Best Consumer Unit set up for TT system

[TheCodboy]

Hi all, I'm just wondering what is people's preferred choice of set up with regards to DB's for a TT system within a domestic setting. I know there's different views on this (Type S uprfront etc). Are most people installing dual rcd boards or DP Rcbo's? Just looking for some views on this. Many thanks!

 

[Strima]

RCBOs suitable for use on TT systems, check manufacturers information, some are, some aren't.

No up front RCD if the tails are correctly supported using tails gland etc. and is correctly earthed if of metallic construction.

 

[pc1966]

Well 'best' can mean different things to different people! Cost usually comes in to it.

If asked I would pretty much go with the above which is always good advice:

• Use a metal enclosure per amendment 3, 18th, etc.
• Always you a proper gland for the tails to provide support as well as protection against insulation damage.
• I would still go with a 100mA delay RCD up-front as it (1) avoids checking RCBO details, (2) avoids them being a single point of failure for fault to earth clearing, and (3) also deals with the rare chance of a SPD fed from the bus failing.
• I would go with all-RCBO so any future faults are easier to find than a dual 30mA board, also less risk of combined leakage from multiple circuits contributing to nuisance trips.
• My own preference would be for the DP RCBO like Wylex/Crabtree/Fusebox but that is more for ease of testing later, however if you have any decent favourite like Heger, Schneider, etc, then stick with them. If you have a DP up-front RCD then even some bizarre reversed polarity fault is still OK even without DP RCBO!

Just my 2p worth here.

 

[mattg4321]

Up front RCD with single pole RCBO's is not a good option IMO.

Neutral-Earth fault will operate RCBO, but fault remains and up front RCD then operates and can't be reset.

I'm fitting Wylex reduced height SP+SN RCBO's on a current job because of this reason.

 

[Wilko]

Yes the newish reduced height Wylex switched L and N RCBO are my favourite but they are a bit exy.

 

[pc1966]

It is the Wylex SP+SN type I have at home, though not TT. Others on here prefer the Crabtree plug-in version but can't say I have used them.

 

[andyb]

Just my 2p worth here.

Don't undersell youeself PC.

Just my opinion, but I don't see the need for the upfront RCD nowdays with all circuits being protected by rcbos. Not sure what it's protecting.
I'd just fit a main switch.

 

[Strima]

Don't undersell youeself PC.

Just my opinion, but I don't see the need for the upfront RCD nowdays with all circuits being protected by rcbos. Not sure what it's protecting.
I'd just fit a main switch.

Some see a risk of the meter tails and internal cabling within the CU causing the metal case to become live under fault conditions, this is extremely unlikely to happen and with the use of tails glands there's no risk of them shorting out on the CU enclosure.

 

[andyb]

Some see a risk of the meter tails and internal cabling within the CU causing the metal case to become live under fault conditions, this is extremely unlikely to happen and with the use of tails glands there's no risk of them shorting out on the CU enclosure.

This is my view also Strima.

 

[buzzlightyear]

I agree with most lads now metal CU being fitted then a tail glad will be survice with out putting a up front rcd..

 

[pc1966]

The reason I would still go for a delay-RCD as a main switch (not as a separate enclosure) is to avoid a single point of failure in the RCD/RCBO electronics.

As mentioned above, an all-RCBO board does not need any up-front RCD to meet disconnection times, so long as the RCD action works. But folk never test them, and they are an order of magnitude more complex than the thermal/magnetic trip of a MCB (or equivalent aspect of a RCBO) that would be the back-up in a TN system (but is not going to trip in a TT system).

So if there is pressure on the budget then a simple incoming DP isolation switch is fine with an all RCBO board (or similar) as it is fully compliant, but to me that is not the best setup.

 

[TheCodboy]

Up front RCD with single pole RCBO's is not a good option IMO.

Neutral-Earth fault will operate RCBO, but fault remains and up front RCD then operates and can't be reset.

I'm fitting Wylex reduced height SP+SN RCBO's on a current job because of this reason.

With a DP main switch (not rcd main switch)?

 

[mattg4321]

With a DP main switch (not rcd main switch)?

In this case I’ve got a 100mA Type S protecting the distribution circuit to the consumer unit.

As mentioned above I think an RCD main switch would be preferable, but is not required.