TT System RCBOs/RCD

[Buzz1980]

Hi All

Trying to do some digging on TT systems. With regards to TT systems, I have had different views/opinions on this stating that TT installations should have double pole RCD when it comes out of the meter so the installation is protected by RCD before tails enter in consumer unit. I also had other sparks state that as long as you have a plastic consumer unit and all circuits protected by RCBOs this meets the requirements. Are both views correct or is one incorrect. Any info and stated regs would be much appreciated. Thanks in advance

 

[Ian1981]

Please read regulation 531.3.5.3.2.201

 

[pc1966]

Also 531.3.5.3.1 says "at the origin" which I would interpenetrate as before the bus-bar feeding the various RCBO, etc.

A plastic CU and all-RCBO might be OK (as the bus-bar would not need protecting due to the non-conductive surround) but would not meet current requirements on fire that have steel CU as the example material all are now using.

 

[Buzz1980]

Hi Thanks for the reply. So reading through the reg is that saying that if all circuits are protected by RCD protection which they are through RCBOs and that all circuits are double insulated, supply and consumer unit are so no need to put RCD in and supply and both views are correct. Thanks

 

[pc1966]

I think if you found an example of plastic CU and all-RCBO installation during an inspection then you could reasonably say it is not a shock risk and meets that aspect.

But it is not something you would consider doing now!

Also worth remembering that most RCBO only switch the phase line (recent Wylex ones do both poles, as do most systems in Europe) whereas the incomer RCD would switch both L & N so is a bit safer in the unlikely, but very serious, case of L/N polarity swap.

 

[Buzz1980]

Hi PC1966

Just noticed your reply. Yeah as the current regs state should have metal CU or a plastic Consumer Unit inside non-combustible enclosure, this wont meet current regs. But if for example you are carrying out a EICR and you find this situation then this would be ok as installed to regs at that time but for to install this now like this would be against the current regs? Thanks for reply

 

[telectrix]

the point of an upfront RCD in a TT system with metal CU is in the event of the tails shorting out to the metal CU, in 40 years i've never seen anything chafe through both the sheath and the heavy insulation of CU tails. takes me ages to cut through it with a sharp knife when scrapping out.

 

[Buzz1980]

Sorry PC1966 we must be typing at the same time. Yeah thats what i thought, that will not be acceptable now but previous editions this would be acceptable , to be RCBOs on individual circuits and then plastic DB. ?

 

[Ian1981]

I find that although not a requirement, an upfront S type 100mA rcd external to the CU helps give the internal rccb links found in dual rcd boards only, fault protection in the event of them coming loose and shorting out against the metal case of the Cu thus disconnecting the power and removing the shock risk.
Above the requirements of bs7671 , but that’s not necessarily a bad thing.

 

[pc1966]

There is another minor advantage to an up-front RCD as normally it would be 100mA delay (S-type) to discriminate against down-stream 30mA RCD tripping, and that would allow using only a MCB for fixed things like immersion heaters or cookers (those without 13A socket feed) that are prone to nuisance tripping 30mA devices due to normal non-trivial leakage currents

 

[Ian1981]

There is another minor advantage to an up-front RCD as normally it would be 100mA delay (S-type) to discriminate against down-stream 30mA RCD tripping, and that would allow using only a MCB for fixed things like immersion heaters or cookers (those without 13A socket feed) that are prone to tripping 30mA devices due to normal non-trivial leakage currents.

However additional protection maybe required for cables concealed in walls less than 50 mm from the surface unless you go through the trouble of using earthed metallic containment etc etc. A time delay gives no additional protection for that particular requirement

 

[pc1966]

However additional protection maybe required for cables concealed in walls less than 50 mm from the surface unless you go through the trouble of using earthed metallic containment etc etc. A time delay gives no additional protection for that particular requirement

Good point there.

Recently discovered Flexishield cable with its foil surround that provides that sort of protection against shock on accidental nailing, great for new installations where you have specific circuits with good reason to skip personnel protection RCD levels.

 

[davesparks]

However additional protection maybe required for cables concealed in walls less than 50 mm from the surface unless you go through the trouble of using earthed metallic containment etc etc. A time delay gives no additional protection for that particular requirement

Bs8436 cables such as flexshield make it relatively cost effective to install circuits without the need for RCD protection for being buried in a wall.

 

[Ian1981]

Bs8436 cables such as flexshield make it relatively cost effective to install circuits without the need for RCD protection for being buried in a wall.

However They do have restrictions on their use with particular sizes of circuit breakers and types.

 

[pc1966]

However They do have restrictions on their use with particular sizes of circuit breakers and types.

For most typical setups it should not be a problem, but you are very right to raise it. The BS8436 (and related Irish standard IS273) cable values are given here:

BS 8436 & IS 273 - Circuit Breakers

BS 8436 & IS 273 - Circuit Breakers

www.flexcables.co.uk

• 1mm "lighting" 16A B MCB (higher current than likely I suspect)
• 1.5mm 20A B MCB
• 2.5mm & 4mm 32A B (or 16A C MCB)

For the Flexishield brand they also say they are good to 200A fault current so up to 40A B or 20A C breakers, but that would be dependent on specifying their higher quality cable.

 

[radiohead]

There is another minor advantage to an up-front RCD as normally it would be 100mA delay (S-type) to discriminate against down-stream 30mA RCD tripping, and that would allow using only a MCB for fixed things like immersion heaters or cookers (those without 13A socket feed) that are prone to nuisance tripping 30mA devices due to normal non-trivial leakage currents

Not a good idea to use single pole devices, either mcb's or SP RCBO's on a TT with an upstream T/D RCD. A fault which trips a SP device on a final circuit will not disconnect the neutral on that circuit, the upstream RCD will possibly still trip disconnecting the entire installation because a N-E fault may remain in place.
In practice DP RCBO's are the only way to achieve compliance on a TT if an upstream RCD protects the DB.

 

[Ian1981]

Not a good idea to use single pole devices, either mcb's or SP RCBO's on a TT with an upstream T/D RCD. A fault which trips a SP device on a final circuit will not disconnect the neutral on that circuit, the upstream RCD will possibly still trip disconnecting the entire installation because a N-E fault may remain in place.
In practice DP RCBO's are the only way to achieve compliance on a TT if an upstream RCD protects the DB.

With an upfront rcd in the installation, then Using a dual rccb CU negates the need for DP rcbos which are rare to find, (I’ve only seen wylex personally that make them)

 

[pc1966]

I don't know who do make DP RCBO for the UK market, any recommendation?

I know Wylex do for their consumer stuff as I have used them, but the majority seem to be SP switching.

My choice was for exactly that reason - so they would isolate a N-E fault as well. Also turned out the electronics is powered from the live side, so an insulation test on the busbar to neutral bar always fails at ~20k, but it means the load side is fully isolated so you can do insulation testing, check for cross-connected circuits, etc, without having to remove wires.

 

[radiohead]

With an upfront rcd in the installation, then Using a dual rccb CU negates the need for DP rcbos which are rare to find, (I’ve only seen wylex personally that make them)

Thats true, but suitable RCBO's are not rare.
Starbreaker RCBO's disconnect the neutral. Widely available. As a bonus Crabtree boards are by far the best in my opinion.

 

[Ian1981]

Thats true, but suitable RCBO's are not rare.
Starbreaker RCBO's disconnect the neutral. Widely available.

Wylex and Crabtree share the same parent company so I suppose that makes sense, I don’t know any other widely available manufacturers that do tho.