Using an front S type RCD on a TT installation

[HappyHippyDad]

I usually install dual RCD consumer units rather than RCBO boards due to cost, however with the cost of RCBO's coming down I have started fitting consumer units with RCBO's.

On an installation with TT as it's means of earthing I would always replace the main switch of the dual RCD consumer unit with an S type RCD. The reason for this is that I wanted the cables from the main switch to the top of the RCD's to have fault protection. With an RCBO board is there still a need to have an up front time delayed RCD as the only part without fault protection will be the busbar which is not going to come into contact with the metal consumer unit.

The RCBO board would have double pole isolation with a normal main switch.

 

[patthehat]

I think RCBO's are fine, but on a TT system they will need to be double pole. The meter tails will also need to be clamped in C.U or use a Gland to secure them on entry.

 

[westward10]

There is no requirement for them to be double-pole.

 

[stevethesparks]

Why not use the time delayed RCD as the main switch?
Having said that then DP rcbo's would be better if you go that option.

 

[1Justin]

OSG suggests RCBO's on a TT without up-front S type. That's a risk you might be prepared to take, but the original reasons still stand. The other thing to think about is you are very likely to be fitting SPD in the same board, and this means more cables connected in before any RCBO's. (Often of the DIY cable variety if manufacturer doesn't provide full SPD cable kit - for example BG doesn't). - Will they fall out?
I've fitted only RCBO boards on all my TN installs recently, but because I always will fit up-front S type on TT, I can't afford to lose the whole house on a neutral fault, so I tend to still still fit split load boards on TT. We could do with cheaper 2 pole RCBO's eh?

 

[HappyHippyDad]

I think RCBO's are fine, but on a TT system they will need to be double pole. The meter tails will also need to be clamped in C.U or use a Gland to secure them on entry.

It needs to have DP isolation, the RCBO’s don’t need to be DP. I always use a gland.

Why not use the time delayed RCD as the main switch?
Having said that then DP rcbo's would be better if you go that option.

I mentioned in my post that I use the time delayed RCD instead of the main switch.

OSG suggests RCBO's on a TT without up-front S type. That's a risk you might be prepared to take, but the original reasons still stand. The other thing to think about is you are very likely to be fitting SPD in the same board, and this means more cables connected in before any RCBO's. (Often of the DIY cable variety if manufacturer doesn't provide full SPD cable kit - for example BG doesn't). - Will they fall out?
I've fitted only RCBO boards on all my TN installs recently, but because I always will fit up-front S type on TT, I can't afford to lose the whole house on a neutral fault, so I tend to still still fit split load boards on TT. We could do with cheaper 2 pole RCBO's eh?

So with an RCBO board, what would the reason for fitting a time delayed RCD be on a TT?
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My question is really just what is the need for a time delayed RCD (in place of the main switch) on an RCBO board with TT earthing?

 

[happysteve]

My question is really just what is the need for a time delayed RCD (in place of the main switch) on an RCBO board with TT earthing?

I don't think there is a need, unless there is a risk that a line conductor could fall out and come into contact with the metal box before any of the RCBOs (such as the feed to the SPD, which 1Justin mentioned. I suppose you could feed the SPD via an RCBO rather than the output of the main switch, but then you've got an RCBO that's not protected by the SPD...)

 

[HappyHippyDad]

I don't think there is a need, unless there is a risk that a line conductor could fall out and come into contact with the metal box before any of the RCBOs (such as the feed to the SPD, which 1Justin mentioned. I suppose you could feed the SPD via an RCBO rather than the output of the main switch, but then you've got an RCBO that's not protected by the SPD...)

Thanks Steve,
I notice you’ve mentioned SPD a few times as if it’s normal to put one in. Are you fitting them as standard? I read the regs as not needing them for domestic.

 

[happysteve]

On the note of SPDs and TT systems (risk of line conductor falling out of the bottom of the SPD), has anyone seen inside a Crabtree board with an integral SPD? Since the Starbreaker boards have a custom bus bar, I wonder if they've managed to neatly connect the SPD with their internal (insulated) busbar, or whether they've just lashed one on with flying leads?

Thanks Steve,
I notice you’ve mentioned SPD a few times as if it’s normal to put one in. Are you fitting them as standard? I read the regs as not needing them for domestic.

I always give the client the option: all-RCBO board = £X, all-RCBO board with SPD = £(X+75) or £(X+100). The usual wording in the quote is something like:

Option (2): As above, additionally with a surge protective device (SPD) protecting the whole installation (it’s installed in the new distribution board, adjacent to the main switch). Recent changes in the Regs require more consideration to be given to the installation of these devices, but they are not mandatory at this stage. They protect the installation from overvoltages (spikes/glitches on the incoming supply) from things like indirect lightning strikes and switching faults on the incoming supply – these can damage things like sensitive electronic devices (anything with a circuit board inside it). It's a piece of mind thing. However, you might find it more cost-effective to use extension leads fitted with their own SPDs (available from Wilco's and the like), instead. This aspect is up to you, really; if you decline it, I’ll just note this on the EIC, it’s no problem.

Landlords tend to choose to save £75 (but not all landlords). Everyone else (so far) has payed the extra. It makes no difference in terms of installation time, and the difference in price is about that much (for Wylex; Hager is about £100).

 

[HappyHippyDad]

On the note of SPDs and TT systems (risk of line conductor falling out of the bottom of the SPD), has anyone seen inside a Crabtree board with an integral SPD? Since the Starbreaker boards have a custom bus bar, I wonder if they've managed to neatly connect the SPD with their internal (insulated) busbar, or whether they've just lashed one on with flying leads?

I always give the client the option: all-RCBO board = £X, all-RCBO board with SPD = £(X+75) or £(X+100). The usual wording in the quote is something like:



Landlords tend to choose to save £75 (but not all landlords). Everyone else (so far) has payed the extra. It makes no difference in terms of installation time, and the difference in price is about that much (for Wylex; Hager is about £100).

Nicely worded. I have to admit, I haven't been giving the option as yet. One reason being I haven't heard of any one having any damage to any equipment (domestic setting) from a lightning strike or other. Is there a reason why you have started to offer it, I guess just interpreting the regs as suggesting it, or perhaps you think it's a good idea?

 

[Risteard]

I'm totally opposed to having an S-type RCCB as the main switch on a Class I distribution board. To my mind it must really be upfront of that to protect against an earth fault there (and I don't personally regard Wiska Sprint glands as an alternative to this), and I also fervently believe that the RCCB should be in an insulating enclosure.

Some may disagree with me on this but that's my opinion and it won't change.

 

[happysteve]

Nicely worded. I have to admit, I haven't been giving the option as yet. One reason being I haven't heard of any one having any damage to any equipment (domestic setting) from a lightning strike or other. Is there a reason why you have started to offer it, I guess just interpreting the regs as suggesting it, or perhaps you think it's a good idea?

It's the wording in the Regs that encouraged me to offer the option, it's the customer's installation, and their money; I don't feel confident making a value judgement on their behalf. Some may be more risk-averse than me, or less so.

If I was changing the DB in my house (which I'll get round to doing... one day...) I'd probably pay the extra £75-100, despite me being confident that the thing will never be needed.

Plus, y'know, my rear end is covered.

 

[1Justin]

.. Are you fitting them as standard? I read the regs as not needing them for domestic.

I am, more or less. And second question.. "Results in serious injury to".. doesn't specify commercial properties.
I've fitted SPD on 9 out of 10 CU's I've done since regs. The one without I did get through the risk assessment without because I could almost see the sub-station. The risk analysis is worded to be a pain in the arse to do properly, and nearly impossible to verify for a quote. This more or less mandates them AFAICT. To be honest, now the prices are down, it's easier just to fit one rather than faff about trying not to.

 

[rolyberkin]

You can buy a Live SPD for £16, coupled with an SBS double pole rcbo board everyones happy.