Seems like a while since I looked at an insulation tester, this one has been around for a some time, but I have only just got to writing it up. This is number 8 in my ongoing quest to see if there is a small handheld insulation tester out there suitable for working on generator rotors. Previous testers reviewed are;


Keysight Insulation Multimeter Failure


Megger MIT420/2 Road Test


Chauvin Arnoux CA6526 Insulation Tester Review


Flir IM75 Insulation Tester Review


Extech MG302 Insulation Tester Review


Di-LOG DL9307 Insulation Tester Review


RS Pro IIT1500 Insulation Tester Review


Uni-T probably needs no introduction as I am sure a lot of people will be familiar with some of their products. A lot are aimed at the hobbyist market, but they do make some instruments aimed more at a serious hobbyist / professional. Their range of insulation testers follows that pattern, with some very basic units at low prices, with some more higher performance items with improved functionality.


Uni-T Insulation Tester Range

I selected to look at a relatively new meter from their UT505 series, that offers functionality similar to the other instruments I have already looked at.


UT505 Series

I would have liked a UT505B as this has extended insulation resistance capability up to 200GOhm at 1000V, but this was not available in the UK at the time that I wanted to purchase, so instead, I looked at the UT505A, assuming that it works in the same manner, but only up to 20GOhm test capability.


Unboxing and Overview


The tester arrived in a branded cardboard box, with everything contained within a soft case to carry the instrument around in.


UT505A Box Contents

The contents are quite basic. The meter itself, red and black test leads, crocodile clips and test probes along with an instruction manual. The manual is quite well written, with step by step instructions for each of the tests to be carried out, certainly enough information if one was unfamiliar with the basics of insulation testing.


Uni-T UT505A Manual


Comparison to Keysight and Megger


The UT505A is of similar size to the Keysight and Megger units, there are however, very few ergonomics to it in comparison to the other two units.


UT505A Functional Comparison


UT505A Technical Comparison


The UT505A is more in line with an insulation tester in terms of the functions offered. It does have voltage and resistance measurement to add to the insulation test capability, but lacks the multimeter functions that are now being seen on some of these instruments.


Interestingly, it does have a variable voltage function, that is lacking in a lot of insulation testers. This is not a ramping function though and has to be adjusted manually by the operator. Its official price  puts it in the mid range of insulation testers available, quite a bit higher than the Exotech and RS Pro units. I say 'official price' as I haven't actually seen anybody selling it for that, and it can usually be picked up for half price or even lower, making it actually the cheapest meter tested to date.


In terms of its insulation test functionality, the 500V test range, that I am predominantly interested in is limited to 500MOhm, similar to the Fluke, Flir and RS Pro units and drastically below the Keysight and Megger units that will reach up to 200GOhm. The UT505B model would go up to 100GOhm, so would have resolved this issue had it been available.


The resistance measurement function offers an automatic test current adjustment, that exceeds 200mA for earth bond testing, as required by the IEC standard.


Performance Tests and Comparison


The UT505A was subjected to the same performance tests as all the other insulation testers. Insulation readings were generally within tolerance, a 10% deviation was seen for a low end value on the 500V test, but accuracy at this end is not too much of an issue of an insulation tester. At the top end of the scale, the tolerances were all found to be within 2%.


UT505A Insulation Test Results


The tolerance devinions plotted produced quite a crazy graph, with a lot of sporadic behaviour seen and not much correlation evident between the five test ranges.


UT505A Insulation Reading Deviation


Overall accuracy comparison


Compared to the other instruments, it doesn't come out too bad in terms of overall tolerance. It is quite a bit better than the DiLOG DL9307, approaching towards a similar accuracy level as the IM75 from Flir and close to the Chauvin Arnoux CA6526, when the two meters are compared over the same test ranges.


The 500V voltage regulation was typical for an insulation tester, the manufacturer specifying it to be within +10%, which it was well within. Compared to the Keysight and Megger units, it shows a much higher deviation, but these two units are just in a class of their own with this test.


UT505A 500V Regulation Plot UT505A Voltage Regulation Data


UT505A Open Circuit Tests


A scope capture of the output of the UT505A shows a sharp initial voltage spike, that then drops down below the test voltage before recovering back up to the specified value. This initial spike is quite significant, measuring as nearly twice the rated test voltage and is some of the worst regulation I have seen on an insulation tester.


UT505A Output voltage

In comparison to other instruments, the steady state voltage seemed to show more fluctuation and noise.


UT505A Short circuit readings


The short circuit current is within the IEC recommendations and in accordance with the manufacturer's specification of less than 2mA. It is also quite even across the test voltage range. Circuit discharge resistance seen below is the highest of all the meters tested. Not a specific problem, but worth while realising that it will take a bit longer to discharge a winding after a test.


UT505A Discharge Resistance


UT505A 1mA Load Tests


There were no problems with the 1mA load test requirements of the IEC standard. The current spread across the ranges was similar to the Chauvin Arnoux CA6526 unit and half that of the Flir IM75 unit, both £600+ testers.


Battery performance is seen in the picture gallery below. Low cut out is at 5.5V, giving a typical battery usage in comparison to the other instruments reviewed.


{gallery} UT505A Battery Performance

UT505A Battery Voltage Test

Battery Voltage Level Warnings

UT505A Battery Consumption

Battery Load Tests

Battery Capacity Comparison

Battery Capacity Comparison

Consumption data for each test shows the much higher load requirement during an insulation test, quite a bit higher than the 200mA earth bond test. Use of the backlight only adds an extra 15mA to the battery load. The backlight works well and enhances the display, to me this display isn't quite as crisp as the Megger unit, the analogue simulation is a straight bar across the bottom of the display, instead of the arc type at the top, which I tend to prefer.


Screen backlight comparison


Winding Simulator Tests


Now to test the meter against the winding simulator looking at the performance of the resistance and insulation test ranges.


{gallery} Winding simulator tests

Winding Resistance Test Results

Phase resistance measurements table

Winding PI measurements

Polarisation Index measurements table

Winding PI Plot

Polarisation Index Plot


Phase resistance measurements were to two decimal places and were some of the most accurate against the nominal values. The polarisation index plot showed very little deviation from the two baseline meters.


The UT505A does seem to have some quirks to its operation. A five and half minute video going through some of the findings.



An alarm beep sounds when insulation levels are too low. Less than 1MOhm on 50V and 100V ranges and less then 4MOhm on 250V, 500V and 1000V ranges. This is built in and automatic, and as it cannot be turned off, it starts to become distracting after a few tests. The UK guidance for minimum insulation resistance is 1MOhm for 230V circuits, so this doesn't really match use in the UK.


There are two options when carrying out timed insulation tests. A time button that allows a number of single durations to be tested. A PI/DAR button that provides the single standard 10min/1min PI ratio and two DAR ratios of 1min/30s and 1min/15s. This latter ratio is also unusual and not recognised as far as I am aware. A 30s/15s ratio is, but this is not one of the options.


During the timed tests, the insulation value is displayed, but there is no time value on the display. During a PI/DAR test, this is opposite, and the count down time is displayed, but there is no insulation resistance reading. At the end of the test, the readings at the two time points and the ratio value can be cycled through, but there are no intermittent values and using the PI/DAR function, you would also not be able to record them manually. I have always recorded insulation readings every 15s for the first minute and every minute after that for a PI test, as if a low PI value is obtained, the shape of the resistance plot can provide further insight into the cause of the low reading. To me, not being able to see the insulation values, is a major flaw in the design of the instrument.


The pictures below show the display for a standard and DAR insulation tests.

UT505A standard insulation test UT505A DAR Test Display


I also found that the 'Lock' function has to be enabled to start a PI / DAR or timed test. If it is off, the meter will not start the test and just beeps. However, whilst the meter will switch off after a timed test and display the resistance value, it does not switch off after a PI or DAR test, whilst the ratio is displayed and you can cycle through the results, the output of the tester remains on. The could easily catch someone out if there was a lapse in concentration or interruption during the test. Whilst a shock from an insulation tester of this nature is not harmful, injury may occur due to the surprise of receiving a shock, dependent upon the work conditions.


Meter lock function


In operation, without a timed test selected, the meter automatically locks on and has to be stopped manually with a second press of the test button. This differs from all the other insulation testers I have looked at, where a second button has to be utilised with the test button to get the output to lock on. During later testing I did find out that I could turn this off by disabling the 'Lock' function, but this seems to be retained based on the last selection, even after the meter has been turned off and back on again, it does not default to being off.


The test leads are PVC insulated and come with a pair of rather bulky test probes and two crocodile clips. Whilst the meter has a connection for a remote operation test probe for both the insulation and continuity functions, one is not provided with the tester and needs to be purchased separately. Rummaging around it my wee collection of insulation testers found a remote probe that would fit in the UT505A and operate it. This is highly unusual, the remote probes up until now have been bespoke designs to each manufacturer. The winner of the matching remote probe, the Fluke 1587FC.


Fluke remote probe operating UT505A


The probes have removable safety caps on them, that conform to GS38 requirements. With the caps on, access to terminations can be restricted. Whilst I could get the probes into an MCB, they would not fit into the output terminals of a Merlin Gerin RCD. Anything smaller than an SAK35 terminal and the probe would not go in with the safety cap on. Removing the cap, reveals a probe tip suited to accessories with 4mm sockets. However, I could still get the probe to fit into all the terminals with the safety cap off.


Probe in MCB Terminal Probe in RCD Terminal


The crocodile clips are a little on the small side for larger terminals. They will just go around an M12 (1/2") nut which is 19mm (3/4") across flats. They will go easily around the M12 stud. This may cause issues when seeking to find an earth to clip on to when testing larger variety motors.


Crocodile clip on M12 stud


Three and half minute video on a further DAR test on the winding simulator and the operation of the Lock function.



During the tests, I checked the insulation level of the leads at 1000V when they were tangled together. The UT505A at 1000V will read up to 20GOhm, so the Megger MIT420/2 was used for this test. As typical with PVC leads, the insulation was lower than the full scale range of the tester, although it is not the worst I have seen. This isn't likely to affect a standard one minute insulation test, but could well impact on a PI ratio test where the ten minute reading reaches high GOhm values.


Lead insulation at 1000V


Quick one minute(ish) video on testing the leads.



Motor Tests


Motor phase resistance


Whilst the motor phase resistance appeared to be balanced across all three phases, all three readings were 0.0 Ohms on the UT505A. Other testers on this motor have measured the resistance to be around 0.2 Ohms. I am not sure what caused this low reading on the instrument.


At a 500V test voltage, the insulation resistance is limited to 500MOhm at full scale, so I was fully expecting the instrument to go over range during the DAR and PI tests. Whilst the meter does go over range, it still returned a DAR ratio of 1.02. During the test, the insulation resistance value is not displayed, only the timer countdown. After the test the 15s, 1min and ratio can be paged through to obtain at least those readings.


Motor DAR Ratio


Overall, the UT505A seems reasonably capable of testing the motor, and results were obtained that would lead the tester to believe the motor was serviceable.


Build Quality


Uni-T is renowned with producing budget style instrumentation, so I was keen to take a closer look at the unit. The case is full moulded, with an outer red moulding, that gives the impression of a protective rubber coating. Whilst not quite as hard as the grey plastic, it does not provide the same protection as a separate rubber boot, such as those found on Keysight instruments. There are probe storage grippers on the back of the instrument, that do work well with the probes supplied, despite their chunky nature.


Probe storage in case


The tilt stand and battery cover are removed as one by undoing three machine screws. With the cover removed, these screws are all retained within the cover, preventing their loss. Quite a nice touch, from a more competitively priced instrument.


Battery cover screw location Battery cover removed Battery cover retained screws


Rear case removed


With the rear cover removed, another nice touch is seen. The battery case leads are held in place on the back cover to prevent strain on the soldered connections and a small JST style connector is used to connect the battery leads to the main PCB, allowing the back cover to be easily removed out of the way.


The meter is marked as having a CAT IV input rating to 600V. Whilst the input jacks are screwed into a full plastic moulding, giving great mechanical support, and there appear to be isolation slots around the positive input terminal, there are only two MOVs and two quite small PTCs to protect the voltage input.

Input MOVs and PTCs


Whilst the PCB is 'busy', its layout does seem quite neat. The pulse transformer for the high voltage is situated close to the insulation test output terminals and looks to be switched through an Omron relay. Two other selection relays, also Omron are located on the opposite side of the PCB of the resistance and continuity function.


Stepup transformer Omron selection relays

The continuity function is protected by a small 20mm HBC fuse, but this will have a low breaking capacity and does not afford the same protection as the voltage inputs if an error is made. All four input jacks are held in place with machine screws. The two remote probe connections clip around stubs, pressed into the plastic case.


Continuity inputMain processor

The main microcontroller is from Silicon Labs, a C8051F340, 8 bit 50Mhz device, and a PCF85176H display driver form the brain boxes of the UT505A.


The main PCB is held in place with 4 self tapping screws into the front case. Inspection of the PCB shows good attention to detail. There were no signs of modifications to the PCB or residue around any of the solder joints. A little bit of mastic type substance was found providing support to a bridge rectifier component.


Main PCB removed


The front side of the PCB contains the main selector switch, function button pads and the display.

Function buttons on PCB Function switch removed Selector switch


A small pice of plastic debris was found underneath the selector switch, other than this, no debris was found inside the case.


Front case


Overall, this is a reasonable quality unit that Uni-T have produced, with some very nice touches to it, that are not found in other more expensive insulation testers.




I find the UT505A to be a bit of a mixed bag. In some respects, this is quite a good instrument. The build quality is good and is close to matching some of the higher end units. It has a nice large clear display, enhanced by the backlight. There are options to select different timed ratios to the standard, DAR and PI seen in the UK. It has the ability to adjust the test voltage, both before and during a test.


Some of the implementation however, leads to be desired. A lack of insulation values displayed during a PI test, leads this test function to be redundant. Recording insulation values during a PI test is fundamental to this style of test. The 500MOhm limitation on the 500V range, whilst being comparable to other insulation testers I have reviewed, just isn't adequate for testing modern insulation systems.


The operation of the lock function needs to be addressed. For me it should default to being off after changing ranges and certainly after switching the meter off and back on again. The meter also needs to switch the output off after a PI / DAR test. I am not sure if this is the way the meter operates, or if it has developed a fault during my time using it.


The quality of the test leads, let the unit down, and could interfere with the test results on a polarisation index test. It would be interesting to see if the UT505B, with the higher resistance capability, comes with the same leads. I also have doubts over the CAT IV rating, there does not appear to be any independent verification of this.


Whilst this has been an interesting unit to look at, given its functionality and accessories, I couldn't honestly say it would be a good purchase at its recommended price. At the discounted price, it can be obtained for, it becomes better value for money. But it only really has the ability to carry out basic insulation testing. The inability to download test results and limited insulation resistance values at 500V means it won't be going near any generator rotor I test. If you are looking for a basic tester though for general use, it could be a worthwhile contender.