<p>A loop impedance tester comprises a clip-on transformer coil (19) for inducing an alternating voltage in a closed, electrically conducting loop (10-13). The current flow around the loop is sensed using another clip-on transformer coil (21). The impedance of the complete loop may be determined by dividing the voltage induced by the sensed current. Alternatively the impedance of part of the loop may be determined by dividing the voltage drop across said part by the sensed current.</p>
