Introduction
I have been utilising the Brymen BM877 alongside the recent MetraHit Coil unit, and with all the extra testing required on the MetraHit, the BM877 was mostly finished testing at the same time, just leaving this blog to write. Previous Insulation Testers Reviewed;
Keysight Insulation Multimeter Failure
Chauvin Arnoux CA6526 Insulation Tester Review
Flir IM75 Insulation Tester Review
Extech MG302 Insulation Tester Review
Di-LOG DL9307 Insulation Tester Review
RS Pro RS-9985 Insulation Tester Review
RS Pro IIT1500 Insulation Tester Review
Uni-T UT505A Insulation Tester Review
Sonel MIC-30 Insulation Tester Review
GMC MetraHit Coil Motor Testing Multimeter Review
Brymen, is a new manufacturer to me. This is the first time I have used one of their instruments. They appear to be a relatively young company, founded in Taiwan in 1993, from their website. They seem to predominantly manufacturers digital multimeters, and have added some insulation multimeters into their range. The also have a good range of clamp-meters and a few electrical testers.
They offer five insulation multimeters in their range. The BM887 and 885 offer more multimeter style functions, included current measurement of up to 10A, but they do not offer DAR / PI or 200mA earth bond tests. The BM876 appears to be their basic instrument and offers DAR / PI testing, but no 200mA earth bond test. The BM878 offers mA current measurement alongside a 200mA earth bond test, but no DAR / PI function. Only the BM877 offers both the DAR / PI and earth bond test, but has limited multimeter functions. As my primary interests are on the insulation testing side, the BM877 is the unit I went for.
Unboxing and Overview
The BM877 arrives boxed, there seems to be reasonable availability in the UK for the Brymen range. The kit that arrives is relatively basic consisting of the meter, a pair of test leads with built in probes, compatible crocodile clips, two screw on adapters for the probes, a remote probe lead and a small instruction manual. Immediately obvious, is the lack of a carry case for the instrument and accessories. One is available separately, along with 4mm screw on adapters for the probe tips. A calibration certificate is also not provided, although the particular supplier I purchased from, could have carried out a calibration check for a further fee.
The BM877 is a little smaller than the Keysight and Megger units, it does have good ergonomics. Most of the operation of the unit can be carried out with the thumb whilst the unit is held in the same hand.
Unlike the Keysight and Megger units, the insulation, earth bond and multimeter functions are split across 4 sets of input jacks. Two of the jacks have the remote probe socket above them to allow the remote probe to control the meter. There is one common jack for the earth bond and multimeter tests and a separate common jack just for the insulation test, this is not my favourite setup and usually ends up with me having the probes in the wrong sockets from time ti time.. The earth bond is fused at 250mA and the insulation test at 400mA.
The screen is large and clear. The two primary numbers are quite large and bold with a few smaller elements used on the screen to display selections. There is no analogue bar on the Brymen, or a battery level indicator, as there is on the Megger unit.
The backlight is not even across the width of the screen, as seen below. It is still usable, but just detracts form the aesthetics a little bit.
The protective boot fits tightly around the meter and contains the tilt stand. Probe holders are located on the back of the rubber boot, that can be used for storage of the probes or assist with one-handed operation. The boot prevents access to the battery and fuse compartment and must be removed to replace either of them.
The probes are moulded onto the leads and re supplied with removable GS38 caps. They are however, quite small in diameter and hardly restrict access into terminals. With the caps removed, crocodile clips can be plugged onto the end of the probe. This isn't my favourite methodology, as I find the weight of the probe can sometimes overcome the crocodile clip and it falls off during testing, quite annoying during a 10 minute polarisation index test.
The crocodile clip are also very small. The maximum they will go up to is an 8mm stud, but would not go around the bolt head. They may be acceptable for working with white goods, but for heavy electrical applications, they would have minimal use. The standard RS alligator clips I have, do plug onto the probe tip in place of the supplied clips.
| {gallery} Brymen BM877 Leads |
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Probes located in holster |
Probes, GS38 Caps and Crocodile Clips |
IMAGE TITLE: THEN IMAGE DESCRIPTION |
Crocodile clip opening comparison |
Threaded adapters for probe tips |
With the GS38 caps removed, small threaded adapters can be screwed onto the probe tips to enlarge them, presumably this is done to strengthen the tips if required.They are slightly larger diameter then 4 mm, so do not allow the probes to be plugged into 4 mm accessories or socket. For this, Brymen do sell a special adapter as an optional extra.
The leads were tested at 1000V for leakage on the Chauvin Arnoux insulation tester, that can read up to 200 GOhms. No issues were found during this test, as the leads are manipulated, there is sometimes a drop in resistance, but it quickly goes back up to 200 GOhms.
As briefly stated above, the GS38 caps are amongst some of the better ones and hardly restrict access to terminal, even on the remote probe. Access was restricted only in DIN 2,5 terminals. Removal of the cap allowed the DIN 2,5 terminal to be accessed. Whilst the GS38 cap on the remote probe had a small enough diameter to allow access to a standard MCB terminal, it was very close to being too short. There are some manufacturer's MCBs with deeper screws for the terminals and I think it would struggle to reach those.
| {gallery} Brymen Probe Tests |
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GS38 Cap restricts connection in DIN 2,5 Terminal |
GS38 Cap permits connection in DIN 4,0 Terminal |
Connection in DIN 2,5 Terminal with Cap removed |
GS38 Cap on Remote Probe permits connection in SAK 2,5 Terminal |
GS38 Cap on Remote Probe stops connection in DIN 2,5 Terminal |
Remote Probe with GS38 Cap just works in an MCB |
Crocodile clips fits on M8 Stud |
Crocodile clip too small for M12 stud |
The crocodile clip maxed out on the M8 stud and had no chance gripping an M12 stud. The rubber boot around the clip also adds to the restriction and makes the crocodile clip a little awkward to use if it needs to be pushed back.
The manual supplied is quite small, but contained all the necessary information with plenty of diagrams showing the connections for the various tests.
| {gallery} BM877 Instruction Manual |
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Instructions for insulation testing |
Guidance of DAR / PI readings |
I did find the guidance for the DAR / PI ratios a little odd and are not the standard interpretations that I am used to and this turned out to be an issue during some of the testing. The actual time stamps for the DAR and PI ratios are the same as I am used to.
In terms of functions, compared against the Keysight unit, the BM877 lacks a variable test voltage and a ramp function, with of which I can make use of for my particular type of testing. Whilst the BM877 has a memory function, it is limited to min, max and average recording and offers no memory facility to recall results with or download to a computer. It does have voltage measurement, coupled with a VFD filter mode for AC voltage, along with resistance and diode tests. The rest of the multi-meter functionality is available in their other units.
One of the big issues for me will be the restricting of the insulation range to 550 MOhms for the 500 V range. The 1000 range can go up to 25 GOhms, so I would have thought that achieving at least 5 GOhms for the 500 V range would have been achieve label for Brymen. Ideally, for a meter with PI ratio test, they should be looking for somewhere near 40 GOhm as a minimum.
60 MOhms for the resistance limit is respectable and above that achieved by the Megger unit and comparable to the Keysight.
Consideration needs to be given for the price of the BM877, that is at the lower end of the scale for the meters reviewed and five times cheaper than the Keysight unit, that is the most expensive to date.
Build Quality
Removing the protective boot reveals a slightly odd shaped meter, that definitely needs the boot to finish of its appearance.
The battery and fuse compartments are covered by the one lid, held on by two machine screws that are captivated into the lid when it is removed. Two cut-out on either side of the lid, aid with its removal. There isn't any seal around the lid edge to prevent moisture or dirt ingress.
Four screws, complimented by two clips on the top edge, then hold the back cover on. The battery pack is connected to the main PCB in the front half of the case by spring contacts on the PCB aligning with contact pads on the back of the rear case. No wires to break off, during the teardown!
A singe screw hold the complete PCB assembly to the front half of the case.
The front case has a set of contact springs to connect the remote probe sockets to the PCB. Protective rubber covers prevent the actual pins of the remote probe sockets from touching the PCB.
The main PCB contains a separate input PCB that takes in the inout jacks and houses the CAT III / CAT IV input protection in the form of a couple of PTCs and three varistors. The inout board also contains a section of the rotary function switch.
The inout jacks are soldered directly onto the PCBs, which may be a weak point in the long term. Each jack has an insulated cap that protrudes through the main PCB, with plenty of clearance from the main PCB.
Removal of the separate PCB reveals one of the contact blocks for the rotary switch.
Behind this contact block is the rotary switch detent mechanism. A plastic spring leaf type system, evident in many of the previous insulation testers reviewed.
Removing the detent mechanism, reveals a second contact block for the rotary switch, with the switch tracks on the main PCB.
The display is a module held onto the main PCB by two plastic legs and two screws and is very easy to remove. Removal reveals the two main micro-controller units underneath. The backlight for the display has two spring contacts and is on one side only, explaining the uneven light across the display when it is in use.
The Hycon HY11P24 mixed signal controller (centre) and a HT1622 LCD controller from Holtek.
The high voltage transformer is on the opposite side of the main PCB and looks to be of nice quality. There is no mechanical restraint for the transformer other than the solder joints of the actual terminations. Next to the transformer is the main mosfet for driving it.
Overall, this is a very nice build from Brymen. The PCBs and cases are all clean, with no signs of rework or modifications to either of the PCBs.
Performance Tests and Comparison
Overall accuracy of the insulation resistance measurements for the BM877 was -0.24% over the 100 tests it is capable of.
The plot shows that the largest deviation is seen in the GOhm range. At the lower ranges, the different voltage levels are a little sporadic compared to one another before evening out in the mid-ranges.
The open circuit voltages were all found to be above their nominal values, the highest tolerance observed was 11 % for the 100 V test range, well within the manufacturer's specifications.
Capturing the open circuit voltage at 500 V, shows a steady rise of 160 ms with no overshoot. This kind of response is typically seen on the more expensive units. The output dropped of fast in 16.5 ms as the test is finished.
Voltage regulation at 500 V was typical of the cheaper units and showed a lot less control than the Keysight and Megger units. However, it remains within the manufacturer's specifications and was better than some of the other units.
The short circuit tests can be seen below and were around 1.6 mA for all of the test voltages, within the manufacturer's specification and in accordance with the requirements of the IEC standard.
Finally the 1 mA load tests were conducted. A spread of 42uA was seen across the different test voltages. Again, not as good as the Keysight and Megger units, but not as bad as a couple of the other units costing well in excess of the BM877.
The basic performance tests did not reveal any underlying issues with regards to the performance of the insulation testing capabilities.
The discharge circuit resistance is the lowest seen out of all the testers reviewed, measuring 3.2 kOhms. The closest to this is 382 kOhms from the CA6526. A lower resistance will provide a faster discharge, particularly useful for capacitive circuits, such as my motor winding simulator.
There is no battery level indicator on the BM877. As the battery voltage drops a low battery symbol appears and will remain until the meter cuts-out. The meter was easily useable down to a battery voltage of 3 V. Below that the display is very dim, it would be seen on good light, but would be come unusable if the ambient light was low.
In terms of battery usage, the BM877 turns out to be one of the more efficient, using up to 52% of the battery capacity, quite a but more than the Megger unit, and only just bettered by the MG302 from Extech.
Current draw on the battery was quite low, with the 200mA and insulation tests drawing the most power, at 1,463W and 0,935W respectively. The backlight added a further 168mW of load, to what ever the test function was drawing.
Winding Simulator Tests
The winding simulator was used to test the resistance, DAR and PI functions. The first monitor issue was the connection with the small crocodile clips. As there is a little bit of threaded stud showing on the connections, this can be utilised and I did not have any issue with the leads falling off during the tests. The DAR test seen below, comes out at 1.6 comparable to the expected value and shows the connection made.
For the timed tests, the display can be switched between displaying the time lapsed during the test and the actual test voltage, allowing readings to be manually recorded during the test. It was observed that the DAR and PI functions did not lock the meter into operation for the duration of the test as a lot of other meters do. The meter still had to be manually locked after starting the test.
Test results were comparable to all the other meters for the Polarisation Index test. The calculated ratio was 7.68, but this was not what was displayed on the instrument. The instruction manual sates that the PI ratio has as a maximum value of 5.0, above this the display should read > 5.0. The > symbol was also not seen. As seen below, the display just showed 5.0 for the PI ratio. There was also no way to page through the results, so the MOhms reading could not be retrieved.
This is a bit of a shortcoming of the meter. The resistance values are equally as important as the PI and DAR ratios and the meter would benefit from being modified to be able to display these. There is also no sense to limiting the PI ratio to 5.0. Modern insulation systems can easily have a PI greater then 5.0, for older insulation systems a PI above 8.0 would be indicative of the insulation drying out and becoming brittle, but this too would be missed by the meter.
It is worth noting that the BM877 does have an incorrect terminals warning when the meter is switched back to voltage or earth bond measurement, with either of the probes left in the insulation test terminal. This visual warning is also accompanied with an audible warning, until the probe in error is removed from the meter jack.
The results of the motor simulator winding resistance tests are seen in the table below. There were made after using the zeroing function to eliminate the lead resistance form the test. The results obtained were very good compared to the nominal values, the worst case being just 1.3% out for the W Phase open winding. Also note the three decimal places of the readings, only matched by the Keysight unit, with the norm being two decimal places for the majority of the meters reviewed.
The test current was measured during the tests to validate the two ranges. The 90 mA range was measured as 89.90 mA and the 200 mA range, 206.7 mA.
The earth bond function is protected with a 250mA HBC fuse, so I removed this to simulate a blown fuse. The meter picks up the blown fuse and shows an 'open' text on the display. I also tried this with the 400mA fuse protecting the insulation resistance function but this does not have a blown fuse warning and the meter just reads the maximum insulation value of that range.
There are also voltage alerts / cut-outs for both the earth bond and insulation resistance functions. For the earth bond, the meter alarms with a voltage above 2 V and above 30 V for the insulation resistance tests. Both functions do actually lockout and will not operate even if the test button is pressed.
Overall, a mixed back of results. The resistance tests produced excellent results, but for me there are fundamental problems with the way the DAR and PI tests function.
Motor Tests
Tests on an actual motor were performed on a 90 kW unit. This was about the usability of the crocodile clips as just as much as the actual readings captured.
The phase resistance reading canoe seen taken above. There is no chance of the crocodile clips going around the stud or nut. As luck would have it, the shorting bars are rectangular, and enough of the corner of a bar could be gripped to get a hands-free reading. At 0.141 ohms though, the reading is higher than that obtained by the Megger and Keysight units, 0.04 Ohms and 0.053 Ohms respectively. All three phases measured between 0.121 and 0.148 Ohms, so it is likely that the connection method is creating the problem.
The connections for an insulation resistance test were just as challenging. The phase connection was the same as the resistance test. For the earth connection , I managed to clip onto a washer on the earth stud, there was no where to clip on to on the motor chassis with these small crocodile clips. However, connections for insulation testing are not as onerous as they are for winding resistance measurements.
All the DAR and PI tests at 500 V failed, as the insulation resistance of the motor is too high for the range of the meter and an 'Err' symbol is displayed at the end of the test.
Both the DAR and PI tests gave successful results at 1000 V, due to the extended range of the instrument up to 25 GOhms for a 1000 V test.
When switching back to the voltage function, to check that the motor winding has discharged, the probe error message was encountered as I had forgotten to swap the probes to the correct input jacks. This is one of the disadvantages of having separate socket for insulation testing from voltage testing. Both the Megger and the Keysight units use the same jacks for the insulation and voltage tests, so this is never an issue with those units.
Conclusions
The Brymen BM877 is a high quality build. The meter is robust and has good ergonomics about it. The protection on the input jacks is far better than a lot of the other units reviewed. I like the safety features of the input jack warnings and blown fuse indication. It is a shame that the blown fuse indication is not added to the insulation test function as well. The meter has a good selection of functions and there are other variants that provide more multimeter functionality at the expense of PI / DAR / Earth Bond. It just depends on the preferences of the user.
Whilst the accessories supplied seems to be of good quality, the crocodile clips are a little on the small side for heavy electrical applications such as mine. A suitable case also has to be purchased as an extra, if required.
The 500 V insulation test range, needs to be extended up to 40 GOhms for effective polarisation index testing and the actual functionality of the insulation test needs to be rectified to allow the insulation readings to be retrieved. The reduced PI ratio of five also needs to be addressed.
I feel the Brymen BM877 is good value for money overall, but probably more suited to a jobbing electrician working on white goods or light commercial installations. With the limited range at 500 V, the meter cannot be used for generator rotor testing and it is a meter that I will look to move on, to make way for other candidates.
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