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Discuss Hi Zs reading through rcd. in the Electrical Forum area at ElectriciansForums.net

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Morning all,

having a bit of a fun time with my tester at the moment. Bear with me as I'm still in training.
Ive got a Dilog 9083p. I know it's not regarded as the best tester out there but like I said, I'm just starting out.

The problem I have is this. Ive tested Ze at the board and all is good. tested my r1+r2 and all is good. when it comes to testing Zs on the circut it comes up really high almost double. for example my sockets came out at 1.5 ohms where as the calculated figure using Ze+(r1+r2) comes out at about 0.7 ohms.

The instructor then says.. now test it with the fluke. And I got a figure of 0.68 ohms. I tried the same test with a metrel and a Megger and they all came up high. only the fluke was anything near accurate. what causes this and is there any way around it? I know theres nothing wrong with doing the calculations but if I didnt have another tester to use, it would leave me wondering if there is a problem with the RCD or breaker if that makes sense
 
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James

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Morning all,

having a bit of a fun time with my tester at the moment. Bear with me as I'm still in training.
Ive got a Dilog 9083p. I know it's not regarded as the best tester out there but like I said, I'm just starting out.

The problem I have is this. Ive tested Ze at the board and all is good. tested my r1+r2 and all is good. when it comes to testing Zs on the circut it comes up really high almost double. for example my sockets came out at 1.5 ohms where as the calculated figure using Ze+(r1+r2) comes out at about 0.7 ohms.

The instructor then says.. now test it with the fluke. And I got a figure of 0.68 ohms. I tried the same test with a metrel and a Megger and they all came up high. only the fluke was anything near accurate. what causes this and is there any way around it? I know theres nothing wrong with doing the calculations but if I didnt have another tester to use, it would leave me wondering if there is a problem with the RCD or breaker if that makes sense
is this on a test rig or real life installation?

sometimes the test rigs will give different results with different testers due to the way they have been configured.
i.e. extra resistors etc hidden at back of rig to simulate different scenarios.

have you asked the instructor why you are getting different results with different testers?
 
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it's in a training centre but you wire your own circuits and then test them. so we do Showers, cookers, ring SWA to a garage and various lightind circuits. there's no faults built into this one although I have that coming with the 3 phase part next week.
I asked the instuctor and he just says "thats because Dilog is rubbish" I asked if Metrel and Megger were also rubbish in that case and he didnt have a reply. when I showed him what I'd found with the other testers he just giggled and said, just use the Fluke in future. lol
 
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James

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it's in a training centre but you wire your own circuits and then test them. so we do Showers, cookers, ring SWA to a garage and various lightind circuits. there's no faults built into this one although I have that coming with the 3 phase part next week.
I asked the instuctor and he just says "thats because Dilog is sh$t" I asked if Metrel and Megger were also sh$t in that case and he didnt have a reply. when I showed him what I'd found with the other testers he just giggled and said, just use the Fluke in future. lol
I have also seen, in places where more than one person is testing at the same time. 1 tester interfering with another and getting some strange test results.
 
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I have also seen, in places where more than one person is testing at the same time. 1 tester interfering with another and getting some strange test results.
yeah could be that I suppose... just leaves me doubting my tester. I know we should always do the calulation to check against the measured but it seems a bit pointless if you cant trust the measured reading. If I did then every single circuit would fail on max Zs :D
 
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Quoted from the July 2018 Megger article...
"RCDs and RCBOs usually incorporate a tiny transformer with a winding connected in series with the live conductor, and another winding in series with the neutral conductor. This transformer detects the difference between the currents in the live and neutral conductors, which indicates the presence of earth leakage."

IMO, Megger of all people should check their terminology is accurate to the latest regs.
 
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this illustrates my problem exactly. looking at the megger site they now reckon uplift can add as much as 1 ohm1?
 

DPG

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this illustrates my problem exactly. looking at the megger site they now reckon uplift can add as much as 1 ohm1?
Yeah, that's a huge difference. And makes the measurement using that method largely pointless.
 

Andy78

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I'd be taking resistance readings across the RCD and measuring Zs at the RCD on the load terminals and the supply terminals. As a matter of curiosity and education.
I have honestly never seen readings skewed this far from predicted (calculated) values. Only small amounts attributable to the inaccuracy of a low current Zs test.
 
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