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Electrical test equipment

In the fullness of time we will be giving an insight into the work of the electrical trades and how it has changed over the years. The tools and techniques and perhaps even a few trade secrets will be on show. To begin, here are some of the testers used by electricians on circuits large and small.

Insulation and continuity testers

Wooden cased bridge Megger

Wooden cased bridge Megger

Inside bridge Megger

Inside bridge Megger

One of the most important tests of the general condition of an electrical circuit is a measurement of its insulation resistance. This can reveal damaged or substandard cable insulation, faulty or damp fittings and those damaged by electrical flashovers etc. Defects of this type are sometimes undetectable until the insulation is subjected to electrical stress equivalent to or exceeding the circuit’s normal working conditions. An ordinary multimeter would be unsuitable for proving the integrity of a 240-volt power circuit, for example, a proper test of which would typically require 500 volts to be applied. Until the advent of transistorised insulation testers, the conventional means of generating this high test voltage was with a small hand-turned generator built into a special testmeter, often generically called a ‘megger’ after the Megger trademark of one of the most popular brands of tester. The name is an allusion to the calibration of the measurement range in megohms. Typical instruments will read from a fraction of a megohm up to hundreds of megohms. An electrician would ‘megger’ a circuit by connecting the leads of the megger first from the circuit cables to earth, and then between the cables, each time cranking the handle for a few seconds while reading the resistance from built-in meter dial. One common feature of these testers was the bridge-connected meter movement. This compares the circuit under test with an internal reference, making the reading almost independent of the generator voltage. A mechanical governor on the generator maintains the speed within reasonable limits in order to standardise the test conditions imposed on the circuit.

Robin insulation tester

Robin insulation tester

Bridge Megger scale

Bridge Megger scale

Bridge Megger

Bridge Megger

Special mains-powered testers are used for high-voltage cables, capable of reading resistances up to thousands of megohms. Another widely used variant of the hand-cranked insulation tester is the so-called bridge Megger, which incorporates, or has facilities of connecting, a decade box and/or multipliers. This enables a wider range of tests to be undertaken, such as Varley loop tests that use a Wheatstone bridge circuit to locate a fault within a cable.

Another common test is that of earth continuity, which is intended to reveal high-resistance joints and disconnections in the protective (earth) conductors. Unlike insulation testing which requires a high voltage, continuity tests take place at a low voltage and the meter displays readings ranging from a small fraction of one ohm up to hundreds of ohms. This can be achieved with a simple battery-powered instrument although some insulation testers included a continuity range also powered by the hand-turned generator. Many battery powered units used a bridge-connected movement, making the reading independent of battery voltage without the need for a zero-adjust knob.

Wee Megger and matching continuity tester

Continuity tester

Continuity tester

Insulation tester

Insulation tester

Megger in case

Megger in case

These instruments were made by Evershed and Vignoles, manufacturers of the famous Megger brand instruments. They have been rebadged for GEC by fixing a plate over the area in which the Evershed name would be moulded into the case. The Wee Megger was a small insulation tester available in both 250 and 500 volt versions. The working principle is identical to that of the larger models, although the meter is not as sensitive and reads up to 50 megohms. The continuity tester is powered by a 4.5 volt battery and uses a bridge circuit to measure from 1 ohm to 300 ohms in two ranges. As such it is remarkably immune to low battery conditions and when obtained, was still working reasonably well despite the battery on-load voltage having fallen to less than half a volt.

Clamp ammeters

This ingenious measurement technique is still used today with digital instruments. It allows currents to be measured without disconnecting anything, so lends itself ideally to heavy work and circuits which cannot be switched off. To take a measurement one simply closes the tong around the cable. The jaws form the magnetic core of a current transformer. Wound on the fixed jaw is a coil, which serves as the secondary, feeding the meter movement directly or through a range selector. The primary is the cable in which the current is to be measured, which is encircled by the tong to form a 'one-turn winding' as the current must complete the circular path via the external circuit. Versions were available with ranges from tens to thousands of amps. This principle will only work for AC, as a DC current will not induce anything in the secondary. The solution lies in the moving-iron DC clamp meter. Here it is the magnetic flux in the tong itself that deflects the meter needle. Naturally the only way to change range on a DC clamp is to replace the actual meter movement, as there is no electrical circuit into which a shunt can be switched. Moving iron instruments also tend to have extremely non-linear scales compared to moving coils, so a rather large number of closely spaced ranges are required for readability.

Ferranti clamp ammeter

Ferranti AC clamp

Ferranti AC clamp

This Ferranti AC instrument has two ranges, 20A and 100A, selected by a switch above the dial. The jaws accommodate a cable approximately two inches in diameter. The steel core is insulated to minimise risk of shock or short-circuit if the jaws touch bare terminals or busbars. As with standard rectifier-type moving coil AC meters, it measures the mean current but is scaled in RMS, and is therefore accurate only on sinusoidal waveforms. It is seen here measuring the load on blue phase of a substation transformer, which at a mere 80A reveals that the picture was taken out of working hours

Crompton Parkinson 'Tong Test' AC/DC clamp ammeter.

Crompton AC/DC clamp

Crompton AC/DC clamp

The kit contains a frame suitable for all ranges and four instrument heads, respectively 75, 200, 400 and 600 amps full-scale, which simply clip into the frame. The meter also works on AC but offers a lower accuracy than a true AC transformer clamp type. The scales can be seen to show compression at both ends due to the characteristics of the moving-iron instrument. The red lines on the jaws show where the cable must lie for the calibration to be correct. This type of meter would have been used by electricians working on industrial installations with large DC circuits, such as electric traction and machinery drive supplies.

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