The resistance property of cable needs to be low enough to ensure that it will not heat up when the maximum rated current is passed through it, says Nathan Barwell business development engineer, Seaward Group, so cable manufacturers need to measure resistance to comply with specifications and international standards.

To ensure meaningful measurement it is necessary to define the conditions: the length of cable to be measured, the temperature at which the cable is to be measured and the current used to measure the resistance are all factors that can influence testing and results – so these need to be clearly defined.

When it comes to cable length, it’s common practice to measure a metre length of cable and a cable clamp featuring connections for the current and connections for the potential points used to hold the wire. This eliminates inconsistent readings due to small changes in the length of cable measured but care should be taken not to stretch the wire when placing it into the clamp, as this will increase its resistance. The clamp should also shield the wire against draughts to avoid temperature variations and include provision for mounting a temperature sensor adjacent to the cable under measurement.

Most modern digital microhmmeters use the four terminal principle of measurement – the ‘Kelvin’ or ‘Thompson’ principle as sometimes known – for determining values below 100 ohms to eliminated lead resistance from the measurement. When measuring, a current is passed through the cable and the volt drop between the potential connections is measured.

The potential leads carry a negligible current and according to ohms law (Resistance = Volts/current), if no current is flowing then the resistance of the potential connection wire is zero. (The resistance of the potential leads is not included in the wire measurement). It is important to position the connections correctly to ensure the correct value is measured – a few centimetres outside the potential connections, positioned exactly at the measurement points.

Cables have a temperature coefficient so resistance values will vary according to the temperature. Some materials have a relatively high temperature coefficient (e.g. copper = 3250ppm/°C) and the measured resistance value will depend upon the temperature. This temperature is influenced either by ambient temperature changes or the heating effect of passing the measurement current through the cable.

The effects of ambient temperature changes can be compensated for when using a microhmmeter, which operates by measuring the ambient temperature with a Pt100 temperature sensor and then correcting the measurement by the samples temperature coefficient and referencing the measurement to 20°C. This ensures all measurements are relative to the same temperature.

Compensating for the heating effects is not quite so easy. The temperature compensation method used for ambient temperature changes can be used but to be effective the cable should be submerged in a tank of liquid (usually water) and the temperature sensor used to monitor the liquid. The liquid bath will dissipate and stabilise the temperature from the cable but this can be costly and messy.

Another approach is to limit the measurement current and the time the current flows through the cable thereby reducing heating effect. A microhmmeter like the Cropico DO5000 has the ability to vary the measurement current and reduce the measurement time to a single shot measurement of approx. 0.5 seconds – or in fast mode 0.02 seconds. Below is a table showing the resistance of wire and cables according to temperature.

 

MATERIAL

TEMP. COEFFICIENT

@ 20 Deg. C

@ 23 Deg. C

Copper

3930 ppm/°C

0.001 O

0.01279 O

Aluminium

4100 ppm/°C

0.001 O

0.0123O

 

It is also important to note other possible causes of measurement errors: inconsistent conditions, incorrect cable clamp, varied positioning of connections, not compensating for ambient temperature variations, draughts affecting cable temperature, the cable being heated by measuring current causing a continual drift in measurement, poor connections and excess oxides or dirt at connection points can skew results and must be taken into account when considering effective measurement solutions.

Using a microhmmeter combined with cable clamps can improve measurement accuracy and consistency. Typically, this is linked to a specialist clamp which holds a standard one metre cable length in place as the copper strands pass along the bobbin extruder. The clamp incorporates separate current and potential measurement connections, which are attached to the cable to measure resistance while accurate and reliable in-line cable resistance measurements enable cable manufacturers to maintain optimum operating efficiencies in terms of copper raw material usage.

The versatile Cropico D05000 for example has programmable current settings in 100 steps from 10µA to 10A and measures from 3mO to 30kO with a resolution of 0.1µO and ±0.03% accuracy. Variable measurement speed settings of 50, 25 or 2.5 per second allows the selection of high speed testing for production line applications, a medium mode where the device under test needs more time to settle or a slow speed for manual operation where display clarity is a priority.

True four-wire resistance measurement eliminates lead resistance errors, while auto averaging and automatic temperature compensation with 20°C referencing or other user-defined settings increases true measurement accuracy. All these options are included in the basic D05000 unit as well as a data logging function which stores up to 4000 readings with date and time stamp. Statistical analysis of these values allows the display of max/min/average values as well as peak to peak and standard deviation. More at www.cropico.co.uk