Time delay and 30Ma RCDs in series

[Clitheroe]

Hello, I am after a bit of help here regarding RCDs.
I have a Wylex split load consumer unit on a TT system (100ma T/D controlling light, cooker,immersion and 30ma controlling most sockets and outdoor circuits) but there isa cable coming from the TD RCD feeding a small two way consumer unit - a light and socket circuit . if a provided RCD protection to the socket circuit from the two way unit whether it be a standalone RCD or RCBO would it affect the T/D RCD in any way due to the fact it already has one RCD already in series with it?

Any help much apprciated.

 

[mapj1]

If the first (incomer) RCD is time delay, then it will not be affected by the fault that trips the power circuit (faster acting, lower trigger current) The slower acting trip is there only for lighting circuit faults, and in case the socket circuit RCD develops a fault.
It sounds like what you have, and propose to extend, is quite correct, as it allows faulty circuits to be discrimminated, instead of plunging everyone into darkness becasue a faulty applience (which actually makes it quite an unusual installation ).

 

[Clitheroe]

Thanks mapj1,
I understand discrimination etc but what I was actually asking about though was if the T/D RCD had two or more RCDs downstream of it which themselves have coils etc in them would there be any bizarre effect in any way or would what I propose to do present no problems in any way and be fine.

Also, is it normal for RCDs to make a very slight humming sound in use if you put your ear by one?

 

[mapj1]

It'll be fine as described. Even if discrimination wasn't correctly done it'd be safe, but irritating to fault find when it tripped.
Yes a mild humming at or near full load is not uncommon, and is no cause for alarm, unless accompanied by thick back oily smoke, or flames.