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. Unfortunately, the vast majority of electricians only have two hands, so this arrangement is rather inconvenient.
But now there’s a better way – two-wire phase-sequence checking, as provided in Megger’s new AVO830 and AVO850 multimeters. Here’s how it’s used. 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. At no stage during the test are more than two hands required!
How does it work? 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. Fifteen seconds is, however, more than enough time to move a test lead from one terminal to another!
Phase sequence checking, which used to involve inconvenient juggling of three test leads, has now been reduced to a simple test that needs only two connections at a time. And, an added bonus is that if you’re using a Megger AVO830 or AVO835 multimeter, the function is built in, so you don’t even need a separate phase sequence tester.