TT Systems and Ze and Zs readings

[Rambhai]

Could some one help me?

What readings should you get when installing new circuits for TT systems?
I am aware that you have to get a reading for the earth rod but what should you have for the following Ze and Zs.?

 

[telectrix]

Ze for TT system should be under 200 ohms. as your Zs reading will be far higher than max. allowed for disconnection times to be met, fit RCD/RCBO to all circuits.

 

[Rambhai]

Thanks for that.

So if the readings Ze was 15 ohms at the earth rod and the installation has a 100mA main RCB protecting the whole installation, what would you suggest I should tell the client.

 

[telectrix]

he should have, apart from the 100mA RCD, 30mA RCDs or RCBOs on all circuits. end of story.

 

[SimpleSimon]

Thanks for that.

So if the readings Ze was 15 ohms at the earth rod and the installation has a 100mA main RCB protecting the whole installation, what would you suggest I should tell the client.

15 ohms!! Good reading .

 

[Rambhai]

Thank you for that and I have told the client about the 30mA and that is in the pipe line.

 

[telectrix]

best way to configure the board is to use the 100mA RCD as main switch and fit 2 additional 30mA RCDs, split board. if this is not practicable, due to space, then RCBO each circuit. expensive but the ideal way as tripping of 1 cct. will not affect any other ccts.

 

[sjm]

If you're using 100ma RCD as main switch make sure it's time delayed S type.

 

[telectrix]

good point. either S type or ditch it for a 2 pole 100A isolator.