Discuss RCD tripping while Zs in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

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nicsin02

Hi guys, what would cause an RCD to trip when carrying out a Zs test, it is a lighting circuit in a new build house and I have it on the "no trip" setting and it still trips the RCD, the RCD itself passed the RCD tests and the IR is 800Mohms so that's fine, I even even replaced the RCD but the problem still exits, just a note it is a dual RCD board with each RCD protecting 4 mcb's and all other Zs test are fine its just the lighting circuit, any ideas guys, thanks in advance
 
"no trip" unfortunately doesnt mean what it says. certain rcds and test meters are incompatible for rcd testing on any scale.
 
What make of meter are you using?
 
It wasn't actually myself doing the testing it was the tester from my old company that fned me to see if I had any ideas, it is a robin ELI tester, he has been using it for years and he tested a few properties after the above mentioned and everything was fine so I don't think the tester is a fault
 
is the non trip test on your meter jut 2 wire, some older meters have to have three leads for the non trip test.
 
It wasn't actually myself doing the testing it was the tester from my old company that fned me to see if I had any ideas, it is a robin ELI tester, he has been using it for years and he tested a few properties after the above mentioned and everything was fine so I don't think the tester is a fault

If the guy was using a Robin yellow brick with D-lok (eg. KMP4120) it will not trip most passive RCDs, but the DC blocking method of 'No-trip' that the Robin uses is likely to be ineffective and trip a lot of modern sensitive RCDs, most RCBOs fall in to this later group.
 
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inb this case, the answer is to bridge out the RCD for testing. don't forget to remove the links after.
 
If everything else is ok, then generate your ZS value through calculation
ZS=ZE + (R1+R2)

Calculation is encouraged as an alternative to live measurements!
 
i usually guess it anyway. NOT. :wink_smile:
 
We had the exact same problem today doing a Zs test kept tripping the circuit breakers in the control panel. Some of the breakers were 2A and 4A when tried with a higher breaker it wouldn't trip.

Sent from my GT-I9300 using Tapatalk 2
 
If your doing a 2 wire Zs test and the RCD trips try swoping the leads over so E on line and L on Earth. That was megger's answer and it works.
 
We had the exact same problem today doing a Zs test kept tripping the circuit breakers in the control panel. Some of the breakers were 2A and 4A when tried with a higher breaker it wouldn't trip.

Sent from my GT-I9300 using Tapatalk 2
The reason why your MCB rated at 6A or less trips is because your loop tester sends a current of around 25A around the circuit during the test. The MCB recognises this as an overload condition and will trip. To overcome this you need to change the scale setting on your tester from 20 ohms upto 200ohms and repeat the test.
Unfortunately the only way to prevent an RCD tripping under test conditions is to use a loop tester with D-lock facility. Under no circumstances should you 'bridge-out' an RCD as this in breach of the EAW & H&S regulations.
 
A clever person on here once pointed out that you can get Zs by having the main switch on, the RCD off and the MCB on. Then bridge between line-cpc at your light fitting (or wherever) and then Zs test across the open RCD.

Laurie
 
if i remember correctly, it was sintra who posted that useful tip.
 
The reason why your MCB rated at 6A or less trips is because your loop tester sends a current of around 25A around the circuit during the test. The MCB recognises this as an overload condition and will trip. To overcome this you need to change the scale setting on your tester from 20 ohms upto 200ohms and repeat the test.
Unfortunately the only way to prevent an RCD tripping under test conditions is to use a loop tester with D-lock facility. Under no circumstances should you 'bridge-out' an RCD as this in breach of the EAW & H&S regulations.

sorry, you're incorrect. the current generated by the tester on no trip is mA, that's 1 reason why the readings are not always accurate.
 

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