r1+r2 too high

captainconduit · Jul 1, 2010

hi folks

Carried out all my r1+r2 tests in db and recorded, carried out ze and zs tests.Comparing the results, my r1+r2 are higher than my zs in some circuits, i have read its earth parrallel paths. What results should i record on test sheet as formula zs= (r1+r2) +ze suggests my results are incorrect, can anyone help

Deleted member 9648 · Jul 1, 2010

Record the measured R1R2 and the measured Zs....that is the actual Zs and so it is that which will determine the fault current and the disconnection time when the circuit is energised....not the calculated value.

caddy · Jul 1, 2010

(R1+R2)+ZE is a "worst case" since it is the actual ZS of that particular circuit. As long as you record the actual R1+R2 value then you can enter either the measured or calculated ZS.

Sparky Ninja · Jul 2, 2010

Always recording the calculated values looks suspect and can cause aggrevation on any assessments from the like of the NICEIC.
As ZS values have to be taken with the main earthing conductor connected to the system, it is always expected for the parallel paths to affect the readings, causeing a lower than expected reading.

However... if carrying out the reading would cause you exposure to live parts.. i.e. a fcu or light fitting, then this can conflict with the building regulations and so should be calculated.

My rule of thumb is to measure ZS on sockets, and calculate on light circuits etc.. NICEIC's Tony Cable explains all this in one of thier DVD's and so I always use it and they never have a problem.

caddy · Jul 2, 2010

widdler said:
Always recording the calculated values looks suspect and can cause aggrevation on any assessments from the like of the NICEIC.

If you can either measure ZS or calculate it why on earth would it look suspect and why would it cause aggrevation..

sledgehammer · Jul 2, 2010

Always record what you measure. No one can criticise if you are truthful. If you took your NICEIC assessor around he'd expect to see exactly what you've recorded. It always provides a good talking point aswell. The only reason you can't disconnect bonds, energize circuit and test to get a "clean" result is because of the safety implications. You'd only do it out of interest and once you've reconnected the bonds you could compare. Domestically this is less relative because of the new plastic water mains and gas service pipes. In large commercial and industrial installs it makes a huge difference.

caddy · Jul 2, 2010

sledgehammer said:
Always record what you measure. No one can criticise if you are truthful. If you took your NICEIC assessor around he'd expect to see exactly what you've recorded. It always provides a good talking point aswell.

I quess all assessors think differently then, on a previous commercial unit install we calculated ZS from R1+R2 this provided a good talking point as we recorded the "actual worst case" ZS and as long as you can explain why caculated and measured ZS values will vary then thats all thats needed.

sledgehammer said:
The only reason you can't disconnect bonds, energize circuit and test to get a "clean" result is because of the safety implications. You'd only do it out of interest and once you've reconnected the bonds you could compare.

You would never ever disconnect all bonds and re-energize the circuit to test.

I would fully agree that measuring ZS of a circuit during a PIR is perhaps more acceptable if you cant isolate circuits. However for a new installation there is no reason that recording a calculated result is fraudulant, suspect, or a source of aggrivation.

sledgehammer · Jul 2, 2010

When I design from plans you have to use the standard Ze qouted in the osg. If you then base all circuits on this,any parallel paths are a bonus and you'll never be over max Zs. Based on that you can't be wrong if you add r1 + r2 to Ze from on site measurements. If your measured Zs differs it's fine because we all know why and I can't think of a reason why it would be worse. I actually have a book telling you to disconnect bonds for Zs readings!! My NIC assessor got me to measure Ze with and without bonds ( iso off of course) to show plastic services have no affect on Ze.

Sparky Ninja · Jul 3, 2010

caddy said:
If you can either measure ZS or calculate it why on earth would it look suspect and why would it cause aggrevation..

Because when I assess guys and realise they've all done calculations, I'm required to ask them to demonstrate obtaining the values by measurment to prove their competance. You'd be surprised how many people don't understand how to carry out live testing. Furthermore it has become apparent some sparks are now predicting these R1&R2 measurments, and using the calculations reason as an excuse for all their readings to be incorrect.

sledgehammer · Jul 3, 2010

Are you saying that people are actually using design figures for their test results? They could fill out the form before the job has begun. Nice!

Sparky Ninja · Jul 6, 2010

I've seen it done before. I collared a guy working for a London loft conversion company. I was inspecting it for an initial verification and asked the guy to hand me the initial design. He gave me the pack issued from the office, which included all the the test certificates post dated prior to the installation being carried completed.

It was upon having reported this, and finding out that the company were fined and not removed from the enrolment that I decided to stop doing inspections for said company and have since switched to two others.

sledgehammer · Jul 6, 2010

Ah HA, i know who you mean by enrolment. You can't make money from somebody being excluded. We've called in the big guns on a couple of instances and they did nothing! Contractor drives around still badges on van etc. They didn't even get him to fix it all.