100ma RCD & 100ma Time Delayed RCD - what's the differen

[dannyboi2003]

Hi Guys,

What's the difference between a 100ma RCD and a 100ma Time Delayed RCD?

The answer seems fairly obvious, the time delayed version is as the name suggests delayed so it doesn’t trip unnecessarily. If I'm wrong about this please correct me. Also what situation would each one be used in, I’m assuming that on a TT supply (for example) you would have one as the main incomer but which one would you use or not use and why.

I'm asking simply out of curiosity, I posted some pics of my CU on here some time ago and they were mentioned then and it would be nice to plug the gap in my knowledge.

Cheers guys
Dan

 

[Spark123]

A time delayed one is used to provide discrimination with another downstream RCD i.e. a 30mA. If there was a fault on a circuit protected by the 30mA RCD which causes say 500mA to flow to earth the 30mA will should react to this pretty quickly and disconnect the supply within 40mS. The upstream 100mA is also likely to operate as there is 500mA flowing for up to 40mS. This is where the time delay ones come into play, they hold off and give the 30mA RCD time to disconnect the fault before they operate themselves.

 

[dannyboi2003]

Hi Spark,

Thanks for the quick reply. This has been on my mind for ages so its nice to finally know the difference between the two and in what sort of situation each one would be used.

Cheers
Dan