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Microhmmeters:
Technical Support.
Micro Ohmmeter - About test current
The value of a primary circuit contact resistance (the resistance between two terminals of any pole of a breaker) is typically measured before a circuit breaker is first put into service and during maintenance inspections. Contact resistance is a value that is specified as a reference for all kinds of circuit breakers. According to IEC 56 (now IEC62271-100) this resistance value should be measured using a test current between 50A and the breaker's nominal current value. According to ANSI C 37.09 the lowest test current is 100A. Other national and international standards specify similar regulations. .
The value of the test current is usually defined by manufacturer. Most of manufacturers (ABB, Alstom, etc.) specify test current at 200A. Some electrical utilities have their own regulations (Vattenfall-Sweden measures contact resistance for breakers and disconnect switches at 300A). Others use 500A or 600A test current, while some do not exceed 100A.
Our opinion is that the current value of 300A (often even 200A) is sufficient for correct and reliable measurement.
If there is a question about the accuracy of measurement or condition of contacts, the measurement can be repeated using the higher current value. Our experience shows better result using the CONT mode - continuous current measurement at 200A to 300A in the duration of 50sec.
Higher test currents increase the accuracy, but the test current above 300A provide insignificant improvement in the accuracy. Higher currents require bigger cables, that are heavy and make the procedure of resistance measurement troublesome and complex, without any significant result improvements.
Conclusion:
We recommend the test current of 300A. For most situations 200A test current is sufficient. In case of doubt, repeat the test using the Cont mode, longer duration measurement at currents of 300A or 200A.
It is recommended to use 400A or 500A for situations where the contact resistance measured is expected to be below 20ľO.
Micro Ohmmeter - Accuracy statement
The accuracy of our Micro Ohmmeters is defined as ±(0,25% rdg + 0,25% FS) provided the following two conditions are met:
- the measurement current is at least 100A
- the measurement range is between 100µΩ and 20mΩ
If the current is below 100A the accuracy can be established following the table below:
| Test current | Error |
| 50 A | 0,25%+1µΩ |
| 20 A | 0,5%+2µΩ |
| 10 A | 1%+5µΩ |
Measurement of specimens below 100µΩ is not recommended with current below 100A.
For specimen with resistance between 20µΩ and 100µΩ the accuracy is defined following the table below:
| Test current | Error |
| 600 A | 0,25%+0,5µΩ |
| 500 A | 0,25%+0,6µΩ |
| 400 A | 0,25%+0,7µΩ |
| 300 A | 0,25%+0,8µΩ |
| 200 A | 0,25%+0,9µΩ |
| 100 A | 0,25%+1,0µΩ |
For specimen with resistance above 20mOhm accuracy is defined using table:
| Test current | Error |
| 50 A | 0,25%+1µΩ |
| 20 A | 0,5%+2µΩ |
| 10 A | 1%+5µΩ |
If you need a higher accuracy, then you need to:
- verify instrument calibration using provided Test Shunt before testing
- always use the max current available for particular range*
- do two measurement in two directions, the second measurement with inverted connection on the VS input. Calculate the average of the two measurements.
*Max current for a given range depends on the supply voltage, length and cross section of cables (cable resistance), and max measurement range on the Voltage sense input. Max measurement range on the Voltage sense input is 2,5V
In case the instrument cannot reach/generate pre-set test current, display will show the value of the maximum current achieved and the result. This result is of the lower accuracy, so we recommend that this test is repeated pre-selecting the first lower current range.
Micro Ohmmeter - About current cables
Selection of current cables depends on:
- Instrument type
- Amplitude of test current
- Required cable length to accomplish the test
Our microohmmeters are very powerful and probably have the highest output power on the market. This means that our Micro Ohmmeters can generate the required, pre-set current even with very long cables.
An indicator that shows the output power of an instrument is the Load voltage at the contacts of the instrument output at the required current value (FLV). For example, our instrument type RMO 400 can produce:
| Supply Voltage | Output Current | Full Load Voltage |
| 230V AC | 400A DC | 5,0V DC |
| 300A DC | 6,0V DC | |
| 115V AC | 400A DC | 4,0V DC |
| 300A DC | 4,8V DC |
a) CABLE SIZE (cross section / thickness)
To select a cable size, you should start from the required test current. Let's do an example of a 300A test current and supply voltage of 115V AC.
The maximum resistance of cables and conductors of power circuit breakers that can be measured is calculated as a ratio of FLV/Current
4,8V/300A = 16mΩ
If we need cables of the total length of 15m (for example 10m + 5m) then the recommended size would be 35mm2. The total resistance of cables is then 9mΩ. Resistance of the measured circuit (leads and contacts) is normally about 1mΩ, making the complete loop resistance below maximum calculated.
If we need cables of the total length of 20m (for example 10m + 10m or 5m + 15m) then the recommended size would be 2x5m, 35mm2 + extension cable 2x5m, 50mm2. The total resistance of cables is 10,2mΩ. Resistance of leads and contacts, as mentioned above is about 1mΩ, making the total within requirements.
Table of cable resistances:
| Current cables | Resistance |
| 1x3m, 70mm2 | 0,9mΩ |
| 1x5m, 70mm2 | 1,5mΩ |
| 1x5m, 50mm2 | 2,1mΩ |
| 1x5 m, 35mm2 | 3,0mΩ |
| 1x5m, 25mm2 | 4,2mΩ |
| 1x10m, 70mm2 | 3,0mΩ |
| 1x10m, 50mm2 | 4,2mΩ |
| 1x10m, 35mm2 | 6,0 mΩ |
| 1x10m, 25mm2 | 8,4 mΩ |
Conclusion:
Based on your particular situation or most common application, you should calculate the length and cross section of cables to be used. Do not buy long and thick cables without a real need for them. Cables can outweigh the instrument itself because of instrument's inherent low weight even at such high power output.
b) CABLE SIZE - Length
Most customers buy both cables of the same length, although one can be either 3m or 5m long. It is a standard practice to connect one cable to a close-by grounding point of the breaker. The other cable, which can be longer is connected to the other end of the breaker under test, and is not grounded. Our experience shows that a combination of one long and one short cable (5m and 10m) works fine in most of the cases.
We recommend for longer cable applications (longer than 2x5m) obtaining extension cables. Most common combination is 1x5m + 1x10m could be achieved with current cables 2x5m + extension cable 1x5m. Combination of current cables 2x5m + extension cables 2x5m can fulfill almost all requirements (2x10m or 1x5m + 1x15m).
Using extension cables one can achieve various combinations of useful lengths. Very often, it is sufficient to use only the basic cables and for those circumstances one should avoid using long and heavy cables.
Using extension cables one can make a combination where basic current cables have lower cross section (i.e. 2x5m 25mm2 ) + extension cables 1x5m 35mm2 or 2x5m 50mm2. Contact resistance between current cables and extension cables is below 100ľO and would not influence the total resistance of the current carrying measurement leads.
Conclusion:
Instead of heavy and cumbersome long cables obtain basic current cables (2x5m) that would provide for light and easy application most of the times, and obtain required extension cables for special situations.
