Megger’s new AVO830 and AVO835 multimeters have a range of functions, including finding hidden cables, measuring capacitance and checking phase sequence

CAPACITANCE MEASUREMENT Capacitors are used in a wide range of applications in today’s electrical installations – including, for example, spike and harmonic suppression, motor starting for single-phase motors and smoothing in power supplies. The ability to measure capacitance with a good degree of accuracy is therefore very useful and is provided by many modern multimeters. In a perfect world, every capacitor


hen it comes to finding hidden cables, there’s no doubt that a

non-contact voltage detector is a very convenient tool. Bring one of these devices close to a live cable, even if it’s buried in a wall or hidden in some other way, and the detector will buzz to alert of the cable’s presence. If necessary, the detector can be moved to track the route of the cable. At least, that’s what should happen. In reality, non-contact voltage

detectors are often overly sensitive, which means they start indicating when they’re tens of centimetres away from the live cable. This makes it difficult to pinpoint the cable’s exact location and it makes cable tracking almost impossible, as the indication from the detector occurs over such a wide area. The solution is surprisingly simple

– a non-contact voltage detector with switchable sensitivity, which is what’s provided in Megger’s new AVO830 and AVO835 multimeters. When low sensitivity is selected, the detector only operates when it’s within a few centimetres of a live cable. This means that cable positions and routes can be determined accurately. When high sensitivity is selected, the

instrument can detect live cables at a much greater distance, which is very useful when trying to find cables that are buried deep in walls or floors. A brief reminder is, however, needed.

Even the best non-contact voltage detector must NEVER be used to prove that a circuit is dead. Correct procedures for isolation and proving isolation must be followed, and these procedures will always involve using an approved form of two-wire tester and making a direct connection to the circuit under test.


would be properly discharged before any attempt was made to measure its capacitance but it sometimes happens that this is not the case. For this reason, manufacturers of the best multimeters provide a voltage-warning feature. If the meter is connected to a charged capacitor when the capacitance range is selected, it will alert the user to the problem and inhibit the test. The capacitor must be discharged, but

not by using the age-old trick of shorting the terminals with a screwdriver. Capacitors can store a lot of energy and releasing this into a short circuit can create a dangerous arc, leading to blobs of molten metal flying out from the screwdriver blade. The capacitor may also be damaged by such a violent discharge. It’s essential to use a safe form of

discharge device, which need be no more than a resistor of a few kΩ connected to two well-insulated probes. The new Megger AVO830 and AVO835 multimeters produce an alarm and inhibit testing if voltage is detected when they’re switched to the capacitance range. However, they then automatically

switch to an input impedance of just 10kΩ, so the capacitor will be discharged safely in a realistically short time. During the discharge, they continue to display the remaining voltage on the capacitor, so the user can be absolutely sure when it’s safe to proceed. This automatic discharge feature

makes testing capacitors safer, easier and more convenient.

PHASE SEQUENCE When working on three-phase systems, it’s important to know the phase sequence. Getting it wrong can mean that motors run backwards, which can cause serious mechanical damage.

Because phase sequence is so

important, there are many instruments available for checking it but most of these have a problem – they need three connections, one to each phase, and these must be made simultaneously. Now, two-wire phase-sequence checking is possible, as provided in the AVO830 and AVO850 multimeters. After selecting the phase sequence

function, the user simply connects the multimeter’s red test lead to Phase 1 of the circuit under test, and the black test lead to Phase 2. Then the user transfers the red test lead to Phase 3, and the instrument instantly shows whether the phase sequence is 1,2,3 or 3,2,1. When the connections are made to

Phase 1 and Phase 2, an internal oscillator within the multimeter instantly synchronises itself with the supply waveform. Even when the connection is broken, the oscillator will maintain synchronism with the supply for a considerable time. This means that when the red

connection is transferred to Phase 3, all the multimeter has to do is compare the phase of the supply waveform it now sees with the phase of the internal oscillator and it can easily work out the phase sequence. In practice, the multimeter will inhibit

the test if the user takes longer than fifteen seconds to transfer the red test lead from Phase 1 to Phase 3, to ensure that the internal oscillator never drifts out of synchronism. Phase sequence checking, which used to

involve juggling of three test leads, has now been reduced to a simple test that needs only two connections at a time.

Megger 

Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44