Old install 100mA RCD main switch but no 30mA

[Gavin John Hyde]

Have to replace this old board for a elderly relative of my soon to be wife, British Gas have been along and used soem of the special triangle stickers to say it is unsafe - but still fitted the new boielr anyway with a FCU from the socket circuit in kitchen, as always money is the priority for them. Somebody else went through effort of putting fairly recent stickers on the oard but didnt fill them in either.

Got me thinking though, I have as yet not been able to locate the earth rod, Zsdb of 26.3 Ohms with water and gas bonds disconnected so potentially an Ra reading.
. there is a bonding to water, bonding to gas and a 3rd cable that disappears off into the wall not with the tails!
So may need to add a new rod to be sure as its all TT around this rural area.

So looking at the board it has 100mA RCD protection as the main switch but why fit just this, why not have a 30mA RCD main switch? especially given the circuits have no other RCD protection.

was this common practice back when this was fitted when the house was built? just seems a strange way of doing things. could be done to 15th maybe 16th edition? before my time eitherway.

Thoughts?

 

[7029 dave]

100ma s type upfont RCD was standard practice for TT earthing arrangements

 

[Gavin John Hyde]

100ma s type upfont RCD was standard practice for TT earthing arrangements

just wondered why it was only 100mA if you are going to have just one rcd make it 30mA?. guess the regs moved forward.

 

[pc1966]

just wondered why it was only 100mA if you are going to have just one rcd make it 30mA?. guess the regs moved forward.

Having a 30mA instant trip as an incomer make the chances of nuisance tripping very high and you lose the lot, including lights, etc which is a real safety hazard. Total leakage on a whole home can easily be close to the ~15mA trip point, the odd thing switching spiking above 30mA

So a 100mA delay incomer is not likely to false trip but is still going to clear a L-E short on TT loop impedances in the 0.2s or so required.

Now that 30mA additional protection RCD are common, and not just for outdoor sockets, it also allows selectivity on faults. Though only really effective separation if the RCD also switches neutral, as otherwise a N-E fault would not be cleared by the 30mA RCD leading to the 100mA deciding to go.

Did you check the trip behaviour of the old RCD? If it is failing to trip that would merit at least a C2 coding, if not then it is mostly C3 unless the likes of a bathroom is not properly supplementary bonded.

 

[7029 dave]

I recommend fitting rcbo's when you change that cu.

 

[SparkySy]

I have seen a fair amount of those exact boards round my way in rural properties, I agree with @pc1966 they were a good get round for nuisance tripping over using a 30ma as the main switch. I did one the other day could not locate the rod although I could see there was clearly a 10mm coming from the garden. A Ze test gave me 28.8 ohms but I ended up putting a new one in because it could have been a horse shoe on the end of it! You know what they say, assumptions the mother of all fups!
I think they are early 16th boards one I took off had 1988 written in the cover.

 

[pc1966]

I recommend fitting rcbo's when you change that cu.

Yes, these days the cost difference for all RCBO versus dual-RCD is not that much so really it ought to be the default. Also many RCBOs are type A now, so better suited to faults on stuff with electronics.

I'm a bit paranoid and like an up-front delay RCD as well, just so no single point of failure, but it is not actually required and most might not want to spend the extra ~£100

 

[SparkySy]

I hate relying on RCD's for sole protection and do exactly the same just in case of failure of one at least there is still some protection.

 

[7029 dave]

Yes, these days the cost difference for all RCBO versus dual-RCD is not that much so really it ought to be the default. Also many RCBOs are type A now, so better suited to faults on stuff with electronics.

I'm a bit paranoid and like an up-front delay RCD as well, just so no single point of failure, but it is not actually required and most might not want to spend the extra ~£100

Totally agree with the upfront delay RCD

 

[123]

TT is very common round here, the way they used to do it (early 80's onwards) was 2 boards. 1 100mA RCD up front - lights, cooker, water heater. 1 30mA RCD up front - sockets

 

[Gavin John Hyde]

See the logic of 100mA rather than 30mA if you have one upfront RCD
As per first post its for the soon to be wifes grandparents. Will be replacing it with nice Hager RCBO board with SPD.

 

[Dave OCD]

I used to fit dual RCD boards then on TT supplies with a 100ma RCD as the main switch which would supply the lighting and cooker, possibly immersion heater then a 30ma RCD for the sockets and shower.

 

[mattg4321]

See the logic of 100mA rather than 30mA if you have one upfront RCD
As per first post its for the soon to be wifes grandparents. Will be replacing it with nice Hager RCBO board with SPD.

The Hager RCBO's don't switch the neutral so won't clear a neutral/earth fault. Not sure if Hager recommend their RCBO's are used on TT. Would be a problem with an upfront S type as this would then trip too.

I'd use RCBO's with a switched neutral - such as Wylex, or ask the DNO if it can be changed to PME.

 

[SparkySy]

@Dave OCD I have seen a few in that combination round this way.

 

[brianmoooore]

Fitted loads like that back in the day. 100mA may be less safe than 30mA, but it was a massive step up from no RCCB at all.
In the dual row MEM boards I used on larger properties that I mentioned in another thread a couple of days ago, I normally fitted a 100mA RCD on the top row, next to the main switch, leaving room for 6 MCBs, and a 30mA RCD on the bottom row. with space for 8 MCBs.
All lighting circuits, except one, would be on the top row.

 

[freddo]

Don't forget this is really quite an old board. 30mA RCD wasn't required when this was installed, like it is today.

 

[Risteard]

TT is very common round here, the way they used to do it (early 80's onwards) was 2 boards. 1 100mA RCD up front - lights, cooker, water heater. 1 30mA RCD up front - sockets

I still fit an upfront S-type for TT installations in an enclosure. At least if the 30mA RCDs fail it offers some redundancy.

 

[radiohead]

The Hager RCBO's don't switch the neutral so won't clear a neutral/earth fault. Not sure if Hager recommend their RCBO's are used on TT. Would be a problem with an upfront S type as this would then trip too.

I'd use RCBO's with a switched neutral - such as Wylex, or ask the DNO if it can be changed to PME.

Indeed, this point seems to be overlooked by most or simply ignored. Fitting SP RCBO's with an up front T/D RCD is effectively non compliant because a single fault may result in the main RCD operating.
DP RCBO's are essential, my preference is crabtree.

 

[UNG]

Before the current operated ELCB's it was quite common to find an upfront VOELCB on TT installations

 

[Risteard]

The Hager RCBO's don't switch the neutral so won't clear a neutral/earth fault. Not sure if Hager recommend their RCBO's are used on TT. Would be a problem with an upfront S type as this would then trip too.

I'd use RCBO's with a switched neutral - such as Wylex, or ask the DNO if it can be changed to PME.

You can get Hager double pole RCBOs - commonly used in the south of Ireland.

AD9XXJ (where XX is the current rating, e.g. AD910J for a 10A type B).

 

[pc1966]

You can get Hager double pole RCBOs - commonly used in the south of Ireland.

AD9XXJ (where XX is the current rating, e.g. AD910J for a 10A type B).

They look like you need a dual-busbar style of board as they don't have flying N. Not sure if that is common in ROI? Certainly it seems to appear in EU for the double-pole protection.

But they actually need double-pole protection (i.e. thermal-magnetic on both L & N) presumably from legacy of not having N very much tied to E.

The Wylex and Crabtree compact RCBO are both neutral-switching and Type A RCD, so they meet the requirements for selectivity with an up-front delay RCD on N-E faults, and the added isolation makes testing a bit simpler as you can flip the breaker and insulation test the final circuit N as well.

I think the recent compact Fusebox RCBO are also neutral-switching and type A, would not be surprised if they all come from the same factory...

 

[Risteard]

I think the recent compact Fusebox RCBO are also neutral-switching and type A

They are indeed.