Sparks Electrical News June 2020

TOOLS OF THE TRADE

7

WORKING KNOWLEDGE WITH TERRY MACKENZIE HOY

Fun things you can do with an analogue meter

A digital volt/amp meter is one which has a display that is in digital numerals – in other words, when you measure some- thing, the value you are measuring is displayed as numbers in on a screen. For this to happen, the signal you are measuring has to be electronically manipulated. If measuring voltage then the voltage has to be converted to a digital signal by means of an ana- logue to digital converter and then this has to be fed to a system which allows the digital value to be displayed as a number on a screen. An analogue volt/amp meter has a needle which indicates a value on a scale, like the speedometer of a car. It does not have any electronics – the voltage is scaled down and supplied to the meter via range selection switches. Digital meters came into wide use in the mid-1980s and were popular since they were thought to be

me to explain how... Other advantages of an analogue meter are that you can use it to synchronise generators and that it can recognisably display slowly changing voltages. A digital meter can do this in that the various voltage levels are displayed over time, but the period of change or oscillation is not. Having said all of this, it is unlikely that any electrical person out there will actually go out a buy and use an analogue meter. But you should think carefully about it. During that part of my life when I was working on switchboards and machinery, I found that reading a value from a meter made me think about what I was measuring and made me more aware of inaccuracies than I would have been had I used a digital meter. r. Give it a try. Analogue meters are available and not expensive.

This indicates an inductance is part of that circuit. This means you can check that current transformer circuits are not open circuit and you can check if all three phases of a motor are connected. You can also check if a capacitor is in circuit: you set the analogue meter to measure resistance and connect the leads to each of the wires of the circuit. The needle will swing over to indicate very little resist- ance and then, as the capacitor charges up, the needle will fall back to indicate high resistance. An analogue meter can also show if there is induced ac current in an electronic signal. Set the meter to measure very low dc amps (0-100 mA). If the needle is steady, there is no induced ac current. If it vibrates, an induced current is present. Something which may not be needed these days is to measure the voltage across a dc shunt. An analogue meter can do this, but you will have to contact

accurate and, importantly, lightweight. This last point may not seem to be very relevant, but the analogue meters, such as the Simpson meter or the AVO Mark 8 were really quite heavy – a digi- tal meter fitted neatly into a pouch on your belt. Digital meters had their problems; they had a high internal impedance which means they could produce measurements that were inaccurately high due to stray electrical charges. Many digital meters do not measure voltages with harmonics very well – an ‘harmonic’ is just a superimposed voltage on the measured voltage. An analogue meter measures true RMS (actual value). Things that an analogue meter can measure that a digital meter cannot are the following: In- ductance in a circuit. Sometimes a circuit is ter- minated in an inductance – specifically and of- ten, a current transformer coil. With an analogue meter you set the meter to measure resistance and connect the leads to each of the wires of the circuit. The needle will swing over to indicate very little resistance. Now, reverse the leads. This time, if there is an inductance present, the nee- dle will hesitate a bit and then only swing over. A ction Can EC-90 is a proven and tested fast evaporating electrical cleaner that does not leave a residue, requires no rinsing or wiping, and quickly cleans hard to reach delicate electronic and electrical components. It is recommended for all professional mechanical and electrical engineers. The product effectively removes deposits and tar- nishes, improving conductivity and penetrating into switches and contacts, preventing electrical faults. It is safe to use on all metals, most rubber, plastics and coatings and it is non-conductive, meaning it can be used on electric motor contacts, generator and transformer windings, electrical parts, switch-gear and fuses. About Action Can Action Can provides a comprehensive range of first-class products. Its ISO 9001 and ISO 14001 accreditations demonstrate the compa- ny’s commitment to research, development and manufacturing quality standards. Electrical contact cleaner aerosol spray

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SPARKS ELECTRICAL NEWS

JUNE 2020