Why is split RCD not appropriate for TT?

[swaRRR]

OSG Pg.40 says:

'[Split CU with 2x RCCBs] is not suitable for an installation forming part of a TT system as there is insufficient fault protection of the single-insulated conductors that connect the load side of the double-pole main switch to the supply-side of the RCCB's.'

Why is there insufficient fault protection here?

I've seen loads of TT systems with RCD protection in them. What would you use instead? I know you can use RCBO's but they're not standard.

 

[cireland]

OSG Pg.40 says:

'[Split CU with 2x RCCBs] is not suitable for an installation forming part of a TT system as there is insufficient fault protection of the single-insulated conductors that connect the load side of the double-pole main switch to the supply-side of the RCCB's.'

Why is there insufficient fault protection here?

I've seen loads of TT systems with RCD protection in them. What would you use instead? I know you can use RCBO's but they're not standard

I think it's because of the split cu. The cable connecting from the bottom of the main switch to the top of the RCCB's aren't RCD protected.
If you have an RCD main switch or Isolator the it's ok.
That's my interpretation of it anyway, I may well be wrong.

 

[mainline]

OSG Pg.40 says:

'[Split CU with 2x RCCBs] is not suitable for an installation forming part of a TT system as there is insufficient fault protection of the single-insulated conductors that connect the load side of the double-pole main switch to the supply-side of the RCCB's.'

Why is there insufficient fault protection here?

I've seen loads of TT systems with RCD protection in them. What would you use instead? I know you can use RCBO's but they're not standard.

Typically, with a split-board consumer unit having one or more 30 mA RCDs for the final circuits, the front-end RCD would be 100 mA, type S (time-delayed) to permit selectivity between RCDs.


However, the cables from the origin of the supply to the input terminals of the RCD were a concern. This was because RCDs don’t protect the cables which supply their input terminals. This means that a line to earth fault on the supply side of the front-end RCD could go undetected.

Where a metal consumer unit is installed, there’s always the risk, however small, that the metal enclosure – and other exposed-conductive-parts – will make contact with a live conductor, causing the metalwork to rise to a potential approaching that of the line voltage of the supply.


The relatively high external earth loop impedance in TT systems might mean that the service cut-out fuse would not operate within the maximum permitted time of five seconds, permitting a dangerously high potential to remain on all the earthed metalwork of the installation.

Consequently, you needed to be sure that any exposed metalwork couldn’t be made live by a fault ‘before’ the RCD, caused either by an insulation fault or by one of the conductors becoming loose.

 

[loz2754]

I know you can use RCBO's but they're not standard.

They are standard for me, and plenty of other sparks. I will not install a split load board anymore, as the RCBO boards are so much cheaper to buy now, and are infinitely superior.

 

[swaRRR]

Typically, with a split-board consumer unit having one or more 30 mA RCDs for the final circuits, the front-end RCD would be 100 mA, type S (time-delayed) to permit selectivity between RCDs.


However, the cables from the origin of the supply to the input terminals of the RCD were a concern. This was because RCDs don’t protect the cables which supply their input terminals. This means that a line to earth fault on the supply side of the front-end RCD could go undetected.

Where a metal consumer unit is installed, there’s always the risk, however small, that the metal enclosure – and other exposed-conductive-parts – will make contact with a live conductor, causing the metalwork to rise to a potential approaching that of the line voltage of the supply.


The relatively high external earth loop impedance in TT systems might mean that the service cut-out fuse would not operate within the maximum permitted time of five seconds, permitting a dangerously high potential to remain on all the earthed metalwork of the installation.

Consequently, you needed to be sure that any exposed metalwork couldn’t be made live by a fault ‘before’ the RCD, caused either by an insulation fault or by one of the conductors becoming loose.

Ok but how is this different with RCBO's?

And what's the difference between a normal 100a main switch in a CU and an S Type RCD?

It's amazing how many of these i've worked on without knowing anything about them.

 

[mainline]

They are standard for me, and plenty of other sparks. I will not install a split load board anymore, as the RCBO boards are so much cheaper to buy now, and are infinitely superior.

The only sparks to be fitting them now are the John Wayne ones.

The likes of screw fix and tool station still punting them out cheap as chips type AC rcds no spd they should be ashamed.

I like the Fusebox range.

 

[cireland]

Ok but how is this different with RCBO's?

With RCBO's the supply is via the busbar. It's in a fixed position so can't come into contact with the casing.

 

[mainline]

Ok but how is this different with RCBO's?

As each RCBO is supplied by a busbar, the risk of the busbar becoming loose and making contact with the metal enclosure is minimal.

 

[swaRRR]

With RCBO's the supply is via the busbar. It's in a fixed position so can't come into contact with the casing.

I dunno about that, some of the installs i've seen...

 

[swaRRR]

The only sparks to be fitting them now are the John Wayne ones.

The likes of screw fix and tool station still punting them out cheap as chips type AC rcds no spd they should be ashamed.

I like the Fusebox range.

I worked on a 1700-home council refit earlier this year where every single board was split RCD.

I also worked on several premium new builds (you know the ones where they use nice insulation instead of the stuff that itches and it comes with triple-glazing?) that used them too.

Scandalous really.

 

[cireland]

I dunno about that, some of the installs i've seen...

Alas, me too.

 

[swaRRR]

Alas, me too.

I worked on one where the busbar had been bent outwards towards the lid, because they couldn't be arsed chopping it down and the main incomers were in the way, the way they'd been brought in. Luckily the CU was still plastic.

 

[westward10]

Does the OSG cite a Regulation.

 

[Baddegg]

They are standard for me, and plenty of other sparks. I will not install a split load board anymore, as the RCBO boards are so much cheaper to buy now, and are infinitely superior.

Agree havnt fitted a split load for at least 4 years I reckon

 

[swaRRR]

Does the OSG cite a Regulation.

No, but this seems to be a regular occurrence where the OSG says something and it turns out it's not actually a reg. Is the OSG fit for purpose?

 

[westward10]

Regulations now require residential to be rcbos.

 

[westward10]

No, but this seems to be a regular occurrence where the OSG says something and it turns out it's not actually a reg. Is the OSG fit for purpose?

Not looked at an OSG for over 20 years your book of choice is BS7671.

 

[cireland]

Does the OSG cite a Regulation.

Wouldn't it be covered by the same regulations stating TT systems have to be RCD protected. This wiring is still on the consumer side of the main switch albeit within the consumer unit itself.

 

[swaRRR]

Not looked at an OSG for over 20 years your book of choice is BS7671.

I know that but OSG is supposed to be a bite-sized, easy reference for whats in the big brown book. Seems like the OSG talks a lot of gibberish.

Here's another one.

OSG page 61.

'Other than at the origin of the installation, every circuit...that may have to be isolated without interrupting the supply to other circuits should be provided with its own isolating device. The device must switch all live conductors in a TT system and all line conductors in a TN system.'

This basically says all breakers have to be double pole in a TT, no?

Yet you go to 132.15.201 which the OSG references and it says absolutely nothing about this.

 

[mainline]

No, but this seems to be a regular occurrence where the OSG says something and it turns out it's not actually a reg. Is the OSG fit for purpose?

Think it stems from
Regulation 531.3.5.3.2.201

 

[JBW175]

No spark worth his salt is fitting split load boards these days IMO.

RCBO & SP is all I quote for.

 

[mainline]

I know that but OSG is supposed to be a bite-sized, easy reference for whats in the big brown book. Seems like the OSG talks a lot of gibberish.

Here's another one.

OSG page 61.

'Other than at the origin of the installation, every circuit...that may have to be isolated without interrupting the supply to other circuits should be provided with its own isolating device. The device must switch all live conductors in a TT system and all line conductors in a TN system.'

This basically says all breakers have to be double pole in a TT, no?

Yet you go to 132.15.201 which the OSG references and it says absolutely nothing about this.

The DP main switch makes it compliant.

 

[swaRRR]

Regulations now require residential to be rcbos.

Which reg is that?

 

[swaRRR]

The DP main switch makes it compliant.

That quote though says that all individual circuits must have their own DP isolating device. Yet the regs don't mention it.

 

[cireland]

The DP main switch makes it compliant.

It does mention 'without interrupting the supply to other circuits'.

 

[swaRRR]

Think it stems from
Regulation 531.3.5.3.2.201

But that reg says supply side of the incoming device eg main switch. **** me these books are confusing. Someone should rewrite them in normal language, they'd make millions.

 

[cireland]

But that reg says supply side of the incoming device eg main switch. **** me these books are confusing. Someone should rewrite them in normal language, they'd make millions.

It does also mention the internal wiring I think. Says something about them being double insulated or
reinforced.

 

[swaRRR]

It does also mention the internal wiring I think. Says something about them being double insulated or
reinforced.

Only as it pertains to a pre-assembled board's link cables as far as i can see.

 

[westward10]

Which reg is that?

531.3.2 (ii)
It does say 'considered'.

 

[cireland]

Only as it pertains to a pre-assembled board's link cables as far as i can see.

'Regulation 531.3.5.3.2.201 requires that where Class I enclosures are installed in installations forming part of a TT system and RCD protection is used on outgoing circuits, all live conductors on the supply side of the incoming device must have double or reinforced insulation'.

Taken from NICEIC website.

 

[mainline]

That quote though says that all individual circuits must have their own DP isolating device. Yet the regs don't mention it.

It does mention 'without interrupting the supply to other circuits'.

I'm sure it says somewhere “that provision may be made for isolation of a group of circuits by a common means”

 

[swaRRR]

531.3.2 (ii)
It does say 'considered'.

The same reg also mentions split RCD though so i don't think it means RCBO's are required in residential premises.

'Regulation 531.3.5.3.2.201 requires that where Class I enclosures are installed in installations forming part of a TT system and RCD protection is used on outgoing circuits, all live conductors on the supply side of the incoming device must have double or reinforced insulation'.

Taken from NICEIC website.

For Class I enclosures in TT systems where RCD protection is used on outgoing circuits, double
or reinforced insulation of all live conductors (incoming cables, extension terminals, etc.) on the supply side of the
incoming device, e.g. main switch, shall be used. Insulated and non-metallic sheathed cables are deemed to meet the
requirements of double or reinforced insulation.

NOTE 1: When selecting equipment, consideration should be given to the assembly manufacturer's internal line interconnecting
cable links on the supply side of an RCD being insulated and non-metallic sheathed, or having reinforced insulation
or equivalent mechanical protection.

From the book.

Supply side of eg main switch is massively ambiguous imo.

 

[swaRRR]

I'm sure it says somewhere “that provision may be made for isolation of a group of circuits by a common means”

It doesn't, not in that quote from the OSG anyway.

 

[mainline]

It doesn't, not in that quote from the OSG anyway.

Welcome to the world of interpretation.

 

[swaRRR]

Welcome to the world of interpretation.

It's all terribly confusing.

 

[cireland]

The same reg also mentions split RCD though so i don't think it means RCBO's are required in residential premises.

For Class I enclosures in TT systems where RCD protection is used on outgoing circuits, double
or reinforced insulation of all live conductors (incoming cables, extension terminals, etc.) on the supply side of the
incoming device, e.g. main switch, shall be used. Insulated and non-metallic sheathed cables are deemed to meet the
requirements of double or reinforced insulation.

NOTE 1: When selecting equipment, consideration should be given to the assembly manufacturer's internal line interconnecting
cable links on the supply side of an RCD being insulated and non-metallic sheathed, or having reinforced insulation
or equivalent mechanical protection.

From the book.

Supply side of eg main switch is massively ambiguous imo.

You were asking about the main tails and argued that :-

Only as it pertains to a pre-assembled board's link cables as far as i can see.

I was merely pointing out that it does mention DI tails or reinforced insulation.

 

[mainline]

That quote though says that all individual circuits must have their own DP isolating device. Yet the regs don't mention it.

If that were the case would it mean that all the 1000s of consumer units that use mcbs are not compliant ?

 

[westward10]

The same reg also mentions split RCD though so i don't think it means RCBO's are required in residential premises.

For Class I enclosures in TT systems where RCD protection is used on outgoing circuits, double
or reinforced insulation of all live conductors (incoming cables, extension terminals, etc.) on the supply side of the
incoming device, e.g. main switch, shall be used. Insulated and non-metallic sheathed cables are deemed to meet the
requirements of double or reinforced insulation.

NOTE 1: When selecting equipment, consideration should be given to the assembly manufacturer's internal line interconnecting
cable links on the supply side of an RCD being insulated and non-metallic sheathed, or having reinforced insulation
or equivalent mechanical protection.

From the book.

Supply side of eg main switch is massively ambiguous imo.

It doesn't say required it suggests them.

 

[swaRRR]

You were asking about the main tails and argued that :-

I was merely pointing out that it does mention DI tails or reinforced insulation.

Nah i'm on about the load side of the main switch to the supply of the RCD's which is what the picture in OSG shows.

 

[swaRRR]

It doesn't say required it suggests them.

You said RCBO's were now required in residential installations.

 

[cireland]

It doesn't say required it suggests them.

It does say 'shall be used'.

 

[swaRRR]

It does say 'shall be used'.

It says residual current devices shall be selected to limit risk of unwanted tripping, and then goes on to list split load RCDs and RCBO's for consideration. It doesn't say RCBO's shall be used.

 

[mainline]

It does say 'shall be used'.

Regulation 531.3.2 (Unwanted Tripping) now points out the use of RCBOs for individual final circuits in residential premises as a method to prevent unwanted tripping of such protective devices.

 

[nicebutdim]

'Other than at the origin of the installation, every circuit...that may have to be isolated without interrupting the supply to other circuits should be provided with its own isolating device. The device must switch all live conductors in a TT system and all line conductors in a TN system.'

This basically says all breakers have to be double pole in a TT, no?

Yet you go to 132.15.201 which the OSG references and it says absolutely nothing about this.

The DP main switch makes it compliant.

I don't believe these two statements can be reconciled.

 

[mainline]

I don't believe these two statements can be reconciled.

It makes it compliant with regard to isolation

 

[westward10]

You said RCBO's were now required in residential installations.

Yes I did later I said 'suggested'.

 

[swaRRR]

Yes I did later I said 'suggested'.

 

[mainline]

It does say 'shall be used'.

And it should really say “Required”

 

[buzzlightyear]

NICEIC says you don't need a up front rcd
On TT main switch the rest of the board rcbo.

 

[mainline]

NICEIC says you don't need a up front rcd
On TT main switch the rest of the board rcbo.

Thats what the thread was about why it's ok with rcbos and not ok using split rcds.