Power line meter: measures volt (RMS), current (RMS) and power (kW), manufacturer Yokogawa Hokushin Electric.
Current is measured using a clamp configuration. This is a current transformer with a closed iron core. The core can be opened to be clamped around the primary, which carries the current I
p to be measured. The primary coil has just one turn. The current ratio equals the ratio of the number of turns of the primary and secondary coil. With n turns of the secondary coil, the current through the latter is 1/n times the current to be measured, or I
s= I
p /n.