Why is split RCD not appropriate for TT?

[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.

 

[westward10]

Prior to the non combustible consumer unit the use of a metal cased consumer unit without prior rcd protection on a TT earthing system was a big no no in fact the NICEIC used to make a big deal of it, it would be Catergory 1 on a periodic. Things change.

 

[buzzlightyear]

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

Where split consumer units are used with a separate double-pole main switch and 30 mA RCCBs to protect the final circuits, the interconnecting cables which connect the load side of the main switch to the supply side of the RCCBs must be provided with double or reinforced insulation to comply with Regulation 531.3.5.3.2.201.
If this isn’t the case, this particular arrangement wouldn’t be suitable for TT installations because the interconnecting cables wouldn’t be provided with adequate fault protection. If the metal enclosure became live due to a fault, it would cause considerable inconvenience as RCCBs don’t have integral overcurrent protection.

A recommended consumer unit arrangement with RCBOs which is suitable for use on TT installations .

 

[mainline]

Where split consumer units are used with a separate double-pole main switch and 30 mA RCCBs to protect the final circuits, the interconnecting cables which connect the load side of the main switch to the supply side of the RCCBs must be provided with double or reinforced insulation to comply with Regulation 531.3.5.3.2.201.
If this isn’t the case, this particular arrangement wouldn’t be suitable for TT installations because the interconnecting cables wouldn’t be provided with adequate fault protection. If the metal enclosure became live due to a fault, it would cause considerable inconvenience as RCCBs don’t have integral overcurrent protection.

A recommended consumer unit arrangement with RCBOs which is suitable for use on TT installations .

Maybe a good time for you to read the complete thread, so as to avoid saying the same things.

 

[buzzlightyear]

Maybe a good time for you to read the complete thread, so as to avoid saying the same things

I have read the post, but some peaple
Would like it clarifyed what should be done.

 

[Dartlec]

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.

The added note shows how often they have to fudge things to clear up inconsistencies, like meter tails not technically being double insulated OR having reinforced insulation.

I wonder whether, if they started again from scratch, they could come up with a book about 1/4 of the length of the current ones, just by being able to not have to clear up all the loopholes that develop over time.

I imagine that is likely impossible now though, with all the cross links to other standards and definitions.

 

[nicebutdim]

It makes it compliant with regard to isolation

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

DP main switch meets requirement for swithcing of all live conductors. Perhaps I'm tired and unable to see what's in front of me, but how does this meet the requirement that every circuit be provided with "its own" isolating device?

 

[Dartlec]

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

DP main switch meets requirement for swithcing of all live conductors. Perhaps I'm tired and unable to see what's in front of me, but how does this meet the requirement that every circuit be provided with "its own" isolating device?

Every circuit that may have to be isolated without interrupting the supply to other circuits

I guess there is an argument as to whether any circuit in a domestic dwelling has to be isolated without interrupting the supply to other circuits.

In the absence of key life support systems etc, is there any circuit that really can't be isolated if necessary by turning everything off? Inconvenient obviously, but is that enough to require anything more in terms of that reg?

Fixed appliances can be isolated with the appropriate switch if needed, (if they have been installed of course), so it's the circuit that's being discussed in that regulation.

Of course RCBOs are the answer in almost all cases - especially now that many of them are changing to 1P & switched neutral. Cost is also becoming less and less of a reason to choose them. But the Regulations specify a minimum standard that is considered acceptable, so it's still useful to discuss where that standard is - something that isn't always as clear as it should be...

 

[mainline]

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

DP main switch meets requirement for swithcing of all live conductors. Perhaps I'm tired and unable to see what's in front of me, but how does this meet the requirement that every circuit be provided with "its own" isolating device?

That statement I think is the OSG interpretation

Isolation: 537.2​

Every circuit / installation must be provided with a method of isolation from the supply. This isolation device should also be provided with a method to prevent unintentional or inadvertent reinstating (usually lockable).

In a TN-S or TN-C-S system it is not necessary to isolate / switch the neutral conductor providing that it is ‘reliably’ connected to Earth on the distributor's incoming supply side.

It’s worth remembering that semiconductor devices can not be used as isolating devices


“Provision may be made for isolation of a group of circuits by a common means”

 

[swaRRR]

That statement I think is the OSG interpretation

Isolation: 537.2​

Every circuit / installation must be provided with a method of isolation from the supply. This isolation device should also be provided with a method to prevent unintentional or inadvertent reinstating (usually lockable).

In a TN-S or TN-C-S system it is not necessary to isolate / switch the neutral conductor providing that it is ‘reliably’ connected to Earth on the distributor's incoming supply side.

It’s worth remembering that semiconductor devices can not be used as isolating devices


“Provision may be made for isolation of a group of circuits by a common means”

Where are you getting this statement from? I've searched the regs and that wording isn't in there.

 

[mainline]

Where are you getting this statement from? I've searched the regs and that wording isn't in there.

Isolation and Switching Chapter 53 BS 7671

Key Points of… Isolation: 537.2 Every circuit / installation must be provided with a method of isolation from the supply. This isolation device should also be provided with a method to preven…

the-regs.co.uk