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Protective Bonding: The 0.1 Ohm Test and What It Really Checks
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On Class I equipment, the protective bond connects every accessible conductive part to the protective earth terminal. The test injects a defined current, typically 25 A from a low voltage source, and requires that the voltage drop corresponds to a resistance below a small limit, conventionally 0.1 ohm.
Why a high current
A low current measurement through a multimeter reads the contact resistance of the probes as much as the bond. A 25 A test is high enough that a poor bond, such as a painted surface under a screw or a single strand of a stranded wire, heats and shows a resistance far above the limit. The test is designed to find the connections that would fail to carry fault current, not to measure a precise value.
Where bonds fail
Under paint and anodising, at screw threads where a star washer was omitted, at hinges and sliding joints that were never bonded at all, and at cable screens terminated by a drain wire that does not make contact. All four are construction issues that a drawing review will not catch and a single measurement will.
Doing it correctly
Measure from the earth pin of the plug or the earth terminal to the accessible part, with the probe tip on bare metal rather than on a coating, and with the test current flowing for the specified time. Null the lead resistance before the measurement, and record the value rather than only the pass verdict.
