Power currents (AC) can be measured simply by using a clampmeter that acts as a current transformer. The instrument’s tongs are clipped round a single insulated conductor and the circuit is not interrupted.
The value of current is obtained from the magnetic flux strength around the conductor and is displayed on a digital display.
Direct current (DC) measurement is also available with clampmeters that have a flux-voltage transducer, known as a ‘Hall-effect’ device.
Many modern clampmeters are virtually multimeters, with the addition of facilities to measure voltage and resistance and currents up to 1000 A.
Care must be taken when measuring the current in uninsulated conductors.
More advanced clamp type meters can indicate power and power factor in single and three-phase AC circuits by using additional connections to measure voltage.
Clampmeter indicate 0 clipped around a 3 core cable what carrying 100 A AC to a motor, because the clampmeter monitors the magnetic flux around the cable, which is produced by the current.
In a balanced 3-core (or 2-core for that matter) cable, the net flux is zero – therefore no indication.
This is why the clampmeter is only connected around a single conductor.