RCD requirements for TT?

HappyHippyDad · Mar 6, 2021

Evening all...

I'm having a complete mind blank on what the mA requirement is for an RCD on domestic household with TT earthing, Ra = 57ohms. I'm not talking about final circuits.

I'm planning on splitting the meter tails into a henly, then into a switched fuse, then 50m SWA to consumer unit in garage which will contain a 30mA RCD or RCBO's.

I simply cannot remember or find in the regs if I need RCD protection for the buried SWA. If it was a TN then It wouldn't need it as it has mechanical protection, but it's TT.

I'm sure a 100mA S type would be OK in the switch fuse as replacement for main switch, but I just want to know where in regs it says this will comply, or even necessary?

Risteard · Mar 7, 2021

nicebutdim said:
That's the conclusion I'm arriving at, if the charger requires Type B RCBO, and why I'd directed the question to Risteard. I figure he might have considered this possibility.

Edit: Just spotted @Ian1981 post, which seems to confirm this thought.

There doesn't yet appear to be a great selection of available 2 pole time delayed 100mA Type S RCCBs, with Type A characterisitcs and I'm surprised that Wylex haven't yet introduced a version of the widely used WRMT100/2. Currently the best priced and most widely available options appear to be those from CP Fusebox and Lewden.

Sorry, was working until 8:30p.m. tonight so only seeing your post now. I haven't come across an RCBO with a Type B RCD characteristic (though they may exist). Any Type B RCD I have fitted has been an RCCB.

If a Type B RCD is needed then any upstream RCD will need to be Type B also. If Type A, then any upstream will need to be at least Type A.

You shouldn't have any issue finding a double pole Type A S-type RCCB - they are widely available.

nicebutdim · Mar 7, 2021

Risteard said:
Sorry, was working until 8:30p.m. tonight so only seeing your post now. I haven't come across an RCBO with a Type B RCD characteristic (though they may exist). Any Type B RCD I have fitted has been an RCCB.

If a Type B RCD is needed then any upstream RCD will need to be Type B also. If Type A, then any upstream will need to be at least Type A.

You shouldn't have any issue finding a double pole Type A S-type RCCB - they are widely available.

No worries about the time and I appreciate the response - it was purely hypothetical and came to mind after Type B requirement for certain chargers was introduced in post #17.