Replacement Insulation tester - UNI-T UT51x series?

[WaveyDipole]

My Robin model 3001 insulation tester failed recently. It was working when last used a few months ago, but recently when I wanted to use it, I found that it was giving me no readings on the HV ranges, and the reading on the Low Ohms setting were erratic. I cannot find a circuit diagram so am a bit lost regarding diagnosing the problem so have been wondering what to replace it with? I don't want to spend more then 200GBP, preferably much less as I am a hobbyist and would only need to use it occasionally.

I found a couple of Fluke 1520's on eBay, but then discovered a thread where someone who had two in storage suddenly found that both had failed. The problem seemed to be a fried IC which apparently was suspiciously close to a HV track, the track also running via the function selection switch. I therefore decided to avoid this handheld. Considering my Robin failed is also a handheld, this also kind dissuaded me from looking at handheld devices. I also looked at used Megger BM11 and similar devices, but these types of devices are huge and for me, space is a consideration.

I then came across the UNI-T UT51x series of devices. These appear to be robust and although larger than a typical multimeter, seem much smaller than those devices in yellow cases. They are cheap enough to fit the budget and Dave's teardown of the UT513 gives it the thumbs up. Of course, not having access to them, he was unable to comment on other models in the range. The only other information I could find was this thread:

https://www.eevblog.com/forum/testgear/uni-t-ut511-insulation-tester-unboxing-and-preview/msg187133/#msg187133

The problem is that the pictures are blurred and with a Photobucket logo, but clicking on the does nothing. The thread is quite old so I suppose photos are no longer available online. The narrative seems relatively positive.

One thing that puzzles me though is that the UT511 seems to be the only model in the range that seems to have the Low Ohms feature mentioned in its description? Is this correct? If so, then why? With its facility to test at up to 1kv, the UT511 might be sufficient for my needs and has the advantage of having the Low Ohms function, but without a clear indication of its build quality I am somewhat hesitant. On the other hand, the UT513 (the UT513A costs rather more and is above my budget) has the Dave's approval and the additional 2.5kV and 5kV modes might be a useful addition, but other than testing things to destruction, would I ever have the need to use them?

Is there anything else on the market, new or used, that I should consider before making a final decision?

[blacksheeplogic]

I've have both a Fluke 1507 and Fluke 1654B for a number of years both without any issue. I also have a Megger MIT485 also a handheld for a number that again has not given me any issues.

I don't have a 1520 but they have been out a long time and I've not seen anything negative written about them. I doubt very much there is anything systemically wrong with them either. Like any piece of equipment you can get a failure but I don't see a single failure as reason to condemn the 1520 as failure prone.

[Messtechniker]

You might also consider the Peaktech 2695 Insulation Tester
I am using to test all newly bought electrical equipment
regardless of any CE TÜV or other kind of approval.
The reason being a severe incident with thankfully no
permanent arm injury to a friend caused by a chinesium
construction site floodlight purchased from a local store.

[threephase]

I have had a couple of failures of handheld insulation testers over a 30 year career, but they are quite rare. A Megger BM223 failed, a DiLog DL9307 popped the varistors and I have just had a MetraHit unit fail. Personally, I wouldn't get too dismissive of handheld devices, especially if you have a limited budget.

I have had a UT513 for coming on 20 years now and it still works fine, it is likely that the design between the 511 and 513 will be very similar, and build quality should also not change. There has been revisions to the unit since mine was built, so you cannot really use these older units as a guide.

They do use size C batteries, which isn't one that I have around a lot. It makes the units a little bit heavier, but on the plus side they last for ages. My UT513 will also operate whilst still plugged into the mains adapter. There is no tilt stand on these units, so if you are using it for a lot of bench work, the viewing angle is not always the best, it is either stood up straight or laid flat. You can of course build a stand for it or prop it up. Out in the field I do not find this an issue. You also have to swap over connections between measuring resistance and voltage.

Generally insulation testers above 1kV do not have a resistance range as the apparatus that you are testing with them, will have very low resistances and a micro-ohmmeter is used to test these that is 4 wire test and higher current for improved accuracy under these circumstances.

I would suggest that if you are purchasing a 5 kV insulation tester for destruction testing, you are likely to be quite disappointed. The insulation testers are designed to push out 1mA max at full voltage, after that the voltage will drop off rapidly and the short circuit current will be under a couple of mA. This limits the energy output, and it just isn't enough to cause any serious destruction. There are exceptions to this of course if the apparatus under test is rated for very low voltage and the test voltage is massively excessive in comparison, then you may get a few sparks.

There are a number of devices out there in your budget range, it just depends what you are testing and the kind of results you want. Are you actually going to use DAR and PI functionality or are you just quick insulation tests for verification?

What kind of insulation resistance values do you want to measure up to? The manufacturers often quote the max insulation reading for the 1kV range, at 500V test voltage and below, this is often less, you need to look at the actual data sheet to get a better idea.

The Extech instruments are quite cheap, but are usually CEM rebrands and the build quality I have found to be a bit of a cross-fingers affair. There are some Mastech units out there that may be of interest to you, but are also cloned quite a bit and offered cheaper. The cheapest insulation tester I have found is a VC60B+ derivative units that are actually not that bad given the price tag, may do you just for intermittent hobbyist use.

Basic insulation testers aren't usually that complex, is there anyone close to you who could assist with fault finding on the Megger?

Most of my insulation tester reviews are on element14, and I haven't really started looking at the cheaper units yet.

[WaveyDipole]

I don't have a 1520 but they have been out a long time and I've not seen anything negative written about them. I doubt very much there is anything systemically wrong with them either. Like any piece of equipment you can get a failure but I don't see a single failure as reason to condemn the 1520 as failure prone.

"Condemn" is a bit strong, but here is the thread about the Fluke 1520 and the reason for my concern:

https://www.eevblog.com/forum/repair/fluke-1520-megohmmeter-service-manual/msg1270548/#msg1270548

I would have purchased the Fluke but for seeing this. (final post by shakalnokturn)

I have had a couple of failures of handheld insulation testers over a 30 year career, but they are quite rare. A Megger BM223 failed, a DiLog DL9307 popped the varistors and I have just had a MetraHit unit fail. Personally, I wouldn't get too dismissive of handheld devices, especially if you have a limited budget.

Perhaps it was a stretch too far on my part to dismiss handheld devices altogether so thanks for the re-assurance.

I would suggest that if you are purchasing a 5 kV insulation tester for destruction testing, you are likely to be quite disappointed. The insulation testers are designed to push out 1mA max at full voltage, after that the voltage will drop off rapidly and the short circuit current will be under a couple of mA. This limits the energy output, and it just isn't enough to cause any serious destruction. There are exceptions to this of course if the apparatus under test is rated for very low voltage and the test voltage is massively excessive in comparison, then you may get a few sparks.

Yes, I was referring to Dave's video where he zapped a VC multimeter with 5kV and there was a nice display of sparks between the tracks:



I guess 'destruction testing' might have been a bit of an exaggeration although I wouldn't expect the DMM in question to have survived intact! I don't expect to be doing that kind of thing myself though.

There are a number of devices out there in your budget range, it just depends what you are testing and the kind of results you want. Are you actually going to use DAR and PI functionality or are you just quick insulation tests for verification?

What kind of insulation resistance values do you want to measure up to? The manufacturers often quote the max insulation reading for the 1kV range, at 500V test voltage and below, this is often less, you need to look at the actual data sheet to get a better idea.

My general usage would be to do insulation tests on any vintage equipment that I repair or restore. Recently had a PSU fail and I wanted to verify the insulation between the TX windings as well as to ground. I have no need for industrial testing, just domestic appliances and vintage equipment. I don't suppose testing at more then 1kV would e required for that? I got the now failed Robin model 3001 with something else and hadn't really considered what features might be required at the time. It had 500V and 1kV test options up to 200M as well as 'low ohms'. Modern new meters seem to go into the G/ohms but I'm not sure what scenarios such ranges and higher test voltages in excess of 1kV might used for.

The Extech instruments are quite cheap, but are usually CEM rebrands and the build quality I have found to be a bit of a cross-fingers affair. There are some Mastech units out there that may be of interest to you, but are also cloned quite a bit and offered cheaper. The cheapest insulation tester I have found is a VC60B+ derivative units that are actually not that bad given the price tag, may do you just for intermittent hobbyist use.

Mastech and VC are the sort of brands that I commentators usually advise to stay away from, although I guess in a very low budget bracket might be considered. Often it is stated to go for a used quality brand but my Robin model 3001 didn't last very long... I will be having another look at it in due course but in the meantime, I have decided to replace it with something new (or possibly used reliable brand) in any case.

These replies have given me some food for thought though.

[WaveyDipole]

Anyone have any thoughts on whether a 1kV insulaltion tester should be sufficient for casual/hobby use? or is it worthwhile spending the extra for instruments with higher test voltages?

[shakalnokturn]

I have a Fluke 1520 and Newlec NLins3 (https://www.eevblog.com/forum/testgear/newlec-nlins3-megohmmeter-quick-teardown-and-mod/)

The Flukes are nice to use, plenty of functionality, still quite expensive and will all be failing soon due to the question able design you mentioned.

The NLins3 often sell cheap on eBay.co.uk, I find they are well designed, get the job done. The only thing I miss is the ability to lock the test on.

Edit: Corrected link.

[WaveyDipole]

I have a Fluke 1520 and Newlec NLins3 (https://www.eevblog.com/forum/testgear/newlec-nlins3-megohmmeter-quick-teardown-and-mod/msg1360734)

The Flukes are nice to use, plenty of functionality, still quite expensive and will all be failing soon due to the question able design you mentioned.

The NLins3 often sell cheap on eBay.co.uk, I find they are well designed, get the job done. The only thing I miss is the ability to lock the test on.

I had not come across one of those NLins before, but they do indeed look like a Megger. Your note about the missing backlight (or not as it turned out) was a point of interest which I will bear that in mind if I happen to buy one. I expect that its a price point differentiator. I couldn't quite make out the brand of electrolytic caps in use, but it was also interesting to note that it uses a Japanese (not Chinese) relay and processor. Seems you got no replies to your detailed post though, but can I say thanks for putting that information up and for the link to the thread. (BTW, a '#' was missing before msg1360734, i.e. .../#msg1360734).

Regarding the Fluke 1520, IF that post is accurate, then the design would indeed be my concern although I sincerely hope that your Fluke does not fail in that way as well! I do not have sufficient knowledge in the matter of multilayer PCB design and its ability to carry high voltage between layers and across switches to make a determination as to the likelihood of failure. For example, might the PCB become a little conductive to HV in a damp environment? Could being stored in a damp shed or garage had led to the failure of both meters as discussed in that thread? I am aware that adequate spacing is required between HV circuitry and LV components to prevent arcing and for this reason sometimes air gaps are made in the PCB. The NLins has such a gap near the output pin on the PCB. I was hoping for a more definite answer from "the experts".

With that in mind, it was interesting to note in Dave's teardown of the UNI-T UT513 that the HV section was completely separated from the LV section and potted for good measure. I did wonder whether the cheaper meters in the range have the same design (including the HV section being potted), bearing in mind that the cost difference between the UT513 (£210-225GBP, note NOT the UT513A which costs even more) and next model down, the UT512 (£110-£150GPB) is significant, not to mention the UT511 (£85-120 GPB). Since they are manufactured to a budget, where have the costs been cut?

BTW, does the NLins3 show the reading as a numerical display form in the centre of the dial like the Megger?

[shakalnokturn]

BTW, does the NLins3 show the reading as a numerical display form in the centre of the dial like the Megger?

Yes, the numerical value is displayed same as the Megger.

I'm editing the link.

[WaveyDipole]

Thanks for confirming that.

[WaveyDipole]

I considered a number of options and in the end did pick up an Nlins3 on eBay complete with case, probes and set of batteries. It arrived yesterday. I checked the battery holder and batteries are good and contacts are clean. One of the probes has a damaged banana plug shroud that has broken away but otherwise the meter seems to work just fine and I am pleased with it. Just needs a good clean up. If you have any more details on how you managed to get the backlight to work this would be appreciated.

[shakalnokturn]

Oh sorry, I thought that was obvious in my short teardown topic.

Disassemble the front case, take a closer look at the last photo on my teardown, there's a faint black arrow on the edge of the function selector notches.
It's trying to say that the stopper pin on that side needs shifting (CCW in the photo) by one position, giving you an extra CW position when looking at it from the front.

After that it should work like the Megger's: Turn to full CW to enable backlight, then turn back to the function you want to use, skipping past OFF position fast enough.

When I get round to it I'll hack mine with something like a fast triple press on the pushbutton to lock the test on.

[WaveyDipole]

Ah, should have spotted that!. Thank you for elaborating on it further. I have just cleaned mine up a bit after which I had a go at getting the backlight working. This has been successful but I did have a couple of scary moments along the way.

I made a big mistake in removing that clip from the back of the knob that secures it to the PCB. It kept popping off and was a pig to get back on. I had to bend the tabs on the inside of the clip downwards a little with a pair of pliers to get a tighter fit and finally managed to get it back on so that it stayed put. The annoying part is that removing that clip is totally unnecessary!

The second scare was that when I had re-assembled everything, the meter did not work. It went through the motions, the relay clicked but no digital reading on the display and no pointer. It turns out this was another mistake on my part. I thought that the two screws on the rear of the PCB were securing screws and removed them first. However, they do not secure the PCB to the case but the clear LCD cover to the PCB. I realised this on removing the PCB from the case by slightly bending the single plastic clip in the centre and then loosely replaced the two screws. The cover puts pressure on the LCD display against the strip connector and they needed tightening a little so that the cover was flush to the PCB. On re-assemble the meter was OK again. Phew!

I see what you mean about turning the knob to the added 'LCD backlight' position and then back to the desired function. The light does not seem to have a timeout but seems to stay on permanently until the knob is turned to the off position. It is a useful addition and was worth the effort.

I did wonder whether the contacts should have some lubrication. i cleaned with Servisol and then cleaned off with a kitchen towel for now.

[shakalnokturn]

Contact lubrication is always hard decision to take for me.
It depends how much you're going to use it and how much dust the grease is likely to gather.
The knob is not that we'll sealed on these, dust does get in, I'd leave it without grease...

[WaveyDipole]

Do you have a manual for this Nlins3 by any chance? I couldn't find one anywhere.

[shakalnokturn]

Sure:
https://docs.rs-online.com/8fc6/0900766b8024ef20.pdf
All you'll have to do is edit the names in appropriate places 

[WaveyDipole]

Brilliant! Thank you.    Megger locks down the manual on their website and requires one to have an account to access it. That answers one or two questions I had. I also found that the meter defaults to a voltmeter mode if a voltage is detected which is another useful feature! Definitely happy with this instrument.