Testing with a CU change

Bradley6969 · Jun 6, 2019

Working with a spark, now when we change a CU no new circuits just old for new, We do all end to ends on rings, IR tests and Zs making sure all the new protective devices that disconnection times are met, would you do R1+R2?

If we do a CU change and add new circuits we do R1+R2 on the new circuits what do you guys do?

Pete999 · Jun 6, 2019

anthonybragg said:
Ze does not = Zs +(R1+R2)

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Sorry the wrong way around ZS = ZE (R1+R2) must leave the red alone sorry Ant you are right my apologies.

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Toneyz · Jun 6, 2019

Pete999 said:
#
Sorry the wrong way around ZS = ZE (R1+R2) must leave the red alone sorry Ant you are right my apologies.

I didn't reply as I thought I would be telling you to suck eggs and you'll realise when you had another look.

9

Toneyz · Jun 6, 2019

I think I replied on Pete's post can't get used to new forum

Toneyz · Jun 6, 2019

Baddegg said:
I thought zs=Ze+ (R1+R2)...but Ze does not = Zs-(R1+R2) or something like that possibly maybe......I’m feeling really uncomfortable with the new look.....the only other time I’ve felt like this was when they spruced up my local! Took me ages to get over that....

You can not do R1 +R2 = Zs -Ze as for the parallel paths

Pete999 · Jun 7, 2019

anthonybragg said:
I think I replied on Pete's post can't get used to new forum

anthonybragg said:
You can not do R1 +R2 = Zs -Ze as for the parallel paths

Gets easier the more you use it

Welchyboy1 · Jun 7, 2019

anthonybragg said:
You can not do R1 +R2 = Zs -Ze as for the parallel paths

If we are talking domestic, mostly twin and earth cabling

You can calculate this fairly accurately, if your in any doubt, if you take your ZS then remove your ze, then have an approximate idea of cable run you can look at the resisstance per meter

Rings are easier to confirm with the exception of a long spur from the furthest point

i only usually find parallell paths in a domestic setting on immersion circuits/ring circuits with heating/boilers etc on and houses with supp bonding

Commercial/ind jobs using a lot of metal work are of course
another matter

Obviously r1 and r2 on TT systems domestic or otherwise i would measure in all circumstances

Toneyz · Jun 7, 2019

Pete999 said:
Gets easier the more you use it

Don't like the emoji's however there is no bad spelling Pete

DPG · Jun 7, 2019

And where's the dumb emoji? Only kidding, calm down.

Charlie_ · Jun 7, 2019

Calm down

[ElectriciansForums.net] Testing with a CU change

HappyHippyDad · Jun 7, 2019

It definitely doesn't feel write to determine R1+R2 by subtracting Ze from Zs.
From a mathematical point of view it works just fine and is 100% correct with respect to rearranging the equation Zs = Ze + (R1+R2), but your Zs is influenced by other earthing paths that your Ze is not influenced by so you could get a biased answer.

Although... that would be true of the initial equation as well! Ze could be 100Ω on a TT. Zs could be much lower than that if the bonding conductors have a good pathway to earth, ∴ in this case Zs ≠ Ze + (R1+R2).

I enjoyed all those mathematical symbols, it's been a while! I miss good old fashioned maths where the answer is black and white.

HappyHippyDad · Jun 7, 2019

Pete999 said:
#
Sorry the wrong way around ZS = ZE (R1+R2) must leave the red alone sorry Ant you are right my apologies.

That is also wrong Pete. Ze(R1+R2) means Ze x (R1+R2). Pedantic I know but I like maths to be correctly written, not too worried about englush!

Charlie_ · Jun 7, 2019

I am also not a fan of determining R1+R2 using Zs.
It’s useful, yes but also misleading..
IMO R1+R2 is to determine the resistance of the respective cables. Furthermore, if the length of the circuits needed to be verified then using Zs - Ze would give incorrect values.

Pete999 · Jun 7, 2019

happyhippydad said:
That is also wrong Pete. Ze(R1+R2) means Ze x (R1+R2). Pedantic I know but I like maths to be correctly written, not too worried about englush!

OK, ZS=ZE+(R1+R2) You pedant you, but correct

telectrix · Jun 7, 2019

Baddegg said:
I thought zs=Ze+ (R1+R2)...but Ze does not = Zs-(R1+R2) or something like that possibly maybe......I’m feeling really uncomfortable with the new look.....the only other time I’ve felt like this was when they spruced up my local! Took me ages to get over that....

they done same with my local back in the 90's. knocked down walls, stables, ripped out woodworm infested bar and all the grime. refurbished into a food orientated, child friendly, monstrosity. what used to be nice friendly pub where I got barred on the Friday and reinstated on the Monday ( massive drop in takings) then became a plastic copy of what a pub might be.

Flanders · Jun 7, 2019

Ian1981 said:
How do you calculate your R1+R2 as that should only be done by measurement only other than at the design stage?

Agree But you also don't calculate R1+R2 and design stage either , you calculate Zs from by inquiring what the Ze will be and adding that your R1+ R2 readings, As others have said you can't back calculate the R1+R2 but I believe loads sparks who do it, As for the ops question, I do all the test except Zs which I calculate .