single pole rcbo's in a tt system

[welchyboy]

is there anyone who can clarify the debate about rcbos on a tt system???

i have a main 300mA s type DP rcd at origin and wanted to use rcbos on sub main cu's in other buildings instead of using a single 30mA device to protect a whole cu

as a whole board going down would be an issue

page 16 of the OSG shows a tt set up with rcbos without a main 100mA rcd being used

 

[sparkytim]

Hi

There has been much debate on this topic.

BS 7671 REG. 537.2.2.1 states " A device for isolation shall isolate ALL live conductors( That means Line and Nuetral) from the circuit concerned, subject to the provision of Regulation 537.1.2 which allows Isolation of the LINE only on TNS and TNCS systems.

However Reg 537.1.4 states " A main switch OR linked circuit breaker shall be provided as near as practible to the origin of every installation as a means of switching the supply on load and AS A MEANS OF ISOLATION. A main switch intended for operation by an ordinary person e.g. of a household or similar installation ( i.e Domestic) shall interrupt BOTH live conductors of a single-phase supply"

So my understanding is that the Main switch is the isolator and RCBOs are acceptable on a TT installation as shown in OSG Page 16

 

[Zeespark]

Tim, that's fine if its an insulated board, if you want to use SP RCBO's on a metal clad board then an RCD as opposed to a main switch should be used. This is nigh on the biggest grey area in the regulations.

AndyGuinness

 

[sparkytim]

Hi Zee

I agree as the OSG states on page 29 " The enclosures of RCDs or Consumer Units incorporating RCDs in TT installations should have an all-insulated or class II construction or ADDITIONAL precautions"

Older Metal clad RCD Main switch Consumer Units such as early version Crabtree Starbreaker Units had an Insulated clamp for the Meter tails and directed grommit strip used for the entry hole into the Unit.

 

[steptoe]

TT requires all live conductors to be isolated in case of a fault,

how do you think SP RCBOs can achieve this?

 

[Zeespark]

Then that's fine as far as I am aware, because the incoming cable has mechanical protection from being damaged. I was taking more of the view of some metal clad boards not having sufficient mechanical protection due to design.

AndyGuinness

 

[sparkytim]

Steps

They don't but the MAIN SWITCH does.

(Isolation BGB Pg 29 Definition No mention of "in event of fault")

I am unable to find any reference in 411.3.2 Automatic disconnection in case of a fault to double pole RCDs.

 

[Zeespark]

Looking at it that way Steps, it can't but My single biggest gripe is needing a 20 way board just to wire something near to a standard home taking into account that a DP RCBO needs 2 module spaces each. If RCBO manufacturers could get it down to a single way, then SP RCBO's would definitely be a thing of the past.

AndyGuinness

 

[steptoe]

StepsThey don't but the MAIN SWITCH does.

(Isolation BGB Pg 29 Definition)

537.2.2.1

so how do we isolate in case of fault?

this has all been done to death in the past,

different people have a different take on how to interpet the regs.

 

[welchyboy]

ok thank you anyway

my problem has been answered.....as i have a 300mA upstream device and i have rcbos downpstream any potential fault on an rcbo protected circuit causing the rcbo to open would not acheive isolation between N-E thus potentially still operating the 300mA device, i should of thought this one out, was just expecting a more complicated answer i think

cheers anyways

 

[sparkytim]

537.2.2.1so how do we isolate in case of fault?

this has all been done to death in the past,

different people have a different take on how to interpet the regs.

My understanding is that a fault is only required to DISCONNECT the supply not ISOLATE the supply.

This is accomplished by disconnection of the LINE by operation of an RCD which may be an RCBO or in certain circumstances if the Zs is low enough can be an MCB 411.5.2

 

[steptoe]

531.2.9 see NOTE

 

[sparkytim]

Thanks for that

So this is to ensure discrimination between RCDs in series, only acheived by use of an S type RCD (Time delay) upstream.

I suppose the reason for DOWNSTREAM Rcds to be double pole would prevent N-E faults bypassing them and tripping main RCD instead.

 

[steptoe]

thats about it,

the way I understand it you can have a daisy chain,

the NE fault could take out the other RCBOs too before tripping the front end RCD,

---------- Post Auto-Merged at 14:37 ---------- Previous post was made at 14:36 ----------

but, as has been said, a lot of differing opinions on this, even from different scam assessors.

 

[Voltimax]

Trouble with good old BS 7671 is that you can find regs to contradict other regs. That's the trouble when you have an old document that keeps getting bolt ons and bits taken out only to be added in again in later issues. The book should have been completely rewritten

 

[steptoe]

thats a very good point volti,

and how many of us on here interpet them differently?

maybe thats why its non statutory, too many variations on how it can be read.

maybe Im over cautious on this issue, but I suppose its down to the individual.

 

[Sidewinder]

We have debated this at length!

I believe that I can use the regs to prove that DP fault protection is not required according to the regs.

However, IF DP RCBO's were available in single module then to me it would be a no brainer.

They will be before the year is out I believe after having had several discussions at ELEX with manufacturers.

Steps believes that he can use the regs to prove that DP protection is required.

I disagree, but DP protection is definitely preferable.

Remember the regs are the MINIMUM requirements.

 

[Zeespark]

Type C and only upto and including 32A Canoeboy, but it does prove that it can be done (Design/Manufacture that is)

AndyGuinness

 

[steptoe]

Techna Double Pole Rcbo Type C 32a 30ma (28-5229)

 

[Sidewinder]

Steps,

The pic shows SPSN!