TT and the protection of metal distribuion boards particullarly 3 phase

Hi,
I am aware of a TT install in a church that has been converted to an arts center. The building used to have an old 3 phase white meter installation and a seperate single phase 24h meter. Both meters have been recenetly replaced and are still live, the DNO has replaced the incoming overhead supply cable and it looks like it would be good for 80 A three phase.

The existing 24hr meter connectects to a a fully insulated 100A switch, this then feeds a metal cased 60A switch fuse that after 5 M connects to a henly block for all the distribution boards all protected with 30mA trips. Given the positon of the metal cased switch should this installation be protected by a 100mA type S.

At some time in the future I would like to be able to make use of the 3 phase supply so I could have one phase for Stage lighting and another for sound and more house lights. A simple way may be to replace the switch fuse with a proper 3 phase distribution board. I am not sure if TNS would be avaliable and given the age of the property a PME conversion would be difficult. What should be used as the main protection device on a TT install in this situation?

Thanks

Ideally, a 3ph DB should replace all the existing single phase boards, if you are intending to make use of the 3ph supply. If the overhead DNO system to which your supply is connected too has been PME'd Then there's no reason why you cannot request your service head to be converted. A time delayed RCD would be an added protection, but not required if the the installations circuits have existing 30mA protection. Any idea as to what your TT systems Ra value is???

The main thing to remember with 3ph distributed installations, is that the 3 phases should be as equally balanced load wise, as far as feasibly possible.

All enclosures for switchgear and protective devices in a TT installation should have an all insulated or Class 2 construction, or additional precautions recommended by the manufacturer need to be taken to prevent earth faults on the supply side of the installation, this will of course apply even when RCDs are used.

MarkieSparkie said:

All enclosures for switchgear and protective devices in a TT installation should have an all insulated or Class 2 construction, or additional precautions recommended by the manufacturer need to be taken to prevent earth faults on the supply side of the installation, this will of course apply even when RCDs are used.

Click to expand...

Not if an RCD protects the supply tails to the enclosure.

Hi,
So I take it that in this case, see the right hand side of the picture the main switch at the bottom should be replaced by an RCD, 100mA type S?

On your picture under your main MEM switch you seem to have an old ELCB which then goes into the henley's. You really should have repalced that, if it is indeed a ELCB.

Take a look at regulation 531.4.1. If your using a single RCD to protect the installation then it should be at the origin, so in yours I would have fitted a 100mA S type or time delayed double pole RCD in the tails coming out of the meter, you then could have got rid of the MEM switch and the ELCB/RCD under it, as that would have gone into your Henleys. You could have got away with any up front 100mA RCD if you had used insulated boards or class II boards.

Because then from your Henleys to the boards that will have had 30mA RCDs/RCBO to protect the final circuits