Anyone know of any manufactures of small recessed DB's or any other options?

[JohnS888]

Hi all

Have a job to replace 6 way recessed DB. Have looked extensively online but smallest can find is 10 way. Seems there's
limited choice with these. Don't mind if have to chisel away abit to but need something preferably near the existing dimensions which are: Width 305, Height 180, Depth 80. Any help appreciated

 

[JohnS888]

Another query for you's. I haven't had a chance to check out supply type for property yet so there is a possibility it could be a TT earthing arrangement although very much doubt it. I find the fact that they installed a 300ma main switch pretty strange. Has 300ma circuit protection ever been a requirement in previous regs? And also as my DB is only supplying a residential property with no distribution circuits or essential services am i correct in thinking even if is a TT installation 30ma rcbos or 30ma main switch would be appropriate? I can't see why it wouldn't be. Thanks

 

[pc1966]

Having an RCD up-front above 30mA is usually a TT thing, or sometimes a fire safety for agriculture sites. 100mA is more common for domestic, but 300mA is just as acceptable provided the earth rod is below 167 ohms.

I don't think there ever has been a requirement on 30mA / 100mA / 300mA (or possibly more) as such. The main point has always been to meet ADS times and for TT that is often only possible via an RCD as you will struggle to get a rod arrangement low enough to trip even a 6A B MCB in the required times.

I suspect 100mA is the 'sweet spot' as less likely to nuisance trip than 30mA for a whole installation, and can had delay so it is selective with downstream 30mA 'additional protection' RCDs, whereas 300mA starts to demand an earth rod impedance that can be a challenge to meet all-year in some soil conditions without quite an effort for several deep rods, etc. Or the no-no of relying on a nice long buried metal water pipe...

 

[JohnS888]

Having an RCD up-front above 30mA is usually a TT thing, or sometimes a fire safety for agriculture sites. 100mA is more common for domestic, but 300mA is just as acceptable provided the earth rod is below 167 ohms.

I don't think there ever has been a requirement on 30mA / 100mA / 300mA (or possibly more) as such. The main point has always been to meet ADS times and for TT that is often only possible via an RCD as you will struggle to get a rod arrangement low enough to trip even a 6A B MCB in the required times.

I suspect 100mA is the 'sweet spot' as less likely to nuisance trip than 30mA for a whole installation, and can had delay so it is selective with downstream 30mA 'additional protection' RCDs, whereas 300mA starts to demand an earth rod impedance that can be a challenge to meet all-year in some soil conditions without quite an effort for several deep rods, etc. Or the no-no of relying on a nice long buried metal water pipe...

Thanks for clarification PC so am right in thinking then if is a TT installation if either install dual rcd board with non rcd main switch or all rcbos I have still achieved the required disconnection times. Thank

 

[pc1966]

Thanks for clarification PC so am right in thinking then if is a TT installation if either install dual rcd board with non rcd main switch or all rcbos I have still achieved the required disconnection times. Thank

If all of your final circuits are off an RCD/MCB or RCBO then you would be able to meet the ADS times on any reasonable TT earth rod setup.

Personally I would still prefer an up-front delay RCD so you don't have a single point of failure for protection in something as complex as an RCD (which is normally seen as "additional protection" along with OCPD based primary ADS protection, and in practice don't get tested by the users), but it is not a requirement as such.

 

[JohnS888]

If all of your final circuits are off an RCD/MCB or RCBO then you would be able to meet the ADS times on any reasonable TT earth rod setup.

Personally I would still prefer an up-front delay RCD so you don't have a single point of failure for protection in something as complex as an RCD (which is normally seen as "additional protection" along with OCPD based primary ADS protection, and in practice don't get tested by the users), but it is not a requirement as such.

Thanks PC very valid points. I think I'm with you on that thinking to