. the insulation resistance between current-carrying conductor and the frame of the apparatus) of each such appliance must not be less than that approved for such appliance by I.S.I. (Indian Standard Institution) or 0.5 megohm. If the insulation resistance of a house wiring is actually less than the required value, it may be concluded that there is some defect in the wiring.
According to Indian Electricity Rule No. 48(1) the supplier shall not connect with his works the installation or apparatus on the premises of any applicant for supply unless he is reasonably satisfied that the connection will not, at the time of making the connection, cause Breakage from that installation or apparatus exceeding one five-thousandth of the maximum current supplied to the applicant’s premises.
This rule should be particularly observed. The sub-rule (2) of this ride states that, if the supplier declines to make a connection under the provisions of sub-rule (1), he shall serve upon the applicant a notice in writing stating his reason for so declining.
The minimum insulation resistance of a house wiring acceptable according to above-mentioned rule may be worked out as follows:
How to Use a Megger:
1. The apparatus is to be placed on a conveniently plane surface. If the handle is now turned before making any connection whatsoever, the pointer will indicate ‘Infinity’ on the scale. (This shows that the instrument is alright).
2. Turn the handle once again after connecting two suitable long pieces of single-core cables with the ‘Earth’ and ‘Line’ terminals of the megger leaving the other two ends open. This time, too, the pointer will show ‘Infinity’ on the scale. If it does not show, one should understand that there is some leakage in the two pieces of cables. This may be due to dirt or dampness in the cable.
3. Now, the open end of one cable connected with the line terminal of the megger is joined with one wire of the wiring system and the open end of the other cable with the earth wire of the wiring. In case of ‘Wee Megger’ the handle is turned at 160 revolutions per minute, while for other two types of megger the handle is turned at 100 revolutions per minute. At this stage the reading indicated by the pointer on scale will be the insulation resistance of the conductor to earth.
(b) To Test the Effectiveness of Earth Connection:
In order to test the effectiveness of earthing, an apparatus named ‘Earth Tester’ is used.
The procedure of this test is as follows:
To measure earth resistance an auxiliary electrode is to be planted temporarily into the earth at some distance from the permanent earth electrode. An alternating current is to be passed between these two electrodes. Permanent earth electrode is denoted as ‘Electrode-X’ and temporary electrode as ‘Electrode-Y’.
During testing care must be taken to see that current flowing between electrodes does not vary. Also the distance between earth electrode and the Y-electrode should be such that the resistance areas of the two electrodes do not overlap. When overlapping is not fully avoidable. Y-electrode must be placed at a distance as far as possible away from earth electrode. Usually this distance is taken to be 46 metres (150 ft).
Now, a third electrode — electrode-Z — is planted temporally midway between X and Y. The current flowing between X and Y and the potential difference between X and Z are then measured (Fig. 194).
Earth resistance is then calculated as follows:
Earth resistance = Potential difference between X and Z/Current flowing between X and Y
At the time of testing alternating current to be used must be of the same frequency as the supply current to the circuit or it should be the current coming out from manually operated earth tester. In case of former type of current the voltmeter used for the test must have very high resistance (at least 200 ohms per volt). But it is advisable to use an earth tester for such a test. The reason is that the current can be easily varied and the result obtained is comparatively more accurate.
If an alternating current of the same frequency as the supply current is to be used, it must not be drawn from the supply mains directly. It may be drawn from a double-wound transformer, and in each case earth electrode X should be kept separated from all circuits during test.
Whether the distance between electrode X and Y is sufficient or not may be checked by performing two more tests with two different positions of Y-electrode. At first Y-electrode is moved 6.1 metres (20 ft.) towards X-electrode and the test is performed. Next, Y-electrode is moved 6.1 metres (20 ft.) farther from the original position away from X-electrode and the test is repeated.
If the results of both these tests are nearly the same as that of the original test, the average of the three results may be taken to be the earth resistance of the permanent electrode X. If, however, the results of the three tests widely differ from one another or at least not nearly the same, the temporary electrode Y is to be removed farther and farther away from X-electrode and the test is performed until the results of the last three tests are nearly the same (Fig. 194).
Methods of Testing Earth Resistance:
(1) When a cold water pipe or any other earthed metal lead having negligible resistance is available at a distance from the main earth electrode of the house (the distance should be such that resistance areas of both do not overlap), it is-possible to find out the resistance of the earth connection by means of an ammeter and a voltmeter).
As shown in Fig. 192, connect an a.c. ammeter with a water pipe and the main earth electrode and send alternating current through this connection. Measure, the voltage between pipe and the electrode with an a.c. voltmeter. If the resistance of water pipe is negligible, earth resistance = voltage/current.
It is very difficult to obtain a water pipe having no resistance at all. Besides, in this way of measuring earth resistance, direct current supply should not be used.
(2) If more accurate result than that obtained from above-mentioned simple test is intended to be measured for earth resistance, two temporary earth electrodes are to be used in place of one, and earth resistance is measured from three different places as shown in Fig. 193.
Make two temporary earth connections at distances away from the permanent earth connection of the house so that the resistance area of one does not overlap with that of another.
Find out earth resistance between A and B by passing alternating current through the circuit as described in item (1) above. Let it be 3 ohms. Similarly, find out resistances between B and C and C and A. Let each of them be 1.5 ohms.
Hence,
resistance of A + B = 3 ohms
resistance of B + C =1.5 ohms
resistance of C + A =1.5 ohms
Adding up we get:
resistance of 2(A + B + C) = 6 ohms;
... resistance of A + B + C = 3 ohms.
But as measured earlier:
resistance of B + C =1.5 ohms;
... resistance of A =3 – 1.5 = 1.5 ohms.
This is the earth resistance of the main earth connection.
A still more accurate result may be obtained from the process described in item (3) below:
(3) At a certain distance (distance should be such that resistance areas of different electrodes do not overlap) from the main earth electrode (X) make a temporary earth connection Y as shown in Fig. 194. Connect an ammeter with X and Y and switch on a.c. supply main so that alternating current flows through this circuit.
At the midway and on the same straight line joining X and Y make another temporary earth connection (Z). Connect a voltmeter between X and Z. Let the voltmeter show V1– volt when connected between X and Z. Similarly, let the voltmeter show V2-volt when connected between X and Y, and V3-volt when connected between Z and Y.
Each time during measurement of voltage since same alternating current flows through the earth, hence,
resistance of X + resistance of Z = V1/current = a ohm (say),
resistance of X + resistance of Y = V2/current = b ohm (say),
and resistance of Z + resistance of Y = V3/current = c ohm (say),
Adding up we get,
2(X +Y + Z) = a + b + c
Or X + Y + Z = a + b + c/2
But resistance of Z + resistance of Y = C
... resistance of X = a + b + c/2 – c = a + b – c/ ohm.
(4) Measurement of Earth Resistance by Meg Earth Tester or Megger Earth Tester:
In both ‘meg earth tester’ and ‘megger earth tester’ the arrangement is such that when the handle of the instrument is turned, alternating current flows through the earth, but direct current flows through the device for measuring the earth resistance. Megger earth tester has four terminals (P1, C1, P2, C2)1, while meg earth tester has three terminals (P, C and Earth)1.
The terminals P1 and C1 of the megger earth tester are joined together by a piece of metal plate. At the time of measuring earth resistance the linked P1 and C1 terminals of the megger tester or ‘Earth’ terminal of the meg earth tester is to be connected to main earth connection under test. Two temporary electrodes are planted into the earth as shown in Fig. 195. Middle electrode is connected to P2 terminal of the megger tester or P terminal of the meg tester and distant electrode is connected to C2 terminal of megger tester or C terminal of meg tester. Now, if the handle of the instrument is turned very fast, the pointer will show the resistance of the permanent earth electrode on the scale.
When a megger earth tester is used, its range-switch should be adjusted for setting the range of the apparatus so that the instrument is not damaged even when the pointer overshoots the range or the terminals become short-circuited inadvertently.
Two hollow iron tubes of conical shape, 3.8 cm in diameter and 76 cm long, are used as temporary electrodes. The distance between permanent electrode and the temporary electrode farthest from it should be about 46 metres.
As the matter is of utmost importance, an illustration is given here to show how electrical resistance of domestic earth electrode is correctly measured.
Let the earth resistance of main earth electrode of a house is to be measured with the help of either megger earth tester or meg earth tester. At first one temporary earth electrode is planted at a distance of about 23 metres (75 ft) from the permanent electrode. A second temporary electrode is then planted at a farther distance of 23 metres (75 ft.) from the first electrode.
There is no such rule that all the electrodes should be placed on the same straight line. In case of meg earth tester ‘P’ terminal of the instrument is connect^ to temporary earth electrode at the middle (called Z-rod) and ‘C’ terminal is connected to distant temporary earth electrode (called Y-rod). Permanent earth electrode is connected to earth terminal of the megger. But if the instrument is megger earth tester, P2 terminal is connected to middle Z-rod and C2 terminal to distant Y-rod. P1 and C1 terminals which are linked together are connected to permanent earth electrode of the house.
Now, on turning the handle properly the pointer indicates a reading, say 2.5 ohms. This 2.5 ohms include the total resistance of the earth electrode and the resistances of the lead wires connecting the instrument with all the permanent and temporary electrodes. If now the equivalent resistance of the lead wires is subtracted from the reading of the instrument, the actual resistance of the earth electrode of the house will be obtained.
A little thought will make it clear that all the lead wires used for connections are in parallel. Therefore, if the lead wires are taken out, joined together in parallel and connected at one end to P and C terminals of a meg tester shorted together and at the other end to the ‘Earth’ terminal of the instrument, then on turning the handle, the pointer will indicate the equivalent resistance of the lead wires.
In case of megger tester one end of the lead wires (joined in parallel) is connected to P2 and C2 terminals of the instrument shorted together and other end to P1 and C1 terminals linked together. Let the equivalent resistance of the leads is found to be 0.5 ohm. Then true resistance of the earth electrode of the house is,
2.5 – 0.5 = 2 ohms.
Generally, a meg earth tester can measure 0 to 50 ohms, and up to 0.1 ohm is easily readable from its scale.
Use of the Test Lamps:
A test lamp is a very useful appliance for an electrician or a wireman. Two pieces of insulated wires connected to a lamp-holder to which a bulb is fitted are sufficient connections for a test lamp. With the help of a test lamp it may be checked whether current-carrying path in a circuit is alright or not.
The existence of voltage between two open leads can also be ascertained. The two open ends of cables joined with the test lamp-holder touch the two open leads, and if the lamp burns, there is voltage. Without voltage test lamp will remain dark. In such a case, however, test lamp should be suitable for line voltage.
If necessary, more lamps of same wattage may be connected in series. Often it is possible to ascertain which terminal is live and which one is dead with the help of a test lamp. Here the term ‘often’ is used to indicate that, if the neutral is earthed, only then it is possible to ascertain the polarity of a terminal by a test lamp.
One wire from a test lamp-holder is touched to live terminal, while the other wire is connected to an earth wire. The test lamp will burn for an earthed neutral system. If the lamp does not burn, the lead under test is the neutral terminal. In such a test it is important to see that the lamp is healthy, i.e. its filament is continuous.
The nature of fault in a circuit may also be estimated by observing carefully the intensity of brightness of a test lamp. If a test lamp is connected in series with a coil (e.g. field circuit of a motor or other machine) and current is passed from the supply, the lamp will burn dimly or it will be only red hot.
The reason is that the resistance of the coil is usually high on account of large number of turns in it. Thus, much of the supply pressure is dropped across the coil leaving only a small voltage for the lamp. In such cases, therefore, the lamp should not burn brightly. It is, however, not expected that the lamp will be equally bright or dim in all cases. The intensity of brightness depends on the amount of resistance in the circuit.
In a motor or a fan let there be four field coils, and let one field coil develop a short-circuit in it. When each coil is, by turn, connected in series with a test lamp and put across a supply line, the test lamp will show equal intensity of brightness for three healthy coils, but it will glow brighter when connected with the short-circuited coil. In other words the coil showing brighter test lamp in comparison to others is the faulty one.
The short-circuit in the armature coils of a generator or a motor can also be similarly detected. The coil connected to each consecutive pair of commutator segments of a generator or a motor or a fan may be tested in series with a test lamp. The lamp will usually glow with similar brightness for all coils except for a short-circuited one when it will glow a bit brighter.
If the armature coil, field coil or commutator of a d.c. machine, the stator or rotor coil of an a.c. machine or H.V. or L.V. coil of a transformer by chance comes in electrical contact with the frame or body of the machine, it is said to have developed ‘earth’ or ‘ground’ fault.
In that case one wire of a test lamp is connected to a supply terminal and the other wire is connected to a terminal of the armature coil or field coil or stator coil or rotor coil etc. The free end of another wire connected to other terminal of the supply line is touched to body or shaft of the machine. If the test lamp now glows, it indicates the existence of an earth fault.
Testing with an Electric Bell:
In cases where tests may be carried out with a test lamp, an electric bell with a battery of suitable voltage may also be used. In such a case the bell will ring in place of glowing of the test lamp.
Proper Sequence of Tests:
Now-a-days the need for testing has become all the more important on account of a.c. being introduced everywhere.
Tests are to be performed one after another in the sequence mentioned below:
(1) Firstly, polarity test is to be conducted for identifying the phase wire or wires and the neutral wire in case of a.c. supply and the positive wire, negative wire and the common neutral wire in case of d.c. supply.
(2) Secondly, the insulation tests are to be conducted.
(3) Thirdly, the earthing test is to be carried out.
As soon as the electrical wiring is over, each wiring shall be tested and fault, if any, has to be set right.
When a new extension of wiring is done and the same is connected to existing permanent wiring, both the extension as well as the joints are to be tested. The owner must be informed of faults if any, which are to be repaired.
The voltage used for testing the insulation resistance should be twice the R.M.S. value of supply voltage in case of a.c. supply, and double the supply voltage in case of d.c. supply. If, however, the supply available is of medium pressure, the testing voltage need not be greater than 500 volts.
If the insulation resistance (overall) indicated by the testing instrument is found to be less than 0.5 megohm, the result should be ignored. The test is then repeated with each sub-circuit separated out. When any such test does not indicate more than 0.5 megohm, it may be concluded that the insulation condition has deteriorated.
Method of Testing Insulation:
Before connecting to supply line whole or part of new wiring of a building or any new extension of the existing wiring, all conductors not connected to earth are joined together and tested for insulation resistance to earth. At the time of testing all fuses would be in position, all switches should-be on (if possible, even the main switch) and all poles of the electrical wiring must remain electrically connected together. Under these conditions insulation resistance to earth must not be less than,
(50/ total number of points) megohm
When entire electrical wiring of a house is tested, an insulation resistance of 1 megohm is considered to be sufficient. The above-mentioned calculation is not then necessary.
At the time of testing as indicated above, appliances like fan, heater etc. having individual insulation resistance to earth not less than 0.5 megohm may remain disconnected from their respective circuit, i.e. excluded from the overall test.
N.B. It is good to perform an additional test with lamps etc. either not fitted or taken out from their respective points. The test is then carried on between conductors, i.e. between live or phase wires and neutral wires.
The minimum acceptable insulation resistance between conductors should be:
(50/ total number of points) megohm
Testing of Polarity of a Single-Pole Switch:
Special inspection is necessary for all single-pole switches which are to be inserted in the conductors having no earth connection. Similar is the case with socket outlet (wall-plug) etc. which should be connected to proper live line or phase wire.
Earth-Leakage Circuit Breaker:
Earth-leakage circuit breaker, where fitted, should be tested for proper performance.
Separate Test for Earth Continuity Conductors:
When it is necessary to test separately the earth continuity conductor of a consumer having supply in a.c., it should be performed with an alternating current of the same frequency as that of supply. The testing current should be about 1.5 times the full load current of the circuit, but must never exceed 25 amperes.
The supply must be disconnected at the time of performing this test. One end of the earth continuity conductor is to be connected to one end of a cable. The cable of a sub-circuit may also be used for this purpose, provided that the resistance of the cable is known.
Now, an electric pressure not more than 40 volts is applied between free end of the earth continuity conductor and the cable, and the current flowing due to this pressure is measured. If the result obtained from dividing the testing pressure by the resulting current does not exceed 1 ohm, the earth continuity conductor is in good condition.
Certificate:
When electrical wiring in a house is finished or extension of an existing wiring in completed, a certificate of completion issued by a contractor or an equally competent person is necessary. This wiring has to be tested from time to time. Earthing system of every house wiring must be tested at intervals not exceeding 5 years.
On or near the main distribution board of every house and within easy view of all, a notice of permanent nature should remain affixed on a plate of size at least 10 cm x 5 cm.
It is highly desirable to follow in practice the notice which reads as follows:
Notice “Important”
In the interest of safety it is strongly recommended that this installation be periodically inspected and tested at intervals not exceeding two years and a report on its condition obtained in a form to be approved by Electric Inspector.