Testing Today.

[securespark]

Yawn, I'm still doing I&T on a property....

There is another DB upstairs.

Now, I understand the principle of testing Zdb with the cpc disconnected, as with it connected it could well present a lower reading due to parallel paths etc...

In fact, I proved it. The Zdb disconnected is 0.4 Ohm.

Connected, it is 0.3 Ohm.

My confusion lies here: my r1+r2 readings are higher than the Zs values for corresponding circuits.

I can't quite get my head round it - is it the same effect?

 

[Adam_151]

Not sure about removing conductors for Zdb of subboards....

Willing to be corrected here, but AFAIK, removing parralel paths and loop testing alone is something thats generally only done for Ze (which then becomes Zdb of main board). But for a subboard, Zdb is basically Zs of the submain circuit. That said, I cannot see any harm in doing it, as long as subboard stays isolated while you do it, so basically do it like you would Ze, I'm just not sure its stictly neccessary, but as I said, willing to be corrected

As to lower Zs than R1+R2 alone, say your furthest point is the bathroom fitting, and say Ze is 0.25ohm and R1+R2 of the light circuit is 1.25ohm, thats a Zs of 1.5ohm of which 0.625 is due to the lighting cpc, now if you added a 4mm conductor in parrallel to this, you could reduce it to 1/5th of its current value (0.125), so 0.625 line + 0.125 cpc + 0.25 external = 1 ohm

 

[GaryMo]

Removing the incoming cpc with the subboard isolated wouldn't present any problems and as proved above, a more accurate result would be obtained.
I think may would just take a loop reading with incoming and outgoing cpc's connected though.