# Earth fault loop impendence

#### Joe100w

How do you measure the earth fault loop impendence zs for the supply. I take is this is not the same as ze with the earth electrode disconnnected.

EFLI (Zs) is measured at the extremity of the circuit between the line conductor and CPC. This is then compared with the values in BS7671 for the circuits OPD.

Also, how would you measure Ze (Ra) if you disconnected the earth electrode? Unless the electrode is adjacent to the intake and your leads reach.
Don't you mean all outgoing PEB's and CPC's with the installation isolated?

You need to measure Ze in isolation i.e. on its own with it disconnected from the Main Earthing Terminal. For a TT system this should be less than 200 ohms for reliability.
When you do the continuity of protective conductors you can add this to your Ze to give you a worst case scenario Zs which you can compare to the tables in the OSG etc, however in a TT system you'll more than likely be relying on an RCD to proved protection to the 50v rule so you shouldn't be looking much more than the 200 ohm figure above - technically you can go up to 1666 ohms for a 30mA device however in reality if I got a figure like that I'd be tightening all my connections!
Before energizing you will have re-connected your electrode to the MET so when you do your EFLI tests at the extremity of the circuit the results will probably be lower than those of Zs=Ze+(R1+R2) above.

hi for ze, by earth electrode, I mean disconnecting the Suppley 16 mm cable from the main earthblock and testing between the consumer unit incoming Phase and the disconnected 16 mm earth cable.

If I understand correctly the zs should be ze + (R1+R2). The (R1+R2) should be the highest that I get when testing all the circuits.

hi for ze, by earth electrode, I mean disconnecting the Suppley 16 mm cable from the main earthblock and testing between the consumer unit incoming Phase and the disconnected 16 mm earth cable.

Yes, as long as the main switch is in the off position.

If I understand correctly the zs should be ze + (R1+R2). The (R1+R2) should be the highest that I get when testing all the circuits.

Yes.

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