2. Polarity Test
3. Electrician’s Test Lamps
4. Resistance Bridge
5. Megger 1) Continuity Tester If the insulation of any of the wires breaks down, the leakage current will start and if the wiring is in metallic conduits they will started giving shock until and unless they are earthed properly. There is a possibility of the conduit joints to be separated or become loose which results in an introduction of high resistance in the earth path. For the earth continuity test, the resistance of the conduit at the extreme end is measured from the earthing electrode with the help of method already explained in article 5. In accordance with IS. specification 732-1963, the total resistance of the conduit from the earthing point to any other position should not be more than I ohm. 2. Polarity Test As long as mains supply is available on a site, testing polarity is quite easy, the only equipment needed being a proper electrician’s lamp. With the supply on and the lamp between phase and earth, .the lamp should light. With the lamp between neutral and earth the lamp should not light. 3. Electrician’s Test Lamps The use of these test lamps is NOT recommended, unless every available part is insulated. These lamps in fact are responsible for more accidents than other item in the electrician’s kit. In its worst form it consists of a bulb to which are soldered two wires, the ends of which are bared ready to hook on to any terminal. The result is of course that they are dangerous in the extreme. The lamps are put across phases and promptly in the electrician’s face, or the wires become unhooked from t terminal and short circuit the lines, often causing serious injuries to the electrician.
Other types of test lamp can be dangerous in other ways. One type has a brass holder for the lamp with no insulation covering it. This is very little better than the lamp with soldered terminals as often no cord grip is included in the holder.
Inevitably the sharp edges of the brass lamp holder cut through insulation of the cables and make the holder live, to the danger of all who use it. Another potential source of danger is having a bare glass bulb dangling on the end of two leads. When the leads are connected to live terminals the lamp could easily hit something metallic and break bringing the filament into contact with the metal and short circuiting the supply with the usual violent consequences.
If test lamps must be used at all, then at’ least they should safe. The bulb should he of low wattage, e.g. 15 watts, and it should be enclosed in a protecting box, a wooden box with a Perspex window is a safe method. The leads should be insulated throughout their length, preferably twisted flexible with about 2 ft. length untwisted and secured. The terminals should be heavily insulted with rubber sleeving and strong crocodile clips should be used. No direct strain to the lamp connections should exist and the leads could be fastened inside the box for this purpose. 4. Resistance Bridge This is a device which measures an unknown resistance very accurately by comparing it with a known resistance incorporated in the instrument. A meter indicates when the unknown resistance is equal to that set up on the instrument panel. 5. Megger The Megger Earth Tester works on the fall of potential method explained earlier, It also requires temporary current and potential electrodes. The instrument is so designed that the readings are not affected either by stray alternating or direct current nor by electrolytic back of e.m.f. and it gives resistance directly without calculations. The Megger Earth ‘fester is essentially a direct reading ohm-meter and a hand driven generator which supplies the testing current. The ohmmeter consists of two coils (current coil and potential coil) mounted at a fixed angle to each other on a common axle. The current coil carries current proportional to the current flowing in the test circuit, while the potential coil carries current proportional to the potential across the resistance under test. Thus the potential coil acts as a voltmeter in the fall of potential method while current coil acts as an ammeter in that experiment. Since the deflection of the needle is proportional to the ratio of the current in the two coils, it gives resistance directly.
The hand operated generator produces the direct current, but to eliminate the effect of electrolytic e.m.f. as said earlier, it is necessary to pass alternating current through the coil, so to change the D.C. into an alternating supply, a rotary current reverser is mounted on the same shaft of the generator. The alternating current in the coil will produce and alternating drop in the coil but potential to be applied across the moving coil must be direct because the ohmmeter is a moving ‘coil instrument working on D.C. alone so for changing the alternating drop into the direct drop, a synchronous rotary rectifier is also attached as shown in Fig.-Although the instrument is immune to stray currents, yet while taking readings sometimes it will be observed that the needle vibrates; this happens only at the instant when the stray alternating current has the same frequency as the frequency generated. For such occasions either increase the speed of the generator a little or lower its speed.
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