TT and RCD

[bloxy]

Read from page 23 of the On Site Guide.

In summary all circuits must be RCD protected.

All of these circuits need 30mA RCD; circuits with buried cables less than 50mm not in earthed steel conduit , all circuits of locations containing bath or shower, circuits with socket outlets not exceeding 20A, mobile equipment not exceeding 32A for use out doors.

All other circuits need 100mA RCD (time delay gives discrimination).

Because of the relatively high impedance of a TT earth rod there may not be sufficiently high current flow to earth to trip an MCB in case of a fault. The 100mA RCD in this situation will trip instead so giving protection to those circuits.

Also the consumer unit must be an all insulated type.

I would think that if you use a dual RCD board with 2x 30mA RCD and no unprotected ways available then you wouldn't need the 100mA and could just use an isolator.

 

[mikhailfaradayski]

One thing im not sure about is, are you just doing some testing for your assesment, fitting a new C/U or doing a rewire?

If a rewire, take the opportunity to split the circuits up a bit more

If you a fitting a new board, then i might go for 5x RCBOs to give minimum inconvience, check 314.1.

If you choose to use that board with 2x RCD and 2 ways off the main switch, go for it.
Other than making the enclosure IP4x/IP2x as required, you don't have to do anything to the unprotected ways, that is the responsiblity of the next guy who comes along.

If all circuits are 30mA protected, then bin the 100mA delayed.

 

[Spark123]

Isn't the max disconnection time 0.2s for circuits not exceeding 32A on a TT?
Anyhoo, a type S should trip within 200ms at 2x rated trip current.
The IET now seem to like general 30mA devices for things like socket outlets and unprotected concealed cables.

 

[securespark]

(1)100ma time delay would not give me the 0.4 disconnection time required by the regulations..!

Are you sure it's 0.4s for TT?

EDIT: Sorry 123, I always post without looking at the end of the thread...

 

[sblakey1]

@ mikhailfaradayski

the site assessment is just a CU change with full testing,

cheers for answers guys very appreciative, did not fancy having to shell out the cost of a time delay RCD (im doing work for free as a favour, plus it helps me out for assessment)


its an old terrace house and its extension was fitted 4 years ago so it got a full periodic done then....hopefully all still well

but tell you what give me Submarine electrics any day!!!!

 

[sblakey1]

@ every body else!!

cheers...


im going with the Dual RCD for final circuits, 2pole isolator at supply end, the box is Insulated and will be IP compliant for unused ways.

also no doubt the elecsa assessor will have his own take on the TT stuff..
must have asked 15 sparks before coming on her and its a 40% yes to no Time delay and 40% you need time delay split with 20% not positively sure what the correct answer is...

cheers guys..


steve

 

[Spark123]

In a nutshell, 30mA RCDs are required for all socket outlets and unprotected concealed cables.
This, amongst others, will mean every circuit wired in twin and earth where the cable is concealed in a wall or partition (without additional protection or buried 50mm> deep) in a domestic dwelling requires 30mA RCD protection.
Cables which have their own protection such as SWA do not need 30mA RCD protection even when concealed, however owing to the increased EFLI in a TT system the cable must be protected by an RCD to achieve automatic disconnection of supply (ADS). This RCD can be larger than 30mA, it can be time delayed (type S or Selective).
When you have one RCD feeding another say in a submain for a garage, for the RCD upstream it is preferable to use a time delayed device so only the one in the garage would trip in the event of a fault on a final circuit.
It is a bit pointless having a 100mA(S) device in a board and using it to feed 30mA devices.
As your CU is placcy then you don't need to RCD protect the tails or apply additional measures as they enter.