Disconnection Times for Inverter Circuits

A matter has been raised recently from the Technical Helpline where a contractor undertaking an Electrical Installation Condition Report has failed the system as not meeting the requirements of BS 7671: 2001 in terms of disconnection times for fault or additional protection. The inverter circuit was found to be fed from a general consumer unit with other final circuits covered by a D.P. RCD. The concern was that the disconnection time of the RCD under fault conditions would be compromised by the inverter, which in theory could take up to 5 seconds to shut down on interuption of supply.

This shut down period will normally exceed the disconnection times stipulated in BS 7671 for additional protection and the inverter will continue to produce an output voltage between Line and Neutral only for the shutdown period. This is provided there is no connection between N - E in the D.B. due to a fault, or a resistance path though a component fed from other connected circuits protected by the RCD - obviously, this situation is difficult to guarantee. Please see page 14 of the ESC's Best Practice Guide 3, on Microgeneration which states that any RCD protecting the Solar pv circuit has to be double pole and shows a diadram illustrating the problems associated with an N-E fault.

To provide a safer solution, it is considered best practice for the Solar pv inverter circuit to be fed from a dedicated consumer unit via a double pole breaker, or alternatively kept separate from other circuits on the non-RCD protected side of the general consumer unit.