R1+R2 Ring Readings

[traineedave]

Hi, Im currently doing my city and guilds training but have started doing some testing on my own house to get head around things...

On my ground floor ring circuit I have recorded the following end to end readings

r1 - 0.50
rn - 0.50
r2 - 0.85

but when I'm testing for r1+r2 im getting 1.35 at the consumer unit and around 0.32 at each of the socket outlets, is this correct or should the reading at the consumer unit be the same as at each socket?

Sorry in advance if this is a silly question!

 

[malcolmsanford]

Why are you doing the R1 + R2 at the consumer unit? You should be having the L1 -CPC2 and L2-CPC1 connected at the CU and doing your R1+ R2 at the socket. what is 1.35/4 ?

 

[traineedave]

Oh I see, so by taking a reading at the consumer unit I'm litterally just adding r1 and r2 together?

1.35/4 is pretty close to my readings of 0.32 so im guessing that apart from taking the reading at the consumer unit my test results seem ok?

 

[malcolmsanford]

I would say so yes. You do know why the r2 was a higher value don't you

 

[traineedave]

Oh yeah I was expecting it to be higher as its a twin and earth cable 2.5mm/1.5mm(cpc), just getting my knickers in a twist over the r1+r2!!

Thanks Malcolm

 

[telectrix]

trainee, please differentiate between r1 and R1 etc. your r1, rn and r2 are end to end readings. R1+R2 is the reading you get when you cross connect L1 to cpc2 and L2 to cpc1 at the CU. then measure at each socket, all readings should be virtually identical, unless you have a spur off the ring. your R1+R2 readings should be(r1+r2)/4. ( divede by 2 to allow for half the ring end/end and divide by 2 again as you have 2 cables in parallel.)

 

[traineedave]

ThanksTelectrix, it makes sense now, I was indeed getting my r1's and my R1's confused, but now it makes perfect sense...thanks again guys. I'm sure this won't be the last of my silly questions!!

 

[traineedave]

One further question....the ring circuit in question is also protected by a bs61008 30mA RCD, so would the maximum permitted Zs be 1667Ohms or would you use the value in the table for a 60898/B 32A MCB? I was thinking that anything upto 1667 would still allow the RCD to operate in the required time but wasn't sure what value should be recorded on a PIR or EIC ?

 

[malcolmsanford]

Yes in TN and TT installations the Zs recorded on your Schedule of Results should be 1667 ohms as per regulation 411.4.9.

Just on a cautionary note though this is the value you put on a Schedule of Results, it sould not detract you from invistigating why a R1 + R2 result, if obtained, was high, as a high R1 + R2 indicates a problem, and though an RCd is providing you that additional protection, please do not go down the road of think

"OK the ring final R1 + R2 is 2.6 ohms (SIC), but it's protected by an RCD, so ok". it is not, check why you have a high reading, it could be a loose connection.