2330 lv3 302 question

[brucelee]

Hi

could someone please help me with a C&G question

when testing a TT earthing system for external earth fault loop impedence using a earth fault loop impedence tester

Why the earth fault loop reading can be recorded as the electrode value

Is it something to do with the resistance of the secondary winding , line conductor as well as the earthing conductor miniscule in comparison to the resistance of the general mass of earth

any help appreciated have my 302 exam and this is one of the questions that is niggling me of sample paper i have

cheers for any help with this

 

[Adam W]

I don't know where 'windings' have come into it but think about the earth loop impedance path.

You're measuring the loop from a point using the live conductor through the whole loop including the earthing conductor(s) via the general mass of earth right back to the point you're testing at.

Therefore if the Ze reads OK without any parallel paths the chances are your earth electrode is ok.

 

[MarkieSparkie]

The EFLI method should only be used as an alternative to the earth electrode resistance test (EERT) method when an earth electrode is used in conjunction with RCDs *, where the electrical impedance is relatively high (typically 10-200 Ohms) and precision is not required. Because the supply transformer secondary winding, line conductor and earthing conductor impedances are small relative to the impedance of the path through the general mass of the earth from the installation earth electrode to the supply earth electrode, the impedance reading taken can be treated as the earth electrode resistance.

* Most TT installations require RCD protection in order to meet the BS7671 maximum disconnection time.