rcd advice

[soulman]

Property with tt earthing system. Tails go into a 100ma rcd then go about 5 to 6 meter to ccu. (tails travel through the building). Consumer is split board has 100ma Time delay as main switch which feeds some circuits. Then a 30ma rcd which feeds sockets etc.

Would i be correct in assuming that after the meter there should be a switched fuse to protect tails and could the 100ma rcd be changed to a time delay before it hits the consumer then two 30ma rcd's in the consumer unit. Therefore there would be some discrimination. my only thought is that the tails should be protected by a 30ma rcd as they dissapear into the floor and appear again at the consumer unit. Currently when ever the rcd trips within the consumer unit it automatically trips the 100ma one by the meter which is a major inconvenience.

Cheers

 

[Technician]

Soulman I have had this conversation recently. An RCD can be

used as a main switch (see the table in the new version) and

if the RCD is an S type (delayed) this should as you say provide

the discrimination that must be preserved.

Will come back about that table.

Table 54.3 on page 149. Look at the RCD entries and read the notes

in small print at the bottom of the table.

 

[soulman]

technician there is no discrimination at all. the ccu has a time delay 100ma as main switch but 6 meters away next to the meter is a 100ma standard rcd so defeats the object of having a time delay in the ccu. In my opinion it would be better to have 2 30ma in ccu and 100ma time delay for the tails but problem is should tails be protected by 30ma as travelling through building also i'm sure there should be a switched fuse at the meter.

cheers

have just ordered new regs for jan

 

[Technician]

Soulman the problem as I see it with the tails is that an

RCD will not protect against short circuit. I think that

the switched fuse for the tails is correct and then an S

type RCD upstream of the 30mA rcd's in the CCU.

HTH.

 

[wozz]

How do these tails travel through the building? will they need additional protection?

RCD first or insulated switch fuse remember.

 

[NozSpark]

Soulman

As it's a TT installation ALL cables need RCDs for protection against fire so use a plastic switch fuse with a 100A 100mA S type at the meter and then a dual 30mA RCD configuration at the CU

 

[steptoe]

you still need fused protection at the incomer too, as well as the time delayed 100mA RCD

 

[NozSpark]

you still need fused protection at the incomer too, as well as the time delayed 100mA RCD

I did say "switch fuse" ; )

 

[Electroglow]

Or use SWA for tails no rcd needed but still put a switched fuse in

 

[soulman]

thank you lads especially NOZSPARK. i was just unsure if the tails needed a 30ma rcd's as they travelled under the floor to the consumer unit. I'm not actually doing anything with this like putting tails in. i was just called to a tripping rcd which is sorted. i just noticed no switched fuse for tails over 3 meters and no discrimination. which has caused the customer problems. And this installation had a periodic a few months ago and none of this picked up as a problem.

 

[NozSpark]

Or use SWA for tails no rcd needed but still put a switched fuse in

It's a TT installation so even a SWA sub main will need RCD protection

 

[Electroglow]

Good point.... For got it was TT

 

[Technician]

Just as an addition to the discussion... I was going to

add something re. the tail length but could not get to

my PC and I had to find the reg.

Reg 434.2.1 refers to tail length to be a maximum of 3

metres where protection against fault current is considered.

Yes, Soulman, why was that not picked up?

HTH