• 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 RMO400 can produce:

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:

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 100µΩ 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.

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