Method of testing a spur short circuit protection system and diagnostic device for performing the method

A diagnostic device for testing a short circuit protection system applied to a spur of an electric circuit, wherein the short circuit protection system comprises a current limiting device comprising a series semiconductor and a current sense/drive circuit which current limiting device applies a current limit to a connection to terminals of said spur if the current thereon exceeds a spur current trip level, and wherein the electric circuit comprises a power supply and an active monitoring and isolation system which fully or partially isolates the power supply from the electric circuit if the current thereon exceeds a power supply trip level for longer than a deadband period, the diagnostic device comprising:

• i) a current demand application device for applying a test current demand to said short circuit protection system which has a current and duration sufficient for said spur current trip level to be exceeded, but a current insufficient for said power supply trip level to be exceeded and/or a duration insufficient to exceed said deadband period;
• and
• ii) a detection device for detecting if the current limiting device applies said current limit or not during said test current demand.