R1+R2 Calculations from ZS

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M

mickeyspark

I am not so clued up on testing, but willing to learn where ever and however I can.

Recently I have doing some maintenance and remedial work. I am working with this guy who has been 'doing this kind of stuff for years'. We replaced a fan and he insisted that not only does it need to be recorded on minor works certificate but we needed an R1+R2 calulation.
I thought an R1+R2 calculation could only be done on a dead circuit, disconnected from the board.
I was told that as long as we had the Ze and Zs, we can take one from another and that would give the R1+R2 calculation required. The Ze was calculated on install and the results were on the board. We done the Zs on the fan and not end of circuit.

Is either really necessary? I thought replacing like for like was ok and didn't need paperwork?

From what I can gather you can't take Ze from Zs and get the true value of R1+R2.

HELP!!!

Thanks guys
 
Last edited by a moderator:
you can't do it this way, you need to measure R1+R2, you can calculate Zs from the formula, but not the other way round. as regards a like for like, no MWCis reqiured. if you look at a MWC it asks for the readings of the modified circuit. you have not modified a circuit. however it is a good idea to do a MWC so that it is on record the work you have done .
 
One of the things that surprised me with the new megger loop tester I have is that you can set it to calculate a value for R1+R2 based on the Ze it will memorise, and the measured Zs.....kinda encourages a shortcut dont it?
 
R1 + R2 is a dead test with no load on the cable therefore the cable will be cool. Zs tests are when there is load (or supposed to be) thus the cable will have a higher resistance and give a different reading, but that depends on the temperature of the cable which is a little awkward to check on site!

So you can't accurately calculate Zs, even if you have the Zs of the DB and of the circuit end. But it will be close.

As for the test that is to be carried out; when replacing like for like a Zs should be sufficient as the R1 + R2 was tested at the time of installation and was deemed to be satisfactory (or was it???). Whereas taking a reading of the Zs will confirm that when you removed the accessory and replaced it, you managed to get the green & yellow coated copper bit all the way in the terminal, and, perhaps more importantly, you can confirm that the circuit you worked on will disconnect under fault conditions.

On the model forms in the Regs there is a tick box for 'earth continuity: satisfactory' so that does not require an R1 + R2 reading to be imputed but a continuity test to be carried out BEFORE you reinstate the supply to carry out the live test (Zs). Some versions of the MW form do ask for a reading to make their certificates look more comprehensive, but that only leads to sloppy testing in most cases. I get the feeling this guy has been shown the short cut way to fudge readings and likes to show others how to.
 
Oh dear the electricians perennial debate rears it's ugly head again, which is the more accurate Ze + (R1+R2) or Zs measured direct, this could be a long thread rivalling exported earths.
 
Simple, Zs. Its the 'active' reading taken when the cores are in normal working conditions, at the highest temperature and highest resistance and the reading is the most onerous. Whereas R1 + R2 ensures you have sufficiently low resistance to safely energise the supply.

If you are checking your cables are intact then that is R1 + R2, if you are checking to see if your work will not kill some poor unsuspecting member of the public then that's your Zs.
 
i cant figure out why all the variants have come from on installation certs, why couldnt they all simply reproduce the ones from the regs without deciding they need to add more info to them.

personal hate is the ammount of points box.
 
keefy said:
Simple, Zs. Its the 'active' reading taken when the cores are in normal working conditions, at the highest temperature and highest resistance and the reading is the most onerous. Whereas R1 + R2 ensures you have sufficiently low resistance to safely energise the supply.

If you are checking your cables are intact then that is R1 + R2, if you are checking to see if your work will not kill some poor unsuspecting member of the public then that's your Zs.
Click to expand...

Sorry I misread the post, R1 + R2 is more ACCURATE, but Zs is more important. Yes the reading of Zs depends on whether the circuit is under load, but normally there is some sort of load, especially on lighting circuits... I love switching them all on for the first time!
 
An experianced tester when carrying out an R1R2 test will have a rough idea of what results to expect , some times when you do a test you can get a high R1R2 and when you carry out a Zs test you get a lower Zs reading than you should get ,this could be due to parrallel earth paths, so realistically you should do a dead R1R2 test at more than one point ,because if your last point is on the bathroom lighting circuit and it has a supplimentary bonding conductor from the light to a shower as it should be to 16th ed then the reading you could get would be incorrect due to the paralle earth path , how ever this should be disconnected prior to the Zs any way , im a firm beliver in doing it properly R1R2 ,IR,Zs,RCD,Ramp , than you know your readings are correct
 
and into the equation comes the internal resistance of MCB and RCD........affecting the value ofZs.
 
Glad im not with the NIC then , they should get a life , so how do they do a dead Ze test , and its good to be a naughty boy ,lol , gee ill have to wear my wellies to do it then , think they need to get out of there offices and come into the real world
 
nickblake said:
Blimy telex you had your sugar puffs this morning then
Click to expand...
just had me brekky. coffee and a smoke. nailed that earth fault. Ze=0.16 TNC-S, Zs 347. RCD internal resistance 347. sheesh. bloody MEM rubbish.
 
telectrix said:
just had me brekky. coffee and a smoke. nailed that earth fault. Ze=0.16 TNC-S, Zs 347. RCD internal resistance 347. sheesh. bloody MEM rubbish.
Click to expand...

Happy days mate you will know to look out for that again.

This is why a Zs test must be carried out after R1 + R2 as it is the only test that actually goes full circuit. (not aimed at you Tel but for general info)


Sent from my iPhone using Tapatalk.
 
Has anyone read the article in Marches Proffessional Electrical magazine regarding 'live working' taken from the HSE document.

Seems we can after all...........
 
Thanks Guys, apparently we have to have a MWC, thought I was right about R1+R2 calcs, this web site is so good for scratching the head and finding info however trivial it may seem. Its a huge world of Electrical knowledge we have to have in our heads, we can't pretend to have all the answers.

Thanks Mickeysparks
 
Because when you measure Zs, you could have parallel paths (pipework etc.). When you measure R1 + R2 you should be measuring just the cable resistance, ie. conductors not terminated at a busbar.
So generally worst case Zs = Ze +( R1 + R2), and when you measure Zs it will probably be less (due to the parallel paths). Thats the theory anyway!
 
except for the aforementioned parallel paths, resulting in a lower Zs than from calc. from R1+R2. if you calc. using R1+R2+Ze, that will give you the expeced value of Zs. if you then measure Zs and get a higher value, that's when you start worrying what effect has the MCB and RCD on the result.
 
then you clip your green lead on to the armour/pipework
 
do you mean testing R1+R2 when due to containment being the cpc, you can't eliminate parallel paths?
 

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