Should internal links in consumer units be double insulated where TT earthing is present

[spud1]

Hi,

So I was reading the notes on regulation 531.3.5.3.2.201 (Page 156) today and noticed Note 1 that consideration should be given (Where TT earthing is present) to the manufacturer's internal line cable links on the supply side of an RCD being insulated AND sheathed! Does anyone know of any distribution equipment manufacturer that either fits double insulated cable links as standard or offers an alternative double insulated kit of leads?

Cheers

 

[radiohead]

Best to use an S type 100ma RCD main switch with a tails gland making sure it enters as close as possible to the incomer. Internal links are then covered. If you are using RCBO's they must be DP or switched N if you have an RCD main switch.

 

[telectrix]

has anybody ever seen internal links damaged and shorting out to a metal case CU? I certainly haven't.

 

[MJPD29]

has anybody ever seen internal links damaged and shorting out to a metal case CU? I certainly haven't.

Neither have I. Only damage I have seen is on factory fitted neutrals not being tightened correctly after fitting,

 

[Wilko]

No I’ve not seen any insulated and sheathed internal cabling. Nor have I seen internal links flailing about, but it is a possibility I guess.

 

[Ian1981]

Taken from Hager’s CU instructions

TNC-S, TN-S when installed in an electrical installation complying with BS 7671
Hager recommends for TT systems a 100A type S time delayed RCCB or a main switch with RCBO protection only on all outgoing circuits.

 

[radiohead]

Taken from Hager’s CU instructions

TNC-S, TN-S when installed in an electrical installation complying with BS 7671
Hager recommends for TT systems a 100A type S time delayed RCCB or a main switch with RCBO protection only on all outgoing circuits.

They are wrong if they are recommending an RCD main switch with SP RCBO's. A fault on a final circuit will not disconnect the neutral and probably take out the main switch as well

 

[Ian1981]

They are wrong if they are recommending an RCD main switch with SP RCBO's. A fault on a final circuit will not disconnect the neutral and probably take out the main switch as well

No it’s a main switch with rcbo’s or dual rccb’s with an S type upfront is what they mean, ( I think).
Not very clear but that would make more sense.

 

[Pete999]

Hi,

So I was reading the notes on regulation 531.3.5.3.2.201 (Page 156) today and noticed Note 1 that consideration should be given (Where TT earthing is present) to the manufacturer's internal line cable links on the supply side of an RCD being insulated AND sheathed! Does anyone know of any distribution equipment manufacturer that either fits double insulated cable links as standard or offers an alternative double insulated kit of leads?

Cheers

They aren't double insulated they are insulated and Mech protected, never seen a cable with the double insulation marked on it.

 

[westward10]

They are wrong if they are recommending an RCD main switch with SP RCBO's. A fault on a final circuit will not disconnect the neutral and probably take out the main switch as well

There is no requirement for a protective device to isolate the neutral under fault conditions, regardless of earthing system type.

 

[pc1966]

There is no requirement for a protective device to isolate the neutral under fault conditions, regardless of earthing system type.

I think radiohead's point is if a final circuit has a N-E fault then you will probably trip the incomer RCD as the SP RCBO will trip but not isolate the neutral fault.

So it is not necessary according to the regs as such, but it is far better to have a N-switching RCBO if you can. I guess it indirectly comes under the requirement for selectivity, etc (off hand don't remember the numbers).

Not that many do unfortunately, but at my last count Wylex/Crabreee/Fusebox (the miniature version) RCBO for consumer DB do switch neutral.

 

[radiohead]

There is no requirement for a protective device to isolate the neutral under fault conditions, regardless of earthing system type.

You miss the point. The requirement is that a single fault on a final circuit does not disconnect the entire installation. If there is an RCD main switch failure to disconnect the N on a faulty final circuit will probably disconnect the entire installation. Because of an indirect consequence of a single fault it is in effect a requirement that where an RCD main switch is used with RCBO's then each RCBO must disconnect both poles.
edit...post crossed with PC1966

 

[pc1966]

I think radiohead's point is if a final circuit has a N-E fault then you will probably trip the incomer RCD as the SP RCBO will trip but not isolate the neutral fault.

Still, at least you know which circuit has the fault.

 

[pc1966]

Back to the OP's point - I really think that any properly installed metal CU is going to be fine.

I.e. where the tails are double-sheathed as expected, they enter by a proper gland for support and protection from damage, and the CU has been installed with care to make sure the busbar is in place and clamped correctly, etc.

Obviously all circuits have to be RCD protected as TT rarely will have a Zs able to disconnect a MCB, and at that point I would state my personal preference for both an up-front 100mA/300mA delay RCD and then whatever 30mA instant RCD/RCBO are needed for "additional protection".

In my case it is less about the risk of an internal CU short, than about the risk of a 30mA RCD failing to trip and so the earth/CPC remains live for a prolonged period. With an incomer RCD there is no single point of failure.

 

[Midwest]

You miss the point. The requirement is that a single fault on a final circuit does not disconnect the entire installation. If there is an RCD main switch failure to disconnect the N on a faulty final circuit will probably disconnect the entire installation. Because of an indirect consequence of a single fault it is in effect a requirement that where an RCD main switch is used with RCBO's then each RCBO must disconnect both poles.
edit...post crossed with PC1966

When were Wylex/Crabtree SP&SN RCBO's first introduce? I have a feeling that many TT installs have been none compliant since July 2015, if that's the case.

 

[Ian1981]

When were Wylex/Crabtree SP&SN RCBO's first introduce? I have a feeling that many TT installs have been none compliant since July 2015, if that's the case.

In what way ?
I’ve never seen an rcbo board with an RCCB as a main switch

 

[davesparks]

There is no requirement for a protective device to isolate the neutral under fault conditions, regardless of earthing system type.

Yes, another oversight in the regulations in my opinion.

 

[Midwest]

In what way ?
I’ve never seen an rcbo board with an RCCB as a main switch

I have sort of. But I've also seen a CU with RCBO's and a main switch, suitable for use with TT supply, as detailed in OSG (yellow) figure 3.6.3. My point is, there were very few if any DP RCBO's or SN back then.