zs for cert

[crow11]

hi all, been doing some testing today on the new lighting circuit and sockets i have put in the garage for my first assesment, everything tests ok my lighting circuit(6A mcb) R1 +R2 is 0.59, and the sockets(radial on 20A mcb) R1 + R2 is 0.28, my ze at origin is 0.14 so calculation of zs for each circuit would be roughly 0.73 for lights and 0.42 for sockets, i use the megger mft1552 which i have to do the zs test on no trip 3 lead test(dual rcd board) i know the reading on the actual test are going to be different to my calculations slightly, they come back at lights 0.83 and sockets 0.52 both circuits are 0.10 higher on the actual test probably because of the impedance in the rcd,s, everything still well within the limits so no probs, what would you advise to put on the cert the calculation or the actual test results, only asking as when the assesor looks at the cert's ze and R1 + R2 and i have put the test figure's in would he understand that they are 0.10 different because i have had to do it on no trip loop because of rcd,s read alot of thing on net saying put the actual test figure's in then lots say just calculate it cheers

 

[BigTom83]

I would use you're measured result. I don't know but I would imagine he would ask why you have done calculation and not measured.

 

[moospark]

Put the measured result in always, the .1 will be a parralel earth.

 

[Deleted member 9648]

TBH I dont think there's any reason to be too picky over Zs results where a 30ma RCD protects the circuit
1. because the max Zs is effectively 1667 ohms....so as long as the measured zs is ballpark with the calculated a fraction of an ohm harly matters.
2. because the two lead no-trip Zs test is wildly inconsistant/unreliable at best.....if the meter manufacturers are prepared to sell us inaccurate instruments and insist they meet the requirements why should we worry about recording accurate results?

 

[telectrix]

you would expect the measured Zs values to be lower than those calculated, due to parallel paths. however, as you said, the values could be higher due to the RCd or MCB affecting the reading. also, the no-trip readings are not always accurate. the whole issue of measured against calculated is a compromise between getting a measured value, and safety. on sockets, it's easy.... use a plug-in lead. on lighting, as you need to partially disassemble a fitting/switch, and clip on leads, h&s recommend a calculated value. boils down to live working vs live testing. best way is to do it the way you feel comfortable with. either way is acceptable.

 

[loop3]

Put in the measured result. You assessor, college or Nic/other? Silly question, but did you null the leads? Also are you using croc clips or the pins? + he he gets you to test it in front of him, better to have the same results, don't go down the road of calculating. It's not good.

 

[SirKit Breaker]

Put your measured results in, no explanation needed to your Assessor, he is an Electrician, and will know why your measured and calculated are a bit different. If he doesn't, then he shouldn't be assessing Electricians.

Cheers.........Howard

 

[crow11]

thanks all for the replies and help, the leads were nulled on the continuty test, no option to null them on the no loop test but i always set to continuty when i first switch megger on and null anyway,sockets where done with kewtech plug adapter, light was done with 1 croc clip on cpc and pins for L & N and then light mate adapter to double check cheers

 

[Octopus]

thanks all for the replies and help, the leads were nulled on the continuty test, no option to null them on the no loop test but i always set to continuty when i first switch megger on and null anyway,sockets where done with kewtech plug adapter, light was done with 1 croc clip on cpc and pins for L & N and then light mate adapter to double check cheers

Those things can throw up strange results too.

 

[SYKRAPS]

Insert your measured results and explain to him the differences you found using calculations, I think he will like the fact that you are aware differences occur and the reasons for why

 

[ipf]

Put in the measured result. You assessor, college or Nic/other? Silly question, but did you null the leads? Also are you using croc clips or the pins? + he he gets you to test it in front of him, better to have the same results, don't go down the road of calculating. It's not good.

Nothing wrong with calculating. What's all this about nulling leads for a loop test ??
Just give him the facts, whichever way you record.

 

[brucelee]

I have found the same with my 1552 always higher than calculated when doing the no trip loop test with 2 leads the megger doesnt need 3 unlike the fluke which requires the neutral lead to be connected to power the meter i believe well was told

you could have jsut input the calculated value either way is acceptable as long as it meets the max measured Zs values of GN3 or OSG

 

[nickblake]

If you have carried out the r1 and r2 as you have correctly stated the ze added to that will give you your calculated zs , when im testing i will calculate the zs by using r1r2 + ze this then gives me an idea of what the zs should be so if the measured test resaults are around that then im happy ,as has been mentioned you can get variants of the measured reading due to RCD's and MCB's and paralel earth paths