Time Delayed RCDs

Application Of MK Sentry Time Delated RCDs:

When two or more MK Sentry RCDs are installed in series with one another, measures must be taken to ensure that they discriminate properly. In event of an earth fault, only the RCD immediately upstream from the fault should operate.
RCDs do not discriminate on rated tripping current alone, i.e. a 100mA rated RCD situated upstream from a 30mA rated RCD, will not offer inherent discrimination.
In order to ensure that discrimination is achieved, a Sentry Time Delayed RCD should be used. The in-built time delay period ensures that the downstream RCD opens the circuit before the upstream RCD starts to operate.
The maximum tripping time of a Sentry Time Delayed RCD is 500ms.

Typical applications are:
i) as main incomers on TT systems where all sockets are already protected by a 30mA instantaneous RCD, but where unwanted tripping may become a problem.
ii) as the main incomer of split load consumer unit arrangement where all circuits are protected by a 10 or 30mA instantaneous RCD or otherwise comply with the direct and indirect contact protection requirements of the Wiring Regulations.

The Sentry Time Delay RCDs are clearly identified with the internationally agreed representative symbol "s".