Find Answers to Common Electrical Questions
Pace Technologies Inc. has started this FAQ to supply answers to common questions we hear regarding power systems testing and the electrical industry. We encourage you to check back often as this page will be updated. Do you have a question for Western Canada’s electrical distribution experts? Please feel free to submit your question using our contact form.
FOUR WIRE MEASUREMENT (KELVIN METHOD) FOR GROUND BOND TEST When measuring low value resistances (for the ground bond test for instance), an important error source is coming from the contact resistance. Actually, in many applications, the contact resistance value can go beyond the value which has to be measured.
The method to cancel this error source, is to supply a high stability permanent current over 2 wires and to combine a voltage measurement on 2 other wires. The voltmeter which is used to measure the voltage drop at the resistance terminals, must have a high input impedance and a good measurement accuracy.
The diagram above shows the 4 wire measurement principle used in our milliohmmeters (ground bond test). Because of the high input impedance of the voltmeter, there is no error due to the leads resistance and the contacts in the voltage measurement circuit. Actually any current is flowing through these wires.
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