calculating Zs with only R2 reading

[egginyourface]

is it possible to calculate zs by only using r2 + ze reading??

 

[Sparky Ninja]

Show me where I did? I'm sure I posted the word 'Not' before the quote to suggest that it was a false truth.
In fact forget it... no point.

 

[Modular1]

is it possible to calculate zs by only using r2 + ze reading??

More to the point, why has this scenario cropped up? If you can get to the end of the circuit to get an r2 reading why can't you just get the r1 too?

 

[Gazap]

Domestic house, i measured just R2 for the lights casing(screws) as these were all fancy and didnt want to cause any damage, would it be ok to do the R1 + R2 from the light switch ? or would you not bother ? Also have a senario were the lighting is a ring, what reading to do, just the highest R2 ?

 

[malcolmsanford]

PIR I assume

The schedule of Results have either R1+R2 or R2 and as the tester only you can really decide if your happy with just doing the R2 test. I would certainly look at doing R2 only on say commercial as in hi bay lights, or a circuit that just can not be isolated. If the installation on your sampling is in good condition and your happy that there is continuity of the CPC throughout then yes R2 is fine.

As for the R1+ R2 as you know it should be taken at each point and the highest value or end of the circuit is the value noted. You can take the R1+R2 at a switch and you could use lightmates and do it at the lights as well if you wished.

I have to admit I dont think I have seen a lighting circuit wired as a ring, certainly not in a domestic situation, are you sure that there is not an addition onto the circuit and they have taken the spur from the MCB, so having 2 conductors in it

 

[Gazap]

hi Malcolm many thanks for the info, have to admit i just noticed the two lines going into the MCB, i'll double check L/L. Just on a steep learning curve at the moment and going through family houses looking at different installations. Just one more question if you dont mind, why would they of taken a spur from the the same MCB when there are 2 spare slots, is this normal and quite common ?

 

[malcolmsanford]

Yes quite often say for a downstairs lighting circuit and there as been an extension built. It can sometimes be easier to take a LINE/NEUTRAL/EARTH from the CU rather than perhaps destroying decor or taking boards up trying to take a spur of the circuit. Basically your using the MCB as a JB, As long as the spur is feeding lights on the same circuit. ie downstairs lights then it is done.

 

[Gazap]

If the MCB is acting as a JB the the spur should only source 1 load, correct ? As also noticed 32a with three lines, i assume this to be 1 final ring circuit and 1 spur, if doing an R1 + R2 how do you test for the spur off the final ring circuit to take the highest reading ?

 

[malcolmsanford]

Look at reg 314.4 where it tells you that each final circuit must have its own individual way in a DB. So for the scenario I told you the downstairs lights which most likely be front room, hall, kitchen, etc and then a downstairs light in an extension that is all classed as a single final circuit as htey are all downstairs light. Which means when you isolate the MCB for the downstair lights, all the downstairs light will be isolated.

Appendix 15 tells us that you can take a single unfused spur double socket from each point in a ring or at the CU. If I found that you had an unfused spur via the CU I would do all my R1+R2, as per normal, at every socket on the final circuit, which is the ring and the spur, and whichever was the greatest value would be my R1 + R2 value, chances are it could actually be a socket on the ring rather than the spur socket, it would depend on the spur length.

 

[Gazap]

many thanks for the advice, really appreciate it, will get reading.

 

[Gazap]

sorry getting a little confused, if i'm doing a r1 + r2 for ring final circuit when taken out of the MCB, i would be doing the highest value but if a circuit has also been added but as a spur in the same MCB, what reading do i give for this spur and would it need to be take as R1 + R2 as a separate entity ? In theory 2 circuits for 1 MCB.

 

[ste mac]

You would enter the highest r1 + r2 measurement onto the cert, whether that is the ring part or the radial spur part of the circuit. It is just the same as a spur from the middle of the ring, the mcb is classed as just another cable junction. If you were to split the ring at a plug socket for your measurements the spur off the MCB would be the same as any other spur off the ring, so therefore they should be classed as the same circuit because they originate from the same MCB. Hope this lessens the confusion.

 

[topquark]

You would enter the highest r1 + r2 measurement onto the cert, whether that is the ring part or the radial spur part of the circuit. They should be classed as the same circuit because they originate from the same MCB. Hope this lessens the confusion.

That will be the case when you come to the Zs test as well. You'll test the sockets on the ring and the spur and record the highest Zs on that one circuit on the cert.

 

[Gazap]

thanks guys, big help