TT System - RCD protection

[PC123]

Hi Spark123,

I will certainly do as you suggest. Joining the NICIEC, while still along way from requiring them to asses me, may not be the normal route chosen. However I have found their advice, access to materials, helpline, training courses etc invaluable up until now and would imagine I will continue to find this until I reach assesment point.
I can understand how my mention of them may have conveyed a different message.

Pensdown,

Cheers for your comments and reasoning behind them. I will call my supplier to look into the conversion route to see if this is possible.
I am hoping that other planned work on the house will mean that I should eventually be able to locate where the all concealed cables run.
However thats for the future, right now ensuring the current installation is safe, correctly earthed and offers adequate protection is my main concern.

Thanks again

 

[GaryMo]

I'm not sure isolation is the issue as the main switch could be used for that (537.2.1.1).
It's more a case of the lump of uninsulated busbar under the RCBO's.

Quote from said topic:

"Firstly, there would be no method of clearing a neutral-to-earth fault. Although the (usually single pole) RCBO would operate, there is a possibility that there could still be current flow, due to the voltage difference between the supplier's neutral conductor and the earth electrode in the property.

Secondly, the connecting busbar to the incoming side of the RCBOs will not be protected. You would need to satisfy yourself that nothing will come into contact with this within the board."

 

[Pensdown]

You raise some good and valid points about the bus bar but that's not really an issue because there are alternatives to exposed bars

Crabtree use the plugin system so their bars are almost completely insulated and MG use the slider to completly isolate the bars.

FMPOV it's about designing every installation on it's own merit and not trying to fudge around regs just because it's always been done like that.

 

[NotHimAgain]

Pensdown wrote

Unless your concealed cables have been installed in accordance with one of the five options (very unlikely) you will need to install 30mA earth leakage protection to every circuit that has concealed cables. So a 100mA type S as a mains switch is unsuitable. Also, it will not meet 314.1

Are you suggesting that a front end device is not required - if so you are on very dangerous ground. This matter is due to be re-evaluated by the Electrical Safety Council in September. The review is taking place because of some problems that have been identified concerning shock protection between the outgoing terminals of a main switch and the supply side terminals of an RCD downstream of a non-RCD main switch.

This is an old chestnut that is similar to the reason for the requirements in 531.4.1.

314 is, as ever, not really relevant and generally unworkable - it is far more important that you achieve shock protection.

I would advise anyone that is considering omitting a front end device on a TT system to confirm with the manufacturer in writing that satisfactory shock protection will be achieved.

Note that if a fault occurs between the outgoing circuit from a non-RCD main switch that is 'behind' any RCD the extraneous-conductive-parts and the exposed-conductive-parts in the installation may attain full supply potential and may not be disconnected.