Electrical Instruments

1 | Practical Marine Electrical Knowledge

Insulation resistance (IR) tester.A measurement of the insulation resistance (IR) gives one of thebest guides to the state of health of electrical equipment. Theresistance should be measured between insulated conductors andearth, and between conductors.An insulation tester is a high reading resistance meter using a hightest voltage - usually 500V d.c. The test voltage is produced eitherby an internal hand-driven generator or by a battery andelectronic voltage charger. A test voltage of 500 V d.c. is suitablefor testing ships' equipment rated at 440 V a.c. Test voltages of 1000V and 5000 V are used for high voltage (HV) systems on board ship.There are several manufacturers of insulation testers available butthe Megger trade name is known worldwide.To prove the basic operation of the tester, short the two probestogether, switch to "M" and press the test button or rockerswitch.The pointer should indicate approximately "0".

Before applying the test, the equipment to be tested must be disconnected from the livepower supply and locked-off according to standard safety procedures.A megger type IR tester can be used to check whether the circuit to be tested is live. Switchthe instrument to "M" and connect the probes to pairs of equipment terminals. DO NOT pressthe button. The meter will now indicate that the circuit is live or not. If the circuit is dead it isthen safe to press the test button. Confirm that a reliable earth connection is obtained byconnecting the probes to two separate earth points on the equipment frame while testing forlow resistance continuity.For an IR test on a three-phase machine, measure and log the phase to-phase insulationresistance values. Three readings should be measured as U-V, V-W, W-U as shown in below.

Measure and log the phase to earth insulation resistance values. Three readings should bemeasured as U-E, V-E, W-E:Note: Insulation resistance decreases with increases of temperature.Continuity TestingAn insulation tester normally also incorporates a low voltage continuity test facility. This is a lowresistance instrument for measuring the continuity (or otherwise) of conductors. It can be usedto measure the low resistance of cables, motor windings, transformer windings, earthing straps,etc. The procedure for use is similar to that for the insulation tester.

Electrical Instruments

2 | Practical Marine Electrical Knowledge

* PROVE the correct operation of the instrument.* ISOLATE and lock off the equipment to be tested.* PROVE the equipment to be dead.* Switch the instrument to "" or "continuity".* Connect the probes to the circuit.* Operate the test switch and check the indication

on the "W" scale. Log all readings.

MultimetersRoutine electrical test work involves measuring current, voltage andresistance i.e. Amps, Volts and Ohms. This is most conveniently doneusing a multimeter with all the necessary functions and ranges. Theinstrument may be the traditional switched-range analogue type(pointer and scale) or the more common digital type with auto-ranging and numerical display.Digital meters have a clear numeric readout which may besupported by a bar-graph display. Where distorted voltagewaveforms are likely (e.g. with variable frequency motor drives) it isnecessary to use a "true-rms" meter for accuracy. Digital meters arealso available which display the test voltage waveform shape with astorage oscilloscope facility on the LCD screen.In all instrument models an internal battery is fitted for use whenmeasuring resistance.Before measuring the resistance of a component it is essential thatthe circuit is switched off, locked off, and any capacitorsdischarged. The instrument is likely to be damaged otherwise.The multimeter should be proved for correct operation before use. The manufacturer'sinstructions should be carefully followed for this but a general procedure is as follows:Use the correct probe leads and insert into the correct sockets on the meter.Current ClampmetersPower currents (a.c.) can be measured simply by meansof a clampmeter which acts as a current transformer. Theinstrument tongs are clipped round a single insulatedconductor - the circuit is not interrupted.The value of current is obtained from the magnetic fluxstrength around the conductor and is usually displayedon a digital display. Direct current (d.c.) measurement isalso available with clampmeters having a flux-voltagetransducer known as a "Hall-effect" device.Many modern clampmeters are virtually multimeters withthe addition of facilities to measure voltage andresistance as well as measuring currents up to 1000 A.