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Discuss 10mA rcd tripping on Zs test in the Electrical Forum area at ElectriciansForums.net

polo1

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As above, taking a Zs measurement on a circuit covered by a 10mA rcd. Tripping the rcd on a no trip, three lead test using a Megger 1721. Is there a change in the settings I’ve forgotten about to allow a Zs to be taken. Oh, testing at a socket rather than source.
TIA
 
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Sintra

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Link out line and cpc at the end of the circuit and test using no trip setting at the device.
 

Ian1981

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If your going to the trouble of linking the line and cpc at the end of the circuit then you may as well measure the R1+R2 and add it to the Ze/zs/zdb or am I missing something ?
 

polo1

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Arms
  • Thread Starter Thread Starter
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For some reason I thought there was an option to reduce the test current on the Megger, but it appears there isn’t.
So Ze +(R1+R2) it is.
Thanks guys.
PS can’t remember the last time I had to test with a 10mA rcd in place!
 

Sintra

Admin
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If your going to the trouble of linking the line and cpc at the end of the circuit then you may as well measure the R1+R2 and add it to the Ze/zs/zdb or am I missing something ?
If you have the line and cpc linked for R1+R2 then for the sake of a few extra seconds just test the Zs at the device instead of calculating.
 

Ian1981

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If you have the line and cpc linked for R1+R2 then for the sake of a few extra seconds just test the Zs at the device instead of calculating.
Is that an approved test method?
Never seen that method before, not in gn3 anyway.
 

Sintra

Admin
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Is that an approved test method?
Never seen that method before, not in gn3 anyway.
Not sure about GN3 but it is in the book "Practical Guide to Inspection, Testing and Certification of Electrical Installations" It was common practice before the introduction of no trip settings on meters.
 

Loki

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For some reason I thought there was an option to reduce the test current on the Megger, but it appears there isn’t.
So Ze +(R1+R2) it is.
Thanks guys.
PS can’t remember the last time I had to test with a 10mA rcd in place!
I know you can change the mA, see 5:40 onwards. 10mA is the minimum setting.




Edit: I've just looked through the megger manual & there's no mention of changing the trip current
 
Last edited:

PEG

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Yep,you would need about 5mA ....not sure that would warrant a good enough test...i think rubbing me wooly jumper,produces more :)
 

SparkySy

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Arms
I tend to favour the link out and test at high current method, just because it's what I've done for years, and it gives me a more accurate measurement than the low current test most of the time, also it amazes me sometimes how much an rcd will influence the result if testing both way. BG rcds seem to be the worst for raising the Zs readings!
 
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