Writing a guide to TT systems and earth fault loop values

[securespark]

Hi

Attempting to write a guide in very simple english explaining the whys and wherefores of EFL readings in the context of TT systems.

The OSG advises the max EFLI outside the consumer's installation for TT is 21 ohms, excluding the consumer's earth electrode.

How is this best translated? I'm coming unstuck trying to make this simple!!

 

[Lectrician]

21?

 

[securespark]

Yup. P11 of OSG...

 

[Dellsmp]

p11 of osg is an introduction to the guide and does not state any regs,(apart from 313-01-01), it only states the installations to which the guide applies.
Try chapter 54 of regs book p111, Earthing arrangements and protective conductors, and 413-02-18 p49 for Ra calculation for tt systems.
Not sure why you would want to write a guide to tt earthing, but hope that helps.

 

[securespark]

cheers, dell.

No, I know that page of the OSG does not mention any regs, just trying to get a handle on what the statement on that page actually means.

I'm writing a guide to EFL readings expected on TT systems for the engineers that work for my company.

 

[BJS_Spark]

As I understand it, it is making a clear restriction that the OSG information (which is a cut down version of the full regs) is based on an assumed best possibly achievable Ze of 21 Ohms on a TT system. For example with the best possible earth spike conditions the resistance from there to the DNO spike would be at worst no higher than 21 Ohms, which is why they say Excluding the spike.
They have rules on how much distance they can travel before spiking their earth to ground and also rules on how they achieve the best earthing into the ground.

 

[Lectrician]

Yes, the 21ohm is the resistance of the DNO part of the loop.

 

[securespark]

Cheers chaps!