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Insulation tester - some guidance please

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gerry madden18/12/2018 18:35:34
48 forum posts
8 photos

Hi all you competent electricians. I'm thinking of buying an insulation tester for Xmas and see that prices vary from around £50 to £500. (...there's even one for £13 which wont make the shortlist for that reason alone !) I don't want frills and already have an AVO. So is going for the lower end of the range sensible or is there something important that that I should be aware of when buying one of these devices ?


Emgee18/12/2018 18:51:19
1110 forum posts
200 photos

Hi Gerry

For your home use you may only need to test at 250v, my Megger tests at 250,500 and 1000v but used for work.
If there is no need for a certificate to show the meter conforms with the testing parameters that will save money.

What use do you envisage for an insulation resistance meter ?


Simon Williams 318/12/2018 19:53:17
386 forum posts
65 photos

If I remember my IEE Regs correctly testing domestic circuits normally energised at 230 volts AC is to be done at 500 volts DC, three phase industrial circuits subjected to 400 volts (line to line) is done using the 1000 volts facility. You might not need the latter but most instruments will offer the facility - they'd be no use to a commercial electrician if they didn't. Which kinds of begs the question of the quality of an instrument which doesn't offer the facility.

Insulation testing is about whether the appliance under test will kill you or not, and the same goes for the continuity tester which will likely be part of the same instrument and which performs an equally important test. At least with a calibration certificate in your hand you know if the information acquired from the measurement can be trusted.

As Emgee notes, if we could have some idea of your intended use for such measurements we could offer better relevant advice.

Seasons Greetings Simon

StephenS18/12/2018 23:33:58
31 forum posts


Some time ago I purchased a MASTECH MS5201 insulation resistance tester (commonly known as a megger) from Ali Express and I am very pleased with it. Good specs and a good price. Some of the cheaper units did not appear to have good low insulation resistance readings on the high voltage setting if that makes sense. So on the 500 volt setting (normal for 230-250 volt countries) the readings that are critical are in the 0.5 to 5 Megohm range as anything above this is largely academic. Some of the cheaper units did not appear to be able to display meaningful readings below 2 or 5 Megohm and this made them useless for me. Recommended unit for my general usage.

Hope this helps. Cheers and Christmas greetings, StephenS.

Mike Poole19/12/2018 00:10:37
1956 forum posts
46 photos

I have a Megger insulation and continuity tester and I can’t remember when I last used it. I would say it is a useful tool for an installation electrician but for maintenance it is rarely required on equipment fault finding but may be more useful on domestic mains wiring. If I didn’t have one it would be well down my list of desirable toys but if I lost or broke my Fluke I would buy another immediately.


gerry madden19/12/2018 18:08:09
48 forum posts
8 photos

Thanks all for your useful and thought provoking comments.

I suppose when I think about it, it wouldn't be the most frequently used tool in my armoury. But I occasionally do small mods to my domestic arrangements and it would nice to be able to say I have made 'that final check and all is ok'.

Also what seems to be popular at the moment here is two washing dryers tripping the ELCBs due to leaky heater elements. When I check these for insulation resistance with a normal Ohmmeter you seem to be able to get any figure you want up to 20Meg depending on the time of day. I was hoping that a higher test voltage would make this kind of check more reliable. (I do hate to spend money on parts when I don't have conclusive proof that they are defective.... of course its always ok to spend on tooling )

Thanks StephenS for your positive comments on the Mastech. That looks like a reasonable device.

SillyOldDuffer19/12/2018 18:24:26
4389 forum posts
956 photos

Forgive me for going slightly off topic, but I've often wondered how Megger insulation tests are applied? By using a high voltage they can measure high resistances whilst also stressing the cable to expose voltage breakdown faults, exposing faults invisible to a low voltage tester. A jolly good test, but brutal.

Is it necessary to disconnect everything before using a Megger? For example, I'd have thought putting 1000Vdc into a anything semi-conductor like a switch mode PSU risked killing it.


Phil Whitley19/12/2018 20:57:54
839 forum posts
105 photos

Simon Williams is correct, for 240v installations you use a 500v tester, I have had Fluke, and Sanwa, and many other electronic testers, to day they have all broken, or been BER at sevice time, and I am still using my AVO8 Mk5, and a "windey" megger, both from my apprenticeship days, you can buy windey meggers on ebay for about £20 or so. I also had a megger made electronic insulation tester, lasted about 5 years. Buy analogue, you will learn nothing from a digital meter. The only reason they went digital is that the analogue meter movements are too expensive to make!

Edited By Phil Whitley on 19/12/2018 20:59:15

Phil Whitley19/12/2018 21:16:29
839 forum posts
105 photos

Mike Poole, an insulation tester is an essential tool when working on electric motors and electric heaters, which almost always fail "down to earth" They are used for final insulation tests on domestic installations, but that is about all. Absolutely essential tool for fault finding, but NOT on electronics!

Andrew Johnston19/12/2018 21:29:19
4689 forum posts
527 photos
Posted by SillyOldDuffer on 19/12/2018 18:24:26:

Is it necessary to disconnect everything before using a Megger? For example, I'd have thought putting 1000Vdc into a anything semi-conductor like a switch mode PSU risked killing it.

Rectified single phase is around 320VDC, full wave rectified 3-phase about 720VDC. So not that far from 500V or 1000V respectively. There are nasty spikes on the mains as well. One of the EMC/EMI mains tests applies a series of spikes on top of the mains, of both polarities, at 500V, 1000V and 2000V to see if anything fails. The whole test sequence can take around 2 hours. Which is long time to hold ones breath and hope the equipment under test doesn't go phut!

Insulation testers will be fairly high impedance so the currents flowing will be low. Any proper mains input equipment should have transient protection on the input against the spikes. These will cope with the low DC currents.


Harry Wilkes19/12/2018 22:18:44
676 forum posts
59 photos

Ah a megger 500v or a 1000v would break the will of any apprentice wink


Simon Williams 319/12/2018 23:05:45
386 forum posts
65 photos

If you want to know if a piece of equipment is safe to energise the insulation tester is the kit to reach for, as a low voltage resistance test using a Fluke DVM as an ohmmeter doesn't cut the mustard. Other makes are available. Quite apart from the regulations, you need that high voltage for the test to be a valid representation of the real life energisation using mains supply. So while the insulation tester isn't the most frequently used bit of kit an anyone's toolbox, when you need it, you need it.

The same goes for the continuity tester. A simple ohms check won't pass 20 mA (the specified minimum test current through a short circuit) through the earth continuity conductor, and won't find loose screws, oxidised connections and the like.

So it's a specialised gadget for a very specific purpose, being the final checks to confirm that an appliance is safe to energise. By all means do the best you can with a digital ohmmeter but bear in mind these are not representative tests of the satisfactory state of the appliance, and if you are taking responsibility for someone else's welfare (don't we all?) the tests had better be done in the approved fashion.

Don't agree about digital vs analogue, digital is much more robust, holds its calibration better, is at least one or maybe two orders of magnitude more sensitive.

Testing stuff with semiconductors exposed to the test voltage is a whole different ball game, well beyond the scope of the casual user. The possibilities for ending up with a piece of equipment which you know is safe to energise but now needs repair are endless.

As Phil has it above, if you are testing motors and heaters, an insulation tester is going to be essential, not least because it's the absolute first question anyone using the equipment should (will) ask. Has it got a valid insulation test? Second question is what is the earth continuity conductor resistance. Without confidence in the answers (for which read sight of a valid test certificate for the measuring instrument) cut the plug off.

HTH Simon

Simon Williams 319/12/2018 23:32:31
386 forum posts
65 photos

FYI measuring something with an ohmmeter and getting a varying reading is absolutely typical of having water in the circuit. Did I mention you need an insulation tester?

Seriously this is very much an example of where the high test voltage of a "Megger" (other makes etc) gives you an answer you can believe. With the high test voltage of a proper insulation tester you will (probably) get a more repeatable answer, though of itself the fact that the measurement is not constant indicates that the equipment should be withdrawn from service, even though the range of readings is otherwise acceptable.

Seasons Greetings to one and all


Michael Gilligan20/12/2018 08:09:18
13230 forum posts
578 photos
Posted by Phil Whitley on 19/12/2018 20:57:54:

... you can buy windey meggers on ebay for about £20 or so.


I would be rather wary of the specific ebay item that you linked, Phil


You are obviously familiar with these things, so ... Can you offer a reasonable explanation of why someone would deface the original moulded Voltage rating, and overwrite it with '100 VOLTS' ?

I agree with your preference for a 'real' Megger, but I would be uncomfortable with using that one.



P.S. for anyone interested, here's the patent

Edited By Michael Gilligan on 20/12/2018 08:13:12

V8Eng20/12/2018 09:40:05
1278 forum posts
27 photos


We used 100 volt wind up Meggers in my BR days specifically for testing electrical systems on trains.

The scale does appear to be printed 100v but the lettering on it’s casiing seems odd.

Edited By V8Eng on 20/12/2018 09:43:00

Robert Atkinson 220/12/2018 09:55:43
260 forum posts
17 photos

Given the difference in colour between the two case halves I'd say the front cover was damaged and it was replaced with one from higher voltage version of the same model. A quick check with a voltmeter will show if the volage is correct.


Robert G8RPI.

Edited By Robert Atkinson 2 on 20/12/2018 09:56:47

Emgee20/12/2018 10:31:12
1110 forum posts
200 photos

I believe the Megger linked to by Michael is not working correctly, the meter is indicating a reading when not being cranked, without operating the handle the needle will be at 1 end of the scale, don't remember which end but I used that type of instrument for a few years before digital meters were available.


Russell Eberhardt20/12/2018 11:43:04
2456 forum posts
83 photos

If you want to test domestic appliances the general test required by the safety standards is a flash test of 2000 V for equipment with an earth connection and 4000 V between mains and any exposed metal parts for equipment without an earth connection.


V8Eng20/12/2018 12:41:55
1278 forum posts
27 photos
Posted by Emgee on 20/12/2018 10:31:12:

I believe the Megger linked to by Michael is not working correctly, the meter is indicating a reading when not being cranked, without operating the handle the needle will be at 1 end of the scale, don't remember which end but I used that type of instrument for a few years before digital meters were available.


Might that depend on which version? I certainly remember the needles settling at about the point shown until the handle was turned they would probably have been ones originating from the 1950s.

There was also a big wooden cased hand wound four wire version (if my memory is right) for testing earth rod systems.

They could certainly make your hand ache if doing a lot of testing.

Stuart Bridger20/12/2018 13:13:11
299 forum posts
17 photos

My first job was testing and repairing such devices.
The 4 wire "bridge meggers" were locally known as "bongo testers" and used for testing electrical bonding of aircraft structures.

Good Information here **LINK**

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