Installation requirements for TT systems

[sparkybird]

Hi all

This is a college work question, I'm afraid! I know you normally send students away with a flea in their ear, but I've been flogging through the regs, books and the internet and have not got very far! I've never seen a TT installation (city girl) let alone installed one, so wondered if someone could give me a quick 'checklist'.

I need to know the precautions that need to be undertaken when installing a TT system. I know about suitable earth electrodes but am stuggling a bit with maximum Ze's, rating of RCD's and when to use them (different ratings for different circuits??)

Any pointers would be much appreciated

SB

 

[ban-all-sheds]

413-02-18 to 20

471-08-06

542-01-04, 07

542-02-02, 03

 

[sparkybird]

Thanks Ban for the chatty reply

What I understnad from these regs is that
Rcd on all socket circuits. The rating of the rcd is not less than 50V/Zs. Not sure how you measure Zs before you have installed the system? Maybe by calcs?

But also seems to indicate that whole installation should be on rcd with 5s operation time where rating is 50V/Ze

But looking in the OSG section 7.2.4 ii) seems to indicate main rcd at 200 mA and 5s disconnection where Ze is more than 200 Ohms (what if it is less? will it be?)

Still rather confused...

SB

 

[Spark123]

Ze (of the electrode) for RCD protected TT systems can be measured with an EFLI tester (test method 2 gn3).
Otherwise the use of an earth electrode resistance tester (test method 1).
Test should be done with no parallel paths i.e. electrode disconnected from the system (and system isolated).
If the efli loop is too high, you can drive a bigger earth rod into the ground.

In theory, for a 30mA RCD where Ra I/\n <50V the electrode resistance can be 1660 ohms, although in reality for reliability the resistance should not exceed 200 ohms.

OSG 2.7.4 is allowing you to use TN-CS figures for the TT system subject to the conditions, not much help to you in theory terms imo.

 

[Big_Spark]

This is what I do..don't take it as red, you need to adapt your method to meet the spirit and the letter of the Regs to the installation your faced with.

Lets take a standard 100A Service Head with a TT supply.

Main Switch of Consumer Unit. 100A 300mA Type S RCD

Sockets: 30mA 32A RCBO
Cooker: 30mA 45A RCBO (Obviously this should be matched to installation requirements)
Ancillery Power circuits: 30A RCBO's of appropriate rating.
Lights: 6A Type B MCB for each circuit.

It should be remembered that electronic equipment can cause normal RCD's to nuisance trip, especially computer equipment, this should always be accounted for, hence the choice of Main switch for the CCU.

 

[sparkybird]

Thanks everyone - esp Big Spark - it's really helpful to see the type of practical arrangement that comes out of the Regs.

However, you say that computer nuisance tripping should be accounted for in the main switch choice, but surely computer equipment will be plugged into the socket circuit, hence will trip if >30mA...? Or am I not understanding this??

SB