TT System

[danny124]

Sorry guys need help with this one,

In 12 years of being a sparks i never come had to deal with TT systems, and for the life of me can remember all the stuff i learnt back in the day and can't find any of my notes from college.


CU methods=

1. 100ma (delayed/slugged) main switch, with 30ma dual rcds with your normal mcb's

OR

2. Just a main switch but all circuits incorporating RCBO @ 30ma (can you do it this way, doubting myself and thinking it can't be done)

OSG doesn't give clear instructions and can't seem to find it in the BRB or BGB.


can someone help if possible, having one hell of a day, and having brain farts too. LOL


cheers guys

 

[Des 56]

Time delayed Rcds are not needed much nowadays
You could use a standard twin 30 m/amp rcd board

Rcbos should ideally be double pole on a TT system
If the proposed board is metal clad,a time delay for the tails may be needed or protection of those tails on entry

 

[Deleted member 26818]

The 30mA RCD regulations apply just the same for TT installations, as for TN installations.
100mA would be required if the tails and the CU are not both double insulated, and as a minimum for any circuits, socket-outlets, etc. that do not require 30mA RCD protection.
If using 100mA and 30mA RCDs in series, the upstream RCD will be required to be time delayed, to provide discrimination.
When setting a time delay, the requirements for TT disconection times must be allowed for. 1s for distribution circuits, and 0.2s for final circuits.

 

[Deleted member 9648]

Sorry guys need help with this one,

In 12 years of being a sparks i never come had to deal with TT systems, and for the life of me can remember all the stuff i learnt back in the day and can't find any of my notes from college.


CU methods=

1. 100ma (delayed/slugged) main switch, with 30ma dual rcds with your normal mcb's

OR

2. Just a main switch but all circuits incorporating RCBO @ 30ma (can you do it this way, doubting myself and thinking it can't be done)

OSG doesn't give clear instructions and can't seem to find it in the BRB or BGB.


can someone help if possible, having one hell of a day, and having brain farts too. LOL


cheers guys

My preference would be the highlighted. A 100ma T/D main switch with all final circuits 30ma RCD protected by either RCBO's or a dual rcd system. Reason being as we all know RCD's fail,and with typical Ra's which will not operate an OCPD a back up RCD will prevent danger if an RCD fails to operate....the chances of two failing is IMO negligable......I believe the IEE have missed a point here,and that back up T/D RCD protection should be made a requirement on all TT systems.
That said any of the good advise by the guys will meet the current requirements.