Does the installation of an RCCB affect fault loop impedance?
Yes. The Wiring Rules require a protective device to operate in not more than 400ms under earth fault conditions. An RCCB always operates in less than 300ms and so meets the earth fault requirements. When conducting an earth fault loop impedance test, an internal connection is made by the test instrument which causes an RCCB to trip. Therefore the fault loop test is satisfactory (see Cl 6.3.4.2 of the wiring rules).
