17th Edition - RCD requirements and concealled cables

[Spark123]

If you didn't want RCD protection on a lighting circuit then you'll need to employ one of the other measures such as ensuring the cable is sufficiently mechanically protected from the penetration of screws or has an earthed metallic sheath suitable for use as a CPC etc.
You'd still need to consider a lamp popping causing the MCB to trip, or a good old fashioned blackout.

 

[RF Lighting]

This is a perfect example of not thinking outside of the briaf. It could be argued that whilst the RCD will protect against electrical problems, it is far more likely that a nuisance trip could put a vulnerable person shut in the bathroom with no light.

Proper circuit design will prevent this from occouring. BS7671 16th edition did not allow a single RCD protecting the entireity of an installation for this very reason.

Unfortunately modern safety treats every one as an idiot and causes other problems on the way.

I can't see any problems that the additional use of RCDs will introduce.

I presume that this is another thing invented by the manufacturers to sell more goods,

No, it is introudced by the IET to make electrical installations safer. They do not manufacture electrical equipment.

what next, emergency lighting in all rooms?

It makes perfect sense to install it in commercial premisis, so why not domestic premisis? I have emergency lighting in my house.

 

[ban-all-sheds]

And I'm going to fit some soon - just landing/stairs/hall.

But back to RCDs - I'm not convinced a proper cost-benefit analysis was done. As I said, I wonder how many people have been injured or worse from drilling into cables in the safe zones, and I wonder how much it will cost and how long it will take for the new RCD rules to save lives/injuries, and of the ones that drill into cables, how many will be saved.

 

[ebee]

In my experience, people often drill/nail above switches/sockets etc and never give a thought to where a cable might be.
In fact some folk are unaware that cables exist at all, they just think that you stick a socket on a wall and electric magically comes out, they can`t get the idea that walls require chasing and floorboards lifting etc.
Exactly the same for pipes for water/gas too.
I had to stop a customer sawing thru the incomming lead sheathed supply to her house because she didn`t like the look of the untidy thing.
Yer can`t make it up!

 

[Spark123]

Might be of interest:
http://download.hager.com/hagergrou...rmation/ConsumerUnitGuideToThe17thEdition.pdf

 

[securespark]

Proper circuit design will prevent this from occouring. BS7671 16th edition did not allow a single RCD protecting the entireity of an installation for this very reason.

However, a split-load board is acceptable.

In the 17th, a solution to this problem being mooted is a "Nottingham" board.

This is a board with an incomer feeding two banks of circuits, each fed via a 30mA RCD.

This is deemed acceptable, like the "old" split board, but will still cause a major blackout if one RCD trips.

For this reason, I believe each RCD should carry mixed circuits, ie on one rcd up lights, down sockets and the other down lights, up sockets etc.. In this way, not all circuits on one floor will go down.

 

[Pensdown]

However, a split-load board is acceptable.

In the 17th, a solution to this problem being mooted is a "Nottingham" board.

This is a board with an incomer feeding two banks of circuits, each fed via a 30mA RCD.

This is deemed acceptable, like the "old" split board, but will still cause a major blackout if one RCD trips.

For this reason, I believe each RCD should carry mixed circuits, ie on one rcd up lights, down sockets and the other down lights, up sockets etc.. In this way, not all circuits on one floor will go down.

So what happens if a neutral to earth fault develops on one of the power circuits? The lighting circuit that is also fed via the RCD will be off until the fault on the power circuit is cleared – IMO that’s bad circuit design.

Personally, I think the common domestic practice of using one RCD to protect more than one circuit has always been bad circuit design and IMO it doesn’t meet the requirements of 314.xx. For example, how can a dead circuit connected to the RCD side of a split load board be modified without running the risk of tripping the RCD neutral to earth?

To meet the new requirements, I can’t see what’s wrong with using a short length of earthed galvanised conduit for lighting switch drops (and maybe the cooker) and RCBO’s for the power & bathroom circuits? It will keep the lighting circuits RCD free and will probably save a few quid.

 

[ban-all-sheds]

For example, how can a dead circuit connected to the RCD side of a split load board be modified without running the risk of tripping the RCD neutral to earth?

DP MCBs?

To meet the new requirements, I can’t see what’s wrong with using a short length of earthed galvanised conduit for lighting switch drops (and maybe the cooker) and RCBO’s for the power & bathroom circuits? It will keep the lighting circuits RCD free and will probably save a few quid.

How would you earth the conduit? Bush it to an earthed galv flush box?

Does this OTT new rule also apply to cables in ceilings?

 

[Spark123]

For example, how can a dead circuit connected to the RCD side of a split load board be modified without running the risk of tripping the RCD neutral to earth?

DP MCBs?

Hager do a switched neutral version too

To meet the new requirements, I can’t see what’s wrong with using a short length of earthed galvanised conduit for lighting switch drops (and maybe the cooker) and RCBO’s for the power & bathroom circuits? It will keep the lighting circuits RCD free and will probably save a few quid.

How would you earth the conduit? Bush it to an earthed galv flush box?

Seems a reasonable option, I suppose you can use a conduit box above to connect the earth to. One disadvantage of using conduit is the need for the correct tools to work with it i.e. pipe clamp, stock + die etc. A lot of DIYers are not going to have access to them.
Other options are to use a cable with an earthed sheath suitable for use as a CPC, though there is still the problem of needing the required tools and knowledge to install the cables correctly.
IMO the easiest option from a DIY pov is to install normal cable in the safe zones which requires RCD protection.

Does this OTT new rule also apply to cables in ceilings?

I don't believe there is a requirement to RCD protect the cables in a similar context in a ceiling.

 

[Pensdown]

That's also my interpretation ….cables concealed in a wall or partition at a depth of less that 50mm.......etc

I agree for a DIY’er conduit will be a problem but I was thinking more about the trade. As long as the back box is earthed I can’t see a problem with bushing straight into it - and assuming a female bush is used on the top to prevent damage I can’t see a need for any conduit boxes.

 

[333rocky333]

Is there no , alternative types of cables that can get round this, or is cost an issue.

I know you CANNOT get it but would like a modern day version of the old lead T+E get round this.

Or like T+E with a sort of co-axial type brading

Aside that swa or pyro
Nice

 

[Spark123]

SWA is going to be a nuisance to terminate and the gland should really be accessable. Pyro is a good bet with the pots with earth tails but that has the same problem for DIY in that it is not straight forwards to terminate.

 

[ban-all-sheds]

I suppose you can use a conduit box above to connect the earth to. One disadvantage of using conduit is the need for the correct tools to work with it i.e. pipe clamp, stock + die etc. A lot of DIYers are not going to have access to them.

Same problem surely with going into conduit boxes?

And as Pensdown said, you need bushes for protection too, although there are those nylon inserts that Kopex do..

But I've got no sympathy for DIYers who won't equip themselves with the proper tools, particularly when the stock & die(s) you need cost less than 1 RCBO saved....

What are you going to do in stud walls where the flush boxes aren't metal?....

Other options are to use a cable with an earthed sheath suitable for use as a CPC, though there is still the problem of needing the required tools and knowledge to install the cables correctly.

Split-con?

 

[Spark123]

Split con doesn't appear to be allowed either - not sure on how effective it will be for a switch drop.

 

[RF Lighting]

Plus it will be pretty difficult to terminate 4.0mm² split con into a standard 10AX switchplate, especially if 2 way switching is utilised, but as Spark123 says, it appears that split con in no longer acceptable for use in this situation.

I wonder if flexible steel conduit would be acceptable? No specialist tools required to install that, although having said that, it is not acceptable to rely on flexible steel conduit to act as a CPC.