RCBO's on a TT system.

[HappyHippyDad]

Morning all..

I am off sick from work, this rarely happens and already I am bored, so I thought I'd start a thread on the above which i have been thinking about.

Question 1: I realise that most on here suggest DP or SPSN RCBO's for a TT, but I am unsure why (with regards the regs)? This is why I am unsure:

537.2.2.1 states " A device for isolation shall isolate ALL live conductors 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 as neutral is effectively connected to earth)

So the above reg is met by having a DP main switch as this reg is in relation to isolation, not disconnection due to fault.

531.2.1 "An RCD shall be capable of disconnecting all the line conductors of the circuit"

A Single pole RCBO accomplishes this reg.

This thread is solely to see if the standard single pole RCBO meets the regulations.

Question 2 : Also, I cant fathom why a SPSN would be acceptable whereas a SPSL RCBO wouldn't, you must be assuming disconnection from line by ADS (and therefore DP disconnection) as you wouldn't just switch the neutral? The benefits and sense of a DP RCBO are clear.

Cheers all :smile5:

 

[Leesparkykent]

It's because if you have an upfront 100ma s type RCD and single pole solid neutral RCBO's and there is a neutral/earth fault it will take out the upfront RCD as the neutral/earth fault won't be cleared when the RCBO has tripped. That's why you should use single pole switched neutral or double pole RCBO's.

 

[R G]

Both SPSN and DP RCBOs are double pole the difference is SPSN RCBO is that it detects overcurrent in the Line only, whereas the Double Pole RCBO detects overcurrent in both line and neutral ...

 

[telectrix]

Both SPSN and DP RCBOs are double pole the difference is SPSN RCBO is that it detects overload in the Line only, whereas the Double Pole RCBO detects overload in both line and neutral ...

are you sure?

 

[R G]

Sorry my mistake Meant overcurrent

Well done Tel for being so observant

 

[telectrix]

surely you meant leakage/imbalance?

 

[HappyHippyDad]

It's because if you have an upfront 100ma s type RCD and single pole solid neutral RCBO's and there is a neutral/earth fault it will take out the upfront RCD as the neutral/earth fault won't be cleared when the RCBO has tripped. That's why you should use single pole switched neutral or double pole RCBO's.

Well lets say we dont have an upfront RCD, would the single pole solid neutral RCBO meet the regulations for disconnection on a TT given the information in the OP.

Both SPSN and DP RCBOs are double pole the difference is SPSN RCBO is that it detects overcurrent in the Line only, whereas the Double Pole RCBO detects overcurrent in both line and neutral ...

You've confused me a bit there RG. Are you saying a SPSN (single pole switched neutral) is double pole?

 

[spark 68]

Well lets say we dont have an upfront RCD, would the single pole solid neutral RCBO meet the regulations for disconnection on a TT given the information in the OP.



You've confused me a bit there RG. Are you saying a SPSN (single pole switched neutral) is double pole?

A SPSN is effectively DP as far as isolation is concerned, but is not the same as a DP RCBO/MCB, as has been pointed out earlier a SPSN has only one overcurrent sensing element in the line side, the N pole is just a linked switch. A true DP device has overcurrent sensing elements in both poles and are linked so either side with a fault will disconnect both.

A SPSN is fine for single phase circuits, a DP is usually used for say a 400V circuit where you have two lines and no N, or a 110V circuit where again you have two lines.

 

[HappyHippyDad]

Thanks S68..

and thanks RG (sorry for not properly understanding how a SPSN RCBO worked).

I'd still be interested in if you/anyone had thoughts on whether a standard RCBO (not SPSN) meets the regulations for use as fault protection for TT given the regs stated in the OP?

 

[Leesparkykent]

happyhippydad;1085193]Well lets say we dont have an upfront RCD, would the single pole solid neutral RCBO meet the regulations for disconnection on a TT given the information in the OP.

There is no requirement for a 100ma S type RCD up front . But if you are relying on the RCBO's as earth fault protection it is prudent to install an up front time delayed RCD because if the RCBO fails you have a back up with the upfront RCD. Think about how many customer actually test their RCD quarterly........ Not many at all! If you do install an upfront s type RCD then you will have to install double pole or single pole switched neutral RCBO's as explained in my other post (IMO it would be silly not to install an upfront time delayed RCD).

 

[Knobhead]

Beat you to it yonks ago HHD http://www.electriciansforums.co.uk...ries-electrical-regulations/64662-rcbo-s.html

 

[spark 68]

Further information to post #8

If we forget about the RCD part of the RCBO for the moment, which senses E faults or imbalances no matter which device is used as this is inherent to the way RCDs work, only the SPSN and DP types will actually disconnect the N, or second line in some cases, in the solid N types although the RCD part will sense a N-E fault and trip the device it cannot disconnect the N thus leaving the N-E fault present, and this will also trip any upstream RCDs.

Looking at the overcurrent parts of the device, in SP applications a SPSN device will suffice as in a L-N fault both the Line and Neutral will both see the same fault current and the device will trip via the overcurrent part in the line side, also a Line to E fault of sufficient magnitude would trip the overcurrent part even if the RCD part failed to operate, these devices have one pole marked as a N, as this is just a linked switch element in that pole.


With true DP types, if we again just consider the overcurrent parts of the device, consider a 400V circuit protected by DP device fed with say L1 and L2, now with a L1-L2 fault, both overcurrent elements in each pole will both see the same fault current and again will trip and disconnect both lines.
On this type of circuit though we have other possibilities for fault current paths, for instance an L1-L3 fault, an L2-L3 fault, an L1 or L2 to N fault, and an L1 or L2 to E fault, clearly the need for overcurrent protection in both poles is required to protect against these "extra" possible fault paths. On DP devices neither pole is marked N.

While the RCD part in a DP RCBO should in theory mitigate some of the conditions above where DP types are needed, the regs say we should not rely on RCD protection alone, plus the fact RCDs are not exactly failsafe or that reliable for that matter.

You could replace a SPSN type with a DP type, but the converse is not true, where DP devices are specified they should only be replaced by DP types.

 

[HappyHippyDad]

Thanks all,

I now feel more ill but at least I understand correctly the different types of RCBO available!

Still cant figure out if anyone has actually stated if an RCBO (solid neutral) meets the regs for a TT though!

I think, judging from everything I've read that it does. Better to use SPSN or DP but it does meet the regs.... I think!

ps.. I can almost hear you taking a big deep breath S68 and thinking, "ok, how can I write this any clearer?" :smile5:

 

[spark 68]

A look at some manufacturers data sheets might help, I am sure I read somewhere that SP RCBOs were not suitable for TT use, it was from a major manufacturer if I recall.

One possible single phase use for true DP types would be for inside mobile caravans, where supplies from the various sites polarity wise may be a bit "suspect"

They (DP types) are used a lot more in countries that don't have polarised plugs and S/Os.

 

[telectrix]

Thanks all,

I now feel more ill but at least I understand correctly the different types of RCBO available!

Still cant figure out if anyone has actually stated if an RCBO (solid neutral) meets the regs for a TT though!

I think, judging from everything I've read that it does. Better to use SPSN or DP but it does meet the regs.... I think!

ps.. I can almost hear you taking a big deep breath S68 and thinking, "ok, how can I write this any clearer?" :smile5:

i think it's to do with both live conductors being isolated on a TT system in case of a fault. (N being regarded as a live conductor)