Zs figures for a TT System

[Gavthesparky]

Did a Periodic today on a TT system. The Ze was a reasonably healthy 38 Ohms and the Ze figures for the various circuits were around the 25 ohms figure. All circuits are protected by a 30mA RCD and the boards main switch is a 100mA time delayed RCD. The Zs figures are way higher than the recommended figures on Pg 103 OSG but am I right in thinking that they comply with 7671 when I look at the table 41.5.

Sorry to ramble on and thanks in advance

 

[pevvers]

Yup, Zs results for a TT system will always be higher than what is allowed when using an MCB/fuse for ADS, hence why the need for RCD protection on the final circuits. There is a calculation for RCD requirements at the front end, but if everthing is protected at 30mA and your Zs are relatively low, then there shouldn't be a problem. Have a look at 411.5.3 for guidance....

 

[Guest123]

Hi.

As it's a TT system then yes it's table 41.5 you need to refer too.

Basically for a 30mA the max permitted Zs in order to achieve disconnection before the safe touch voltage of 50V is exceeded is 1667ohms. For a 100mA it's 500ohms.

The Ze (Ra) should be no more than 200ohms but as you have 38 then jobs a good un.