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Topic Title: Low current Zs test tripping RCBO
Topic Summary:
Created On: 24 March 2019 08:53 PM
Status: Read Only
Linear : Threading : Single : Branch
 Low current Zs test tripping RCBO   - Sparkymania - 24 March 2019 08:53 PM  
 Low current Zs test tripping RCBO   - mapj1 - 25 March 2019 09:24 AM  
 Low current Zs test tripping RCBO   - chrispearson - 25 March 2019 11:52 AM  
 Low current Zs test tripping RCBO   - RB1981 - 25 March 2019 03:26 PM  
 Low current Zs test tripping RCBO   - KFH - 25 March 2019 04:37 PM  
 Low current Zs test tripping RCBO   - RB1981 - 25 March 2019 04:41 PM  
 Low current Zs test tripping RCBO   - Sparkymania - 25 March 2019 08:03 PM  
 Low current Zs test tripping RCBO   - mapj1 - 25 March 2019 09:14 PM  
 Low current Zs test tripping RCBO   - Sparkymania - 25 March 2019 09:29 PM  
 Low current Zs test tripping RCBO   - AJJewsbury - 26 March 2019 09:34 AM  
 Low current Zs test tripping RCBO   - Sparkymania - 26 March 2019 10:46 PM  
 Low current Zs test tripping RCBO   - Sparkymania - 27 March 2019 06:55 PM  
 Low current Zs test tripping RCBO   - RB1981 - 27 March 2019 08:02 PM  
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 26 March 2019 09:34 AM
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AJJewsbury

Posts: 17795
Joined: 13 August 2003

I don't see why this would happen as the tester is only supposed to pass a 15mA test current on low whether 2 or 3 wire.

I'm guessing now, but I suspect it's really a bit more than 15mA (to make the voltage measurement easier) - but only as a very short pulse which makes it sort of equivalent to <15mA r.m.s. when averaged over a decent chunk of the waveform. Traditional RCDs were easily fooled by such tricks, but the newer ones with lots of extra sensitive electronics in them so they can detect partial waveform residual currents (i.e. A types and better) are more likely to spot this kind of thing and trip - although of course there'll be a lot of variation in these edge cases from one design to another.. As we move away from AC type RCDs to A, and B types (not to mention and F and K and EV etc) well see a lot more of these kinds of problems. Maybe we'll give up on live loop tests on final circuits and just test for Zdb live and use R1+R2 or similar to prove continuity & Zs to the far ends.

- Andy.
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