R1+R2 on 2 core Hi Tuff

You could not (R2) is the cpc value, and you do not have one. R1+R2 is to establish 1.Earth continuity 2. Calculate Zs if required.


Colin

What is the conduit made of? From the sound of it, even though the garage may be adequately protected, the cable is not. Therefore that's a C2 from me. 522.8.10

It might be in metal conduit?

Where does the TT island start - is the feed between the buildings also TT, or is that part of the PME ?

(and if it is PME, does it have an earth with it - is there metal conduit or similar - if not how does the fuse blow if a garden fork finds it ? This may not be the safest choice of cable.)


There are two things to verify -

1) will the wiring before the RCD be protected by the MCB or fuse upstream ? - this is normal sums for you I think.


2) Will the wiring after the RCD be protected by it?

Here the load side CPC stops at the electrode, so you could wander lead from there to inside the garage to verify the earth path from a socket front or whatever, or short CPC and L at a socket and measure R1+ R2  from the external electrode to the live at the start of the garage circuit. (only with it off, of course...)

Then you need to verify the electrode separately.


Far better if it is already up and running do a Zs check instead, and see if that would fire the RCD, which I imagine is 30mA. That not only checks the wiring is present to the electrode but also that the electrode is actually in the earth, and not cut off short, or corroded or inside a buried  plastic flower pot or something silly. expect 100 ohms or so for the electrode, and 3/5 of SFA for the R1 + R2 so Zs is really the electrode reading as the R1+R2 is lost in the flicker of the last digit.

Mike.

What is the conduit made of?

Good question. If it's earthed steel then it's acting as a c.p.c. and so you can test R2 or R1+R2 at the far end as normal. But then there's the risk of having two different earthing systems within reach of each other at the garage end.


If it's plastic it might or might not comply with 522.8.10 - all depending if it gives equivalent mechanical protection - which rather depends on the circumstances - where it's buried and what other mechanical protection/risks are present.


The lack of a c,p,c, on the distribution circuit sort of feels it should fall foul of 412.1.2 and 412.2.3.2 as it's in effect a circuit that relies on shock protection by double or reinforced insulation rather than ADS in an uncontrolled environment - but I think it just scrapes though as the equipment being fed isn't class II and a c.p.c is provided at the far end (if from a different earthing system).


   - Andy.