Best practice choice for a CU?

[Monkey Tennis]

Got a call from a customer with tripping problems on their CU... Went out to see what the issue was: decrepit teasmaid causing the tripping so easily recified, however seeing the current installation made me wonder what decision making had gone on by the installer.

The CU has been in for less than 6 months - was a replacement for a fuse board when the client had PV installed this summer - the CU was not a 17th edition as you would expect, but a 'build it yourself' Proteus with all 7 circuits on 'C' curve RCBO's - this is a house, the guy does not use commercial lighting set-ups or have a workshop with tooling needing a high draw on start-up... The cooker whilst wired on 6mm was also on a 32A rather than the 40A you would expect, but that is another matter! Just looking at it, the cost for this set-up alone must be nudging £300 as opposed to £70 for a 17th Edition CU!

Now I realise that the safety aspect of the installation is not in question, but my question to you guys are firstly, why would you NOT put a 17th edition board in with a split loaded RCD's?.. Secondly, Why would you install all RCBO's rather than RCD provision?.. Thirdly Why 'C' curve rather than 'B' in a purely domestic setting?

FYI the installers are NICEIC so not a case of a clueless cowboy!

 

[Andy78]

Got a call from a customer with tripping problems on their CU... Went out to see what the issue was: decrepit teasmaid causing the tripping so easily recified, however seeing the current installation made me wonder what decision making had gone on by the installer.

The CU has been in for less than 6 months - was a replacement for a fuse board when the client had PV installed this summer - the CU was not a 17th edition as you would expect, but a 'build it yourself' Proteus with all 7 circuits on 'C' curve RCBO's - this is a house, the guy does not use commercial lighting set-ups or have a workshop with tooling needing a high draw on start-up... The cooker whilst wired on 6mm was also on a 32A rather than the 40A you would expect, but that is another matter! Just looking at it, the cost for this set-up alone must be nudging £300 as opposed to £70 for a 17th Edition CU!

Now I realise that the safety aspect of the installation is not in question, but my question to you guys are firstly, why would you NOT put a 17th edition board in with a split loaded RCD's?.. Secondly, Why would you install all RCBO's rather than RCD provision?.. Thirdly Why 'C' curve rather than 'B' in a purely domestic setting?

FYI the installers are NICEIC so not a case of a clueless cowboy!

Is not more division of circuits better than just some ? To have half the house trip out in the event of a blown light bulb could be argued to still not fully satisfy the regs regarding nuisance tripping.

some manufacturers only produce C curve rcbos.... not sure of proteus as I wouldn't go near it with yours

That's a good one

 

[Leesparkykent]

maybe used rcbos as didn't want the pv on the same rcds as the other circuits as a G83 inverter can take up to 500ms to disconnect, could of used a high integrity board tho with a rcbo inbetween the main switch and rcd for the PV. type c are fine as long as zs readings are correct and cant see whats wrong with a cooker on a 32amp if that is all the cooker circuit requires.

that said I would not usually use type c on a standard domestic set up

 

[Octopus]

Would you put a 6mm cooker circuit on a 40A breaker? - I wouldn't if I didn't install the cable and knew the installation method,

As for the RCBO board - maybe the preinstall tests showed up earth leakage and the (good) decision was made to have seperate RCBO's

My guess is that the C curve were spare from another job

 

[R G]

You can't just fit a dual RCD board with solar panels in use , its got to be either a high integrity board or an RCBO unit ...

beat me to it (post3)...lol...

 

[davesparks]

Why would you expect a cooker to be on a 40A? Domestic cookers have been fed at 32A for decades!

You would install RCBOs rather than RCDs because it gives better discrimination and causes the least nuisance in the event of a fault, but unless they are SPSN or DP RCBOs then they won't give protection agains N-E faults.

For reference I always fit SPSN RCBOs as standard for all domestic CUs these days.

 

[7029 dave]

Nothing wrong with 6mm on 32amp breaker.

 

[Pete999]

Is not more division of circuits better than just some ? To have half the house trip out in the event of a blown light bulb could be argued to still not fully satisfy the regs regarding nuisance tripping.

some manufacturers only produce C curve rcbos.... not sure of proteus as I wouldn't go near it with yours

That's a good one

That's what I thought.

 

[Engineer54]

Why have you been called out when the CU replacement is just 6 months old?? It's all still under guarantee/warranty surely...

As for the cooker being wired in 6mm T&E sitting on a 32A OCPD, ...Well that's been the norm for well over 60 years if taking into account the old imperial 7/044 and 30A MCB's / 3036 fuses. You could even go as far as saying that 99% of all households old and new in the UK will be installed using exactly that same set-up..

 

[telectrix]

the only potential problem that i can see is the Zs readings not being low enough for a C type breaker. i would assume that the readings will be on the EIC, so you can check them for compliance.

 

[jedisparks]

That set up sounds okay to me. I often provide a choice to customers of an RCBO board (main switch and individual RCBOs for each circuit) or 17th edition dual rcd type. Despite the cost they often go for the former with the reduced disruption in event of fault.

I hope the original contractor hasn't been slagged off too much because on the face of it it sounds okay.

Best

 

[Octopus]

I offer Hi-Integrity boards or RCBO boards only.

Had to fit a MK board today that the client had bought - what flimsy rubbish!

 

[R G]

I offer Hi-Integrity boards or RCBO boards only.

Had to fit a MK board today that the client had bought - what flimsy rubbish!

Agree with you there , with the MK boards , compared to what MK boards were like in the 80s and 90s , Screwfix has 16 way Wylex high integrity boards fully loaded with all MCBs and 5 RCBOs and some metal boxes thrown in for £80 + vat , I notice ..

 

[Deleted member 9648]

Why is it people panic as soon as they see a type C on a domestic?

 

[R G]

Why is it people panic as soon as they see a type C on a domestic?

Never found them to be a problem really , just check max Zs readings comply.