OTS PB and OTS AF series of insulating oil test sets
PB models are small and light, starting at 16.8 kg in weight for field use. The AF range has a larger test chamber for increased test capacity for lab use
Test vessels are shatterproof, easy to clean, and reusable. This reduces waste whilst achieving repeatable results
All of the current test standards worldwide are pre-loaded in the instrument for convenient automatic operation
A convenient and precise thumbwheel adjustment for electrode spacing incorporates a locking mechanism that completely eliminates accidental electrode movement during testing
About the product
The OTS PB and OTS AF insulating oil test sets are a range of automatic oil test sets that perform accurate dielectric breakdown voltage tests on mineral, ester, and silicone insulating liquids. This critical test indicates the ability of a fluid to withstand electric stress. All models have precision, shatterproof test vessels that are easy to clean and provide repeatable results, whether used in the field or laboratory. They also have a transparent, shielded lid and a large test chamber, giving easy access to the test vessel and allowing you to see what is happening within it.
Test results are identified either by a serial number or asset ID and are time and date stamped. OTS units come with PowerDB Lite, Megger's asset and data management software, at no extra cost, giving you an excellent tool for downloading and printing results. The units have an internal printer so that you can have a hard copy of your results, if needed. In addition, the AF model includes a barcode scanner.
We have designed these test sets with your safety in mind. During a test, you can terminate the measurement at any time by pressing any button on the keyboard. Such a keyboard stroke will remove the high voltage immediately and abort the test. Additionally, the transparent lid provides ample visibility within the chamber yet is protected and electrically shielded by a screen with multiple links to the instrument's ground.
All existing test standards worldwide are pre-loaded in the instrument for convenient automatic operation. However, should a new test standard be accepted or a current standard amended, you can configure three custom tests to the new requirements. This flexibility enables you to continue testing for the short period over which Megger updates the test procedure files. New updated files are then downloaded by the user and installed into the test instrument via a USB drive.
OTS PB models
These 60 kV and 80 kV oil test sets are the smallest and the lightest on the market, with weights ranging from 16.8 kg to 20.8 kg, depending on the model configuration. These units can be mains powered or battery operated for additional flexibility in portable applications. All PBs are fitted with NiMH batteries and are also supplied with an internal 12 V DC charger and a vehicle adaptor cable as standard issue. The transport case and carry bag are optional accessories. The carry bag has pouches for the electrode accessory pack, leads, a quick user guide, and a paper roll.
OTS AF models
These 60 kV, 80 kV, and 100 kV models have a much larger test chamber for even easier access and cleaning, which is particularly useful in a lab environment. They are fitted with a 12-key alphanumeric keypad to facilitate the entry of test IDs, file names, and notes. Alpha characters are entered by repetitively pressing a key. The AF models also can use a USB barcode reader to scan oil sample barcode labels, which is ideal for better integration within a laboratory.
Technical specifications
- Test type
- Oil dielectric breakdown
FAQ / Frequently Asked Questions
Put simply, a dielectric breakdown voltage test is a measure of the electrical stress that an insulating oil can withstand without breakdown. The test is performed using a test vessel that has two electrodes mounted in it, with a gap between them. A sample of the oil to be tested is put into the vessel and an AC voltage is applied to the electrodes. This voltage is increased until the oil breaks down – that is, until a spark passes between the electrodes. The test voltage is then immediately turned off. The voltage at which breakdown occurred is the test result, and is typically evaluated by comparing it with guidelines set out in various standards, or in the oil manufacturer’s specifications.The exact method of performing the test is determined by the standard that is being used. The standard typically defines parameters such as the size and shape of the electrodes, the gap between them, the rate at which the test voltage is increased, how many times the test is repeated, and whether or not the oil is stirred during the test
There are many types of organisation that benefit from carrying out tests on insulating oil. These include:
- Utility contractors (principally in substations)
- Utility companies (principally in power stations and substations)
- Rail companies (locomotive high voltage step-down transformers and switchgear)
- Oil test laboratories (providing testing services)
- Transformer and switchgear manufacturers (quality control of oil)
- Oil companies (testing new oil during manufacture)
- Heavy industry and manufacturing (asset maintenance programs)
While the generic term ‘oil’ is almost universally used to describe insulating fluids, there are currently five different types of insulating fluid in common use. These are:
- Mineral oil
- High molecular weight hydrocarbon (HMWH) fluids
- Silicone fluids
- Synthetic ester fluids
- Natural ester (vegetable oil) fluids
All of these oil types can be tested for dielectric breakdown voltage and tested with Megger OTS range test sets. Mineral oil is the most common insulating fluid and has been in use since the late 19th century. There are many mineral oil filled transformers that have been in continuous use for more than 50 years. Mineral oils are refined from either naphthenic crude or, more recently, from paraffinic crude. HWMH, silicon, synthetic ester and natural ester fluids are more recent developments and are often preferred because they are much less flammable than mineral oil. ASTM D5222 specifies that for insulating fluids to qualify as ‘less flammable’ they must have a fire point of at least 300 ºC. The five fluids differ significantly in the way they behave in the presence of moisture. Mineral oil is the least satisfactory, and even small amounts of water significantly reduce its breakdown voltage. Silicone fluid is also quickly affected by small amounts of moisture, whereas ester fluids behave very well in the presence of moisture and can typically maintain a breakdown voltage of greater than 30 kV with more than 400 ppm water content. This is one of the reasons that esters last much longer in service.
The dielectric breakdown voltage test is a relatively quick and easy way of determining the amount of contamination in insulating oil. Usually the contaminant is water, but it can also be conductive particles, dirt, debris, insulating particles and the by-products of oxidation, and ageing of the oil.For in-service equipment, the dielectric breakdown voltage test offers a useful and convenient way to detect moisture and other contamination in the oil before it leads to a catastrophic failure. The information gained from the test can also be used as an aid to:
- Predicting the remaining life of a transformer
- Enhancing operational safety
- Preventing equipment fires
- Maintaining reliability
Dielectric breakdown voltage testing is also carried out on new oil before it is used to fill equipment, and as part of the acceptance testing for deliveries of new and reprocessed oil.
Dielectric breakdown voltage testing is an important element in the maintenance programme of any item of oil-insulated electrical equipment. However, to get the maximum benefit from this type of testing, Megger strongly recommends that the oil is tested at least once a year and preferably twice a year. The results should be recorded, as trending the data will make it easier to identify sudden or unexpected changes. If a sudden change in the results is found, the transformer can be inspected for leaks, the oil level can be checked, and the water content of the oil evaluated. If contamination is confirmed, it will often be possible to dry and filter the oil, thereby reconditioning it rather than having to replace it with expensive new oil.
ASTM D877 is an older standard and is generally not very sensitive to the presence of moisture. For that reason, it is not widely used for in-service applications. In 2002, IEEE revised C51.106, Guide for the Acceptance and Maintenance of Insulating Oil in Equipment. IEEE removed the values for D877 from their criteria for evaluating in-service oil in transformers. Generally, ASTM D877 is recommended only for acceptance testing of new oil received from a supplier in bulk loads or containers to ensure the oil was correctly stored and transported. Typically, a minimum breakdown value of 30 kV is specified. The ASTM D877 standard specifies the use of disc-shaped electrodes that are 25.4 mm (1 inch) in diameter and at least 3.18 mm (0.125 inch) thick. These electrodes are made of polished brass and are mounted to have their faces parallel and horizontally in line in the test vessel. The edges are specified to be sharp with no more than a 0.254 mm (0.010 inch) radius. It is good practice to inspect the sharp edges regularly to ensure they have not become too rounded. Excessively rounded edges will have the effect of falsely raising the breakdown voltage, possibly passing oil that should have failed the test. It is also important that the electrodes are kept very clean, with no pitting or signs of corrosion; otherwise, breakdown values can be falsely low.
ASTM D1816 has become widely used in North America, even outside the standard’s stated application of petroleum-origin insulating oils and viscosity limits. D1816 is more sensitive than D877 to moisture, oil ageing, and oxidisation, and is more affected by the presence of particles in the oil. The IEEE revision of C51.106 in 2002 added breakdown voltage limits for new and in-service oil using D1816. ASTM D1816 specifies the use of mushroom-shaped electrodes 36 mm in diameter. As with D877, the electrodes are made of brass and must be polished to be free of any etching, scratching, pitting, or carbon accumulation. The oil is stirred throughout the test sequence, and a two-bladed motor-driven impeller is specified. The standard prescribes the impeller dimensions and pitch as well as the operating speed, which must be between 200 rpm and 300 rpm. The test vessel must have a cover or baffle to prevent air from coming into contact with the circulating oil. The D1816 standard, although generally accepted as more valuable than D877, has one significant limitation: when testing in-service oil, this test method is very sensitive to dissolved gases. Excessive amounts of gas in the oil can lower the test results to the point that a perfectly good sample of oil, with low moisture and particle content, will fail the test. It is important to bear this in mind when testing oil from small gas-blanketed transformers and, in some cases, free-breathing transformers.
IEC 60156 is an international standard that appears in many forms as IEC member national committees from various countries have adopted it. Examples are British Standard BS EN 60156 and German VDE 0370 part 5. IEC 60156 specifies the use of either spherical or mushroom-shaped electrodes, the same as those used in the ASTM D1816 standard. The IEC standard differs in several ways from D1816, but the main difference is that the IEC standard allows the optional use of a stirring impeller, the use of a magnetic bead stirrer, or even no stirring at all. The standard states that differences between tests with or without stirring have not been found to be statistically significant. A magnetic stirrer is only permitted when there is no risk of removing magnetic particles from the oil sample under test. When oil is used as a coolant, in which case it circulates, it would be stirred during testing. For example, typically, transformer oil circulates when it is used as a coolant. Therefore, an oil sample from a transformer should be stirred to ensure the best chance of detecting particle contamination. Oil from a circuit breaker is ordinarily static, so particles would naturally fall to the bottom of the tank, where they are unlikely to cause a problem. So in static use applications, an oil sample is usually not stirred. The dielectric breakdown values from the IEC 60156 method are usually higher than those from the ASTM methods. The higher dielectric breakdown values are partly because of the differences in voltage ramp-up speed and electrode gap compared with D1816, and electrode shape compared with D877 (the IEC electrode shape provides a more uniform electric field). The result is that for well-maintained transformers, the breakdown voltages may be higher than a 60 kV test instrument can reach. The inability to quantify a breakdown voltage higher than 60 kV may not be a problem when evaluating new oil from a supplier or even for in-service oil. However, frequently an actual breakdown voltage value is required. Therefore, when testing according to IEC 60156, an instrument capable of applying a higher voltage is advisable. As with D1816, dissolved gas in the oil sample may reduce breakdown values, but the effect is much less pronounced with the IEC 60156 standard.
Further reading and webinars
Related products
Troubleshooting
Check the gap between the electrodes and make sure the vessel is cleaned according to the standards.
Megger offers a voltage check metre that can be fitted to the instrument in place of the measuring vessel. Doing so allows you to compare the voltage shown on the check metre with that shown on the instrument display. Check meters are not sufficiently accurate to use as a calibration standard. Still, they provide an excellent way of detecting changes in instrument calibration. You should record the check meter readings each time you carry out a voltage check to identify changes quickly. If any significant change is detected, you should not use the instrument until you have returned it to Megger or an accredited service centre for servicing and recalibration.
Indicators that you need to send your OTS into Megger or to an accredited service centre for repair include your OTS not booting up or not building voltage.
Interpreting test results
There are several key factors to consider to carry out effective and reliable insulating oil dielectric breakdown testing. You’ll need to know that your results are valid, considering standards and their specific conditions that must be met. You also need to know if your insulating fluid meets manufacturing standards.
This extract of a chart comparing standards shows that each standard specifies different conditions that must be met if the test results are to be accepted as valid. You can find the full chart in our ‘Guide to insulating oil dielectric breakdown testing’.
| Standards | ASTM D1816 | ASTM D877 | IEC 60156 | |
|---|---|---|---|---|
| Procedure A | Procedure B | |||
| Valid test conditions | If breakdown does not Tests must be repeated if range of BD voltages recorded | Tests must be repeated if the range of BD voltages recorded are more than 92 % of mean. If the range of 10 BD voltages is more than 151 % investigate why. | Expected range of standard deviation/mean ratio as a function of the mean provided as a chart. | |
Mean is the average of the breakdown values recorded in the test sequence. For example, if the breakdown values are 33 kV, 37 kV, 32 kV, 35 kV, 38 kV, and 34 kV, the mean value would be the total of these results – 209 – divided by the number of results – 6 – which gives a mean value of 209/6 = 34.83 kV. (Note that in this example, there are six results as required by the IEC standard. The ASTM standards require either five or ten results.)
Range of breakdown voltage is referred to in the ASTM standards. For example, D877 specifies that the test sequence must be repeated if the range of breakdown voltages recorded is more than 92 % of their mean value. Two examples will make this easier to understand.
In the first example, the breakdown voltages recorded are 43, 45, 52, 40, and 38 kV. The lowest value is 40 kV and the highest is 52 kV, so the range is 12 kV. The mean of the recorded values is 43.6 kV, so the range is only 12/43.6 x 100 % = 27.5 % of the mean value. These test results are, therefore, valid.
In the second example, the breakdown voltages recorded are 33, 45, 52, 18, and 20 kV. The lowest value is 18 kV and the highest is 52 kV, so the range is 34 kV. The mean of the recorded values is 33.6 kV, so the range is 34/33.6 x 100 % = 101 %. This is above the 92 % limit, which means that the test must be repeated.
Standard deviation: IEC 60156, there is a graphical representation of standard deviation – otherwise known as the coefficient of variation – versus the mean breakdown voltage. Calculation of the mean has already been covered, but what about the standard deviation? IEC 60156 does not explain how to calculate this. The procedure, however, is to calculate the difference between each of the six test results and the mean value of those test results, then square each of the differences and add them together. Divide the figure obtained by 2, and then take the square root. The final answer is the standard deviation for the set of test results.
IEC 60156 states that, for the test results to be considered valid, the following procedure must be followed:
- Perform six tests
- Calculate the mean of the results
- Calculate the standard deviation (see above)
- Divide the standard deviation by the average value, noting that scatter is expected and acceptable (see the chart at the end of IEC 60156)
- If the value is acceptable, conclude testing
- If not, perform six more tests
- Repeat the calculations using all 12 results
An insulating fluid manufacturer normally quotes typical new and in-service fluid breakdown values in its data sheets. In addition, the test standards refer to oil condition standards that provide guidance about the acceptability of results.
D877 is usually only recommended to accept new oil from a supplier. However, some oil testing laboratories still recommend its use for specific in-service applications. In these cases, a breakdown voltage of 30 kV or more is usually considered acceptable, with values below 25 kV unacceptable. Values between 25 and 30 kV are considered questionable. For new oil, a minimum value of 30 kV is normally specified.
| Oil type | New oil |
|---|---|
| Mineral oil | 45 kV |
| Silicone oil | 40 kV |
| HMWM | 52 kV |
| Synthetic ester | 43 kV |
| Natural ester | 56 kV |
D1816 is more widely used and is accepted by the IEEE as the test method to be used for dielectric breakdown testing for the acceptance and maintenance of insulating oil. The IEEE C57.106 standard incorporates the D1816 limits – which are shown below – for new and in-service oil. Note that the values provided in this table are for mineral oil.
IEEE C57.106-2006
IEEE Guide for acceptance and maintenance of insulating oil in equipment
| Applications | Voltages class/group | D1816 (1 mm gap) | D1816 (2 mm gap) |
|---|---|---|---|
| New mineral insulating oil as received from supplier | Not specified | >20 kV | >35 kV |
| New mineral insulating oil received in new equipment, prior to energisation | ≤69 kV | >25 kV | >45 kV |
| 69 to 230 kV | >30 kV | >52 kV | |
| New mineral insulating oil - processed from equipment, prior to energisation | 230 to 345 kV | >32 kV | >55 kV |
| ≥345 kV | >35 kV | >60 kV | |
| Service-aged insulating oil - for continued use (Group 1) | ≥69 kV | >23 kV | >40 kV |
| 69 to 230 kV | >28 kV | >47 kV | |
| ≥230 kV | >30 kV | >50 kV | |
| Shipments if new mineral insulating oils, oil circuit breaker (OCB) | OCB | >20 kV | >30 kV |
| New OCB insulating oil - after processing, prior to energisation | OCB | >30 kV | >60 kV |
| Service-aged OCB insulating oil - for continued use | OCB | >20 kV | >27 kV |
| New mineral oil for load tap changer (LTC), prior to energisation | LTC | >35 kV | >55 kV |
| Service-aged LTC insulating oil - for continued use | LTC - Neutral | >20 kV | >27 kV |
| LTC - ≤69 kV | >25 kV | >35 kV | |
| LTC - >69 kV | >28 kV | >45 kV |
IEC 60156 uses acceptance values that are contained in two further standards: IEC 60296 and IEC 60422.
IEC 60296, fluids for electrotechnical applications: Unused mineral insulating oils for transformers and switchgear. As its title indicates, this standard applies only to new, unused oil as received from the manufacturer, which must have a dielectric breakdown voltage of 30 kV or more, determined using the IEC 60156 test method. Oil that has been vacuum filtered in a laboratory must have a minimum dielectric breakdown voltage of 70 kV.
IEC 60422, mineral insulating oils in electrical equipment: Supervision and maintenance guide. This standard prescribes acceptable dielectric breakdown values for new oil (after filling but before energising) and for in-service oil. The values are:
| Equipment voltage | Dielectric BD voltage |
|---|---|
| ≥72.5 kV | >55 kV |
| >72.5 kV ≤170 kV | >60 kV |
| >270 kV | >60 kV |
| Equipment voltage | Dielectric BD voltage | ||
|---|---|---|---|
| Good | Fair | Poor | |
| ≥72.5 kV | >40 kV | 30 - 40 kV | >30 kV |
| >72.5 kV ≤170 kV | >50 kV | 40 - 50 kV | >30 kV |
| >270 kV | >60 kV | 50 - 60 kV | >50 kV |
The IEC recommends that if values are in the ‘fair’ range, testing should be performed more frequently, and that the test results should be cross checked with other testing methods. If the test results are in the ‘poor’ range, the oil must be brought back into a good state by reconditioning. This might, for example, involve filtering and drying the oil.
User guides and documents
Software and firmware updates
OTS Test Standards
The attached file will update all the test standards of your OTS to the latest versions. Do not change the file name or it will not work. Please follow the instructions below:
- Extract the attached file (stdSeqs.db) to a USB memory stick
- Insert the memory stick into the Type A USB port on the front panel of the OTS (or the Type A USB port on the rear of the OTS)
- On the OTS, navigate to the Tools menu with the Hammer & Wrench symbol
- Scroll down and select Manage test standards
- On the next screen select Update Standards (USB) and the instrument will upload the new file from the USB stick.
- The instrument will now have the latest standards installed ready to use.
For Older OTS (Firmware version 1.15) use "OTS-Test-Standards-V0-10.zip". For updated OTS (Firmware version 3.xxx) use "OTS-Test-Standards-V0-30.zip"
IMPORTANT NOTE:
- OTS-Test-Standards-V0-30.zip is not compatible with OTS Firmware version 1.15
- OTS-Test-Standards-V0-10.zip is not compatible with OTS Firmware version 3.xxx
FAQ / Frequently Asked Questions
Two things are particularly important when taking oil samples. The first is to ensure that the proper sampling procedure is followed, and the second is to ensure that all of the essential information is properly recorded. If the sample is to be sent to a test house for testing, the test house should be able to advise on the information needed, but it’s important to bear in mind that the condition diagnosis will only be as good as the information supplied. The test house should also advise on the volume of the sample and the type of container to use. For oil samples from transformers, the information that oil test laboratories generally require is:
- Description of the sample
- List of tests to be performed
- Transformer nameplate information
- Type of transformer
- Type of insulating fluid
- Any leaks noted
- Insulating fluid service history (has it been dried, etc)
- Transformer service history (has it been rewound, etc)
- Type of breather
- Type of insulation, including temperature rise rating
- Details of cooling equipment (fans, radiators, etc)
- Temperature of top of fluid, read from gauge
- Actual fluid temperature measured
- Fluid level
- Vacuum and pressure gauge readings
For load tap changers, it is also advisable to record the counter reading, the selector range, and the sweep range. Sampling should be performed in accordance with the appropriate standard. Hints and tips for taking oil samples:
- For a sample to be truly useful, it must be representative of the oil in the equipment. This means that cleanliness is extremely important.
- Samples are normally drawn from a drain valve or sampling cock. This must be cleaned both inside and out before the sample is taken to ensure that dirt does not fall into the sampling container.
- The drain valve is at the bottom of the equipment, where all of the sludge, water and contaminant particles collect. It is important therefore, to flush the system thoroughly to ensure that the sample is drawn from the main bulk of the oil. This may involve removing two litres of oil, and even more if the equipment has been out of service for some time.
- Do not be tempted to use old engine oil bottles. Even a few p.p.m of engine oil will cause the sample to fail a breakdown test.
- Do let the oil flow down the side of the sample bottle, or use a clean tube run to the bottom of the bottle; it will prevent air being mixed with the oil.
- Do store the oil samples in glass or clear plastic bottles in the dark, mineral oil will deteriorate if exposed to UV light.
Safety
- Before taking samples, ensure that you have all of the required permissions and permits.
- Have everything you need to lock out/tag out to hand.
- Make sure that the PCB (polychlorinated biphenyl) content of the oil, if any, is known and that the equipment is labelled. PCB is very hazardous and requires special handling.
- Use all of the correct personal protective equipment (PPE) and correctly rated tools.
- Check the area for electrical and tripping hazards.
- Check for wildlife – snakes, bees, etc like transformers!
- Check that the transformer is under positive pressure – are the pressure gauges reliable? Could they be blocked or broken? NEVER try to take a sample from a transformer under negative pressure. Air could be drawn into the transformer and cause it to fail.
Sampling equipment
- Take extra sample bottles and syringes – they’re often needed
- Ensure that the sample bottle seals are airtight
- Use only ground glass syringes
- If rubber hose is used, discard after each sample is taken
Flushing the systemWhen flushing the system, a spare sample bottle is usually repeatedly filled and emptied into the waste. It is good practice to measure the oil temperature using the last bottle that will be discarded, as this avoids having to put the thermometer into the actual sample. Taking the sampleWherever possible, try to take samples during times of relatively steady loads and temperature – in other words, when the equipment is at equilibrium. This is particularly important with transformers as, if the sample happens to be taken after the transformer has cooled following a long period of running at full load, the breakdown voltage of the oil will be much lower than normal. This is because moisture in the paper insulation will have migrated to the oil during the period of full load, and will not yet have had time to migrate back. This is usually considered to be a normal phenomenon, but it is possible that it may also be a factor in so-called ‘sudden death’ transformer incidents where, for no apparent reason, a seemingly healthy transformer suddenly fails. This is another good reason for recording as much information about the transformer as possible and for trending results to look for unexplained changes. Do not take samples when it is raining or snowing, or when the relative humidity is above 50 %, as there is a high probability that samples taken in these conditions will be contaminated. Do not take samples when it is windy, as dust blown by the wind may contaminate the sample. Try not to take samples when the ambient temperature is high, as perspiration is a common source of contamination problems.
Successful dielectric breakdown voltage testing depends not only on obtaining a good sample, as discussed in the previous section, but also on ensuring that the test vessel is properly prepared. The preparation of the test vessel can be divided into two key elements: the first is storing, cleaning, and filling, and the second is setting the electrode gap. Storing, filling, and cleaning test vesselsIEC 60156 recommends that a separate test vessel assembly is used for each type of insulating fluid that it is required to test. This standard requires that the test vessels are filled with dry insulating fluid of the type that they will be used to test, then covered and stored in a dry place. ASTM offers an alternative option of storing the vessels empty in dust-free cabinets. Immediately prior to testing, vessels stored full must be drained and then all internal surfaces, including the electrodes, rinsed with fluid taken from the sample to be tested. The vessel should then be drained again, and carefully filled with the test sample, taking particular care to avoid the formation of bubbles. If the vessel was stored empty, or if it is to be used for a different type of fluid from that with which it was filled during storage, it should be cleaned with an appropriate solvent before the rinsing and filling procedures described above are followed. ASTM D1816 specifies the use of a dry hydrocarbon solvent such as kerosene, which meets the requirements of D235. Solvents with a low boiling point should not be used as these evaporate rapidly, cooling the vessel and giving rise to the risk of condensation. Solvents commonly used include acetone and, in the USA, toluene. Toluene is banned in Europe. Use lint-free, clean room wipes to clean the vessel. Do not use paper towels as they may introduce particles that hold moisture, causing breakdown values to be dramatically reduced. Touching the electrodes or the inside of the vessel should be avoided and during cleaning, the electrodes should be checked for pitting or scratches that may cause breakdown voltage values to be decreased. Setting the electrode gapSetting the electrode gap accurately is very important, as the results obtained are only valid if the gap is correct. A big problem is movement of the electrodes after the gap has been set and for this reason, many users of oil test sets check the electrode gap frequently – sometimes before every test. A better solution is to use test sets where the electrodes can be locked in position, such as the instruments in Megger’s latest OTS range. Megger recommends the use of flat, smooth gap gauges. The latest Megger gauges have a black anodised coating, which not only provides a smooth surface but also shows when the gauge is worn, as the shiny aluminium starts to show through the coating. Hints and tips for vessel preparation:
- If rinsing the test vessel with the sample oil before testing, it is most important to immediately fill the test vessel with the oil sample to be tested. Any significant delay will result in the oil film on the vessel’s walls absorbing water from the air, and since the walls have a large surface area, this will contaminate the oil sample and reduce the breakdown voltage once it has been mixed with the sample.
- Pour the oil into the vessel swiftly with minimum turbulence so as not to entrap air.
- Allow the sample to stand for a few minutes before the testing to allow air bubbles to clear.
- Do not leave the sample in the vessel to stand for too long before testing as it will absorb water from the air in the headspace above it. This will reduce the breakdown voltage.
- If you are using an impeller stirrer that utilises a baffle plate to exclude air from the oil sample ensure that:
- Oil does not pass over the upper surface of the baffle plate
- Oil is in full contact with the underside of the baffle plate
- The use of a magnetic bead for IEC60156 will circulate oil in the lower portion of the test vessel, whereas the impeller will circulate all of the oil in the test vessel. The magnetic bead therefore has the advantage that moisture absorbed by oil in contact with air is not stirred into the sample, avoiding unwanted contamination.
- Remember that the rules of cleaning and preparing the vessel also apply to the magnetic bead, impeller, baffle plate and electrodes, not just the vessel walls.
- When performing continuous testing of many oil samples, such as in laboratory environments it is important to clean or rinse the test vessel between every sample tested.
- Always refer to the appropriate test standard to ensure the preparation is performed as specified.
You should:
- Store electrodes in a suitable container
- Immerse electrodes in clean mineral insulating oil
You can keep electrodes in a test vessel, left to stand overnight, with the last oil sample tested.
The 400 ml vessel supplied meets the requirements for most testing standards. A 100 ml vessel is also available that complies with ASTM D877.
The breakdown voltage of an oil sample increases significantly with temperature. For example, a natural ester sample with a breakdown voltage of around 35 kV at 30 ºC could easily have a breakdown voltage of nearly 60 kV at 70 ºC. For this reason, all oil test standards specify that the temperature of the sample must be recorded in the test report. The trending of test results to identify changes in breakdown voltage is only valid if the sample and ambient temperatures for all results have been taken into account. Some breakdown testers measure oil temperature automatically. This helps to ensure that the sample temperature has been measured and avoids the possibility of introducing contamination by placing a thermometer into the oil sample.
The simple answer is yes; new oil can fail a breakdown test. Sometimes, users suspect their test set is faulty because it is failing new oil. When the test set is checked, however, almost invariably no fault is found.
IEC 60156 recommends that a separate test vessel assembly is used for each type of insulating fluid required to test. This standard requires that the test vessels are filled with dry insulating fluid of the type you will use them to test, then covered and stored in a dry place. ASTM offers an alternative option of storing the vessels empty in dust-free cabinets.
Cleaning the outer surfacesYou should:
- Disconnect the instrument
- Wipe the instrument using a clean damp cloth with isopropyl alcohol
Cleaning the test chamberMake sure the test chamber is always kept clean, particularly before a test.You should:
- Wipe away any spilled oil
- In the chamber
- Outside of the test vessel using a lint-free cloth
- Use the drain facility at the rear when lots of spilled oil is in the test chamber
- Unclip the clear tube and drain the oil into a beaker or other suitable container
Cleaning the inside of the test vesselYou should:
- Follow the instructions given in the relevant test specification
- Use a small volume of the next sample of oil you are measuring in the case of no instructions
You should:
- Use isopropyl alcohol
- Immerse the electrodes in clean insulating oil for a couple of hours before use
You should:
- Use a clean soft cloth and brass cleaner
- Use minimal pressure to avoid removal of excessive electrode material
- Use a clean cloth with isopropyl alcohol after removing the dirt
- Immerse the electrodes in clean insulating oil for a couple of hours before use
- Discard pitted or scratched electrodes and fit new electrodes
