Can't test RCBO as main RCD trips first - what to do?

[terminator]

Terminator please quote the reg that tells us we have to have an upfront RCD if all circuits have 30mA protection, I'm not saying your wrong but I can't find that reg.
ATB J

Jimmy - I said 'ought to have' because you can never be sure that someone may come along and change or add a circuit just using an MCB - and if the main earthing resistance had deteriorated (TT is notoriously unreliable) you would lose your disconnection times. It does say in the BGB "the preferred protective device is an RCD" 411.5.2. Then Note 1 says "where discrimination between RCDs is necessary refer to Reg 531.2.9. That is about compliance with the Regs for fault protection using two or more RCD,s in series and that discrimination in their operation is necessary to prevent danger.

In this case I think you need an upfront S type RCD to meet disconnection times and downstream RCBO,s to ensure disrimination and to prevent danger by the whole installation being knocked out.

 

[Engineer54]

Jimmy - I said 'ought to have' because you can never be sure that someone may come along and change or add a circuit just using an MCB - and if the main earthing resistance had deteriorated (TT is notoriously unreliable) you would lose your disconnection times. It does say in the BGB "the preferred protective device is an RCD" 411.5.2. Then Note 1 says "where discrimination between RCDs is necessary refer to Reg 531.2.9. That is about compiance with the Regs for fault protection using two or more RCD,s in series and that discrimination in their operation is necessary to prevent danger.

In this case I think you need an upfront RCD to meet disconnection times and downstream RCBO,s to prevent danger by the whole installation being knocked out.

Not sure if i've read this right, ....but an up-front S type RCD won't meet TT disconnection times, (delayed trip time) it's there solely as additional protection to any RCD device that fails, for one reason or another to operate, (they are not the most reliable of protective devices). At the very least, it will trip out a faulty circuit, even if it is slightly outside of the set times of a TT system...

 

[terminator]

Yes, agreed Engineer - A touch of dyslexia - I meant the other way round, RCBO's to meet disconnection and and upfront for backup and discrimination, as you have pointed out. Been a long day.

 

[Jimmy Boy]

I don't disagree with extra protection I was just curious to see if it was an interpretation of the regs or a reg
Eng 54 could you explain to me how you interpret the regs on protecting tails in to an enclosure on a TT ? if the 100mA RCD is in the board, and not external to it how does/can it protect the tails that are from DNO fuse to incomer on RCD ? Not a trick question just a genuine enquiry fella.

ATB J

 

[Guest55]

I don't disagree with extra protection I was just curious to see if it was an interpretation of the regs or a reg
Eng 54 could you explain to me how you interpret the regs on protecting tails in to an enclosure on a TT ? if the 100mA RCD is in the board, and not external to it how does/can it protect the tails that are from DNO fuse to incomer on RCD ? Not a trick question just a genuine enquiry fella.

ATB J

My interpretation of this situation is that the enclosure itself cannot be metallic due to no rcd upstream of the CU main switch - ie. being fully insulated provides that extra level of protection for TT wiring prior to the rcd.
If you get my drift lol.

 

[Jimmy Boy]

Yea I would go with that, but OSG doesn't prohibit 'Metal' enclosures as long as the tails are mechanically protected, I guess fed in to the board in conduit or similar.

ATB J

 

[Guest55]

Im fairly certain that metal CU's were / are prohibited for domestic TT installs.
Cant remember the reg , might even be from the 16th regs.
Its just something that i remember reading somewhere.

 

[Jimmy Boy]

Yea I can't find it in the BGB only the OSG Green page 30 still thumbing the BGB
J

 

[Engineer54]

I don't disagree with extra protection I was just curious to see if it was an interpretation of the regs or a reg
Eng 54 could you explain to me how you interpret the regs on protecting tails in to an enclosure on a TT ? if the 100mA RCD is in the board, and not external to it how does/can it protect the tails that are from DNO fuse to incomer on RCD ? Not a trick question just a genuine enquiry fella.

ATB J

To be honest with you, for the life of me i cannot understand why you would need an RCD to protect the insulated and sheathed tails entering a metal CU/DB on a TT installation!! If the installation is correctly installed by a competent electrician, that suitably ''Bushes'' the entry of any tails entering a metal CU/DB, ...why would you need protection from an RCD??

I'm not the best person to ask for interpretation of BS7671 regarding TT systems. As to my mind, that whole section needs completely revamping in it's entirety... lol!!

 

[drew35]

I would have thought you could have gunned the RCD out and stuck a main switch in from stock in about thirty seconds, tested the RCBO, and then gunned the RCD back in. If you were that worried.

 

[ackbarthestar]

I think it relates to normal leakage currents and high Ze(Ra) resistance. A voltage, albeit small and non existent to dry hands/bodies fully clothed, will be constantly present.

I have measured up to 27V, excessive, between earth and MET before now without the RCD tripping.

When/if a fault develops then you are looking at a potential divider created by a small R1 with a very large Ra but then that's what the RCD is for....