Insulation tester - some guidance please

That's the normal position for the needle to be when not being used, my 500V model which is the same as shown works perfectly, a quote from the manual describing the how they work,

"there is no appreciable restoring force to make the pointer come to rest in any particular position"

Measuring insulation resistance with high DC (relative to normal operating voltage) is pointless if all you are trying to ascertain is whether the equipment is safe to energize. Take a typical domestic 240V installation with MCB overcurrent protection and 30mA RCD earth fault protection. To operate the MCB the phase to neutral or earth insulation would need to have an resistance of just a few ohms and to operate the RCD an resistance of a few Kohms. Both these values are easily detectable with a multimeter and a deciscon on the safety of energizing can be made. Once the system is energized the safety of the system and its insulation is down to its design, maintenance and operation.

However if you are trying to ascertain the condition of the insulation then its resistance (measured with high DC voltage) may be helpful but interpreting the results will be difficult.

At higher system voltages (above 11kV) there is evidence that high DC test voltages cause damage to plastic insulated cables.

My view is use a decent multimeter to check your insulation.

A long time ago I used to drive for a firm that repaired fridges. One of the engineers used to get a kick out of asking an apprentice to hold a wire from a 'windy' megger which he then wound up and shocked the guy.

The apprentices learnt to keep away from him when he was in a 'joking' mood and recognised the megger as the source of his fun.

So what happens next - engineer buys a push button megger that they don't recognise and the cycle starts over!

Michael Gilligan, yes, I noticed the lettering does seem odd on that megger, there was a mine version which was lower voltage (I have one) but they were black.I alsao have an older wooden one with "Royal Engineers" branded on it! Now we know there was also an aircraft, and a train model too! The needle position is random till the handle is wound, so could be anywhere on the scale. Sorry Simon Williams< I can't agree on robustnes, I have spent literally thousands on modern digital equipment, and have chucked it all out when it failed, and not replaced it, but I am still using an AVO8 and a megger from the early seventies! As to digital equipment being more accurate, don't believe it, check your specs and you will find that your test gear has a given accuracy at a fixed temperature only, because its electronics is sensitive to ambient changes, the AVOs and meggers use wirewound resistors which are unnafected by temperature, and anyway, the actual difference in claimed accuracy is so small as to be irrelevant, but the AVO will hold its accuracy over a much wider range of ambient temp. As to sensetivity, they all measure within the parameters of the selected scale. My problem with digital meters is that they are trying to measure values which are very often analogue! try watching a capacitor charge up with a digital meter, and looking for the little needle dips which indicate breakdowns at certain voltages, or going around tapping and rattling to find a loose connection or broken flex, ok, you can press hold and see a reading for a varying voltage, but on an analogue meter, you can watch it happen.

In the late 50s as an apprentice in the YEB meter test room it was a joke to get the lad to hold the wires of the 1000volt megger, but they could not understand why I could hold on as they wound.

So long as I held tightly I only got a tickle, laugh on them.

It was a different story with the later electronic version though, I could not hold that.

Hi Phil, after over 40 years of electrical and electronic fault finding I stand by my comment on the need for an insulation tester, it spent most of its life in the equipment cupboard whereas my fluke multimeter spent most of its life in my pocket. As an apprentice motor failures were much more common but as with most things they have become much more reliable so that failure is now very rare, the last facilty I worked on probably has 10,000 motors installed and motor failure is very rare and determining failure rarely required an insulation tester or a bridge megger. As small motors are not economical to repair the precise nature of failure is not worth the time and effort to diagnose. I am not proposing that a multimeter does what an insulation tester is for, just that it is a more useful tool most of the time. My immersion heater didn’t need anything more than a multimeter to inform me the element was down to earth and open circuit but that was a particularly catastrophic mode of failure. Tracing the condensation in a socket box that was tripping the RCD very definitely did need an insulation tester, it’s always useful to have the right tool for the job but in my experience an insulation tester is rarely required but when it is it nothing else will do.

Mike

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Thanks for that, Stuart

MichaelG.

Yes Michael Giligan, good info, my wooden case model is the safety bridge megger, the second of the three pics. Mike Poole, not disagreeing with you, all you really need to tell a motor is gone is your nose! It may be that more modern installations than the ones I worked on in the sixties and seventies have better insulation, and fault to earth less, but a lot of what I did was in factories where the common denominator was wet! turkey and other food processing, and gravel pits, to mention a couple. earth leakages were very common. Today I have thank goodness retired from all that, and spend my time building, repairing and playing with machines and machine tools, much more rewarding, and a damn sight warmer and drier.

Edited By Phil Whitley on 20/12/2018 20:20:12

If I remember it right, the IEE Wiring reg's say the test source for testing insulation of low voltage equipment energised at 230 V AC must provide not less than 500 volts DC into a load of 1 megohm, other loads pro rata. Sorry gents but I don't think your windey Meggers do this.

I take Phil's point about liking analogue meters - I happen to prefer my digital, but it's horses for courses. However I can't let the assertion that a simple multimeter is sufficient to prove the equipment is safe to energise go unchallenged, the Regs say otherwise. By all means go looking for a fault with a DVM, but the testing to prove that it's safe to plug it in to the mains has to be done at an elevated voltage. Phil's right, all you often need to find there is a fault is a sense of smell!

The use of 5KV (flash testing) would only be warranted if the equipment was being re-designed or modified, or there was some reason to believe that a simple insulation test was insufficient. Or you've got a particularly difficult customer!

Mince Pies out for Santa + 4 days!

That is what my windey megger produces Simon!

This is not a fault. The meter movements in Meggers do not have a return spring and the pointer "floats" when the handle is not turned. The movment has two coils, a voltage coil that drives the pointer towards infinity and a current coil that drives it towards zero. It's a ratiometric instrument so the voltage does not have to be spot-on or zeroed like you do on an AVO.

Robert G8RPI.

1000 volts is chicken feed! We were often shocked by 6kV pulses. Now, the Van der Graff generator could easily make your hair stand on end.

We once had car seat covers that would cause a flash to earth across one’s shoe soles, after extracting oneself from the car, especially if one was wearing a synthetic shirt. Had to hold the car door, while getting out, to dissipate the static. They did not last long.

They don’t recommend playing with high voltages, these days, as it can reduce one’s favourable response in the case of ever needing to be defibrillated.