TT can be done well, and is useful for situations where an offset voltage between CPC and terra-firma is not desirable - animals on earth floored barns are one example, and humans in hot tubs outdoors are another.
I agree on the reliability of RCDs being a concern, but it does not have to be perfect, just as good as, or better than, the alternative.
In a TNs system, the classic double independent fault to danger is the live wire to the case of the washing machine, and an undetected break in CPC path back to the substation. Earthing rotting off is not common, but not unknown, especially on underground cables.
There are easy ways to reduce the impact of a defective RCD, though we'd need data I do not have to say at what risk level the failed CPC becomes more likely that a dead RCD.
One way to reduce the risk of un-detected RCD fault is regular checking, every 3 months, perhaps 6 months , or as is more likely in a domestic setting, never in a million years, unless there is a real fault. The regs do include this, but I do not think it actually works in many situations.
If an RCD has say a 5% chance of failure in the first 10 years of its life (and this is a pure PIDOOMA figure for the sake of explanation define PIDOOMA - I do try and give my sources....- if we have better figures we should use them,) then we can reduce the chance of failure from 1 in 20 to 1 in 400 by having 2 independent RCDS in cascade.
Of course if the RCDS were the same age and make and in the same box this advantage is partly vitiated as they may suffer the same design weakness and the failures are no longer independent.
This is the approach taken on on a UK campsite, where the site hookup, and the caravan, are both supposed to contain a double pole RCD, and are almost always of different age and make.
For the same reason I am in favour of the 100mA ~ 1/10 second time delay (S type) in an up-front plastic box, and then the 30mA instant types or RCBOs in the final circuits, though this is only really mandated in agricultural settings.
In a large TT system you may see things tiered, with perhaps 300mA 300mS earth fault detection at the origin, then a few 100mA S types feeding various sub-mains, then 30mA instant devices on final circuits. the chance of a failure at all levels in the chain is very low, probably lower than that of an undetected loss of CPC.
Although precluded by UK regs at the moment, the use of sockets with a built in RCD feature are also good as another layer to the safety onion, and more likely to be tested, as the end user is less worried about just tripping off one appliance.
Done well such an approach can be incredibly reliable.
Mike.
PS and some developed world places idea of how to handle earthed neutrals in a non TT way is not ideal.
This would not raise eyebrows in NZ/Oz. I presume you can see the problem.
TT can be done well, and is useful for situations where an offset voltage between CPC and terra-firma is not desirable - animals on earth floored barns are one example, and humans in hot tubs outdoors are another.
I agree on the reliability of RCDs being a concern, but it does not have to be perfect, just as good as, or better than, the alternative.
In a TNs system, the classic double independent fault to danger is the live wire to the case of the washing machine, and an undetected break in CPC path back to the substation. Earthing rotting off is not common, but not unknown, especially on underground cables.
There are easy ways to reduce the impact of a defective RCD, though we'd need data I do not have to say at what risk level the failed CPC becomes more likely that a dead RCD.
One way to reduce the risk of un-detected RCD fault is regular checking, every 3 months, perhaps 6 months , or as is more likely in a domestic setting, never in a million years, unless there is a real fault. The regs do include this, but I do not think it actually works in many situations.
If an RCD has say a 5% chance of failure in the first 10 years of its life (and this is a pure PIDOOMA figure for the sake of explanation define PIDOOMA - I do try and give my sources....- if we have better figures we should use them,) then we can reduce the chance of failure from 1 in 20 to 1 in 400 by having 2 independent RCDS in cascade.
Of course if the RCDS were the same age and make and in the same box this advantage is partly vitiated as they may suffer the same design weakness and the failures are no longer independent.
This is the approach taken on on a UK campsite, where the site hookup, and the caravan, are both supposed to contain a double pole RCD, and are almost always of different age and make.
For the same reason I am in favour of the 100mA ~ 1/10 second time delay (S type) in an up-front plastic box, and then the 30mA instant types or RCBOs in the final circuits, though this is only really mandated in agricultural settings.
In a large TT system you may see things tiered, with perhaps 300mA 300mS earth fault detection at the origin, then a few 100mA S types feeding various sub-mains, then 30mA instant devices on final circuits. the chance of a failure at all levels in the chain is very low, probably lower than that of an undetected loss of CPC.
Although precluded by UK regs at the moment, the use of sockets with a built in RCD feature are also good as another layer to the safety onion, and more likely to be tested, as the end user is less worried about just tripping off one appliance.
Done well such an approach can be incredibly reliable.
Mike.
PS and some developed world places idea of how to handle earthed neutrals in a non TT way is not ideal.
This would not raise eyebrows in NZ/Oz. I presume you can see the problem.
