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The ground bond test (also called PE resistance test, ground continuity test), is the first test required by the electrical safety testing standards. The ground bond test consists in testing whether the ground points of a device under test are well connected in between each other, and also to the mains ground. The measured result of a ground bond test is a resistance value, which has to be lower than the indicated limit from the international standards. 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.