In an electrical test instrument for determining a loop impedance in a circuit breaker protected AC circuit, a test current of less than the circuit breaker trigger current is supplied to the circuit and the test voltage which develops due to the test current is isolated and detected. For testing a 240 volts 50 Hertz mains supply circuit, the test instrument includes a two-path cross synchronised filter (4) which rejects the 240 volts mains signal to isolate the test voltage signal to enable an accurate determination of the loop impedance to be made.