Low current Zs test tripping RCBO

I think most (usually A-type) RCBOs these days are much more sensitive to residual currents that aren't nice pure sine waves than the traditional AC type RCCBs. That makes life much harder for loop testers - it's difficult to get an accurately measurable change in voltage from a maximum 15mA leakage current - so they usually resort to all sorts of tricks to use larger currents for very short durations to try to work out the loop impedance. In 3-wire mode where the meter has access to the L-N voltage and L-N loop it can deduce a certain amount from high current tests on that loop without risking tripping any RCD (even if it's only monitoring the changes in voltage caused by other loads) - in 2 wire mode everything has to be done on the L-PE loop so tests might have to be slightly more agressive for the L-PE loop or repeated more times to get a plausible reading - all of which gives an RCBO extra chances to trip.


I've also had problems using 3-wire tests on some low rated (<=6A) RCBOs - seemingly (and annoyingly) they seem to be tripping on overcurrent rather than residual current - i.e. it's the :large currents on the L-N loop that's tripping them rather than the small L-PE test currents.


  - Andy.

My Megger 1741 trips Wylex RCBOs. My Megger LTW and LRCD220 do not if that helps.


JP

Thanks John.


Have you come to a view as to why this is the case?

Thanks for the replies everyone.


The RCBOs are all MK type AC (tested on AC setting). Fairly new as they are in a 3rd amd DB. I always test using the correct AC or A setting. Never had a problem like this until now. 


Why would a low test trip even a < 6A RCBO? The L-E test current is only 15mA. How much neutral current is there on the 3 Lo test? I never thought if it as being high. I thought it was just using the neutral to drive the tester but, as AJJewsbury says, it is testing the L-N as part of the L-E test. It seems, then, that the L-N current is in amps rather than mAs.


Still can't see how 15mA L-N is tripping all the RCBOs on the 2 wire though. They all passed the 1/2 test when RCD tested. If they are doing this on the test surely there would be a problem with nuisance tripping on loads with earth leakage up to that same amount which would render those RCBOs useless in many applications.

Still can't see how 15mA L-N is tripping all the RCBOs on the 2 wire though.

I suspect it's actually pulses that peak at a something above 15mA - but short enough to be 'sort of equivalent' to <15mA r.m.s. when viewed over a fair chunk of the waveform. Traditional RCD designs were easily fooled by such tricks, but some of the newer designs (intended to be more sensitive to pulsed residual currents) can spot the pulse and trip.


In some ways its sounding like a re-run of the D-loc problems - some earlier RCD testers added a large d.c. component to the loop test current to 'lock out' traditional RCD designs (by saturating the torroid) and so stop them tripping when the large loop test current flowed - but some designs (even though they were nominally still "AC" RCD types) weren't fooled and tripped anyway.


   - Andy.

Hello,  I have also had this issue whilst Zs testing 2 lead low with a megger MFT1730.  Just bought a 1741 for the works kit and I've not used it for Zs testing on two lead yet.  I'm curious now.

I'm going to throw confusion into the works though, because I was advised to swap the two leads over at the Megger - so plug the green into the red slot and the red into the green slot.  I don't know what the machine does differently down the cores but it works a treat.  Also with the plug tester, plugged in back to front on two leads.

Zs

I suspect the meter does nothing different with the leads crossed, but the electronics inside that particular model of  RCD or RCBO will be more sensitive measuring asymmetric wave-forms one way than the other.

AJJewsbury

I remember around 2003 using a Robin D-Lok on a new build hospital after using it to test Zs of circuits on basic RCDs. I then started to test Zs on circuits with RCBOs. These testers didn't have batteries and the display came to life when connected to the circuit under test. Every time I pressed the test button the display would disappear and come back after a delay of 1 to 10 seconds.

It took an hour to work out what was happening.

I suddenly occurred to me that the DC lock would not be working on a device that is using electronics to amplify the residual imbalance. I couldn't work out why, though, the power was going off then back on after a delay.

So I decided to go down to the DB from the roof and there was one of the apprentices standing there. "The breakers keep on tripping so I reset them". I could have killed him.


Zs

I have sometimes tested with the probes the wrong way round accidentally but found it still tested out the same when I realised and swapped them. I wouldn't have thought of swapping them round in the case of this spurious tripping. Thanks for that. Unfortunately, I may not be going back there so I will be unable to try that out. As this is the first time this has happened it must be quite rare and I would need to try it on that specific DB were I know it's happening.

Today I tried the L and E swap on an RCBO type A after installing some sockets in a container. It didn't work. 2 lo test worked fine, gave me the result. Then I thought I'd try Zs's idea or swapping the leads (or just changing the position of the probes; same thing). It tripped the RCBO. So i can't see how that method can be used to overcome the original issue.


Also, got someone to look at the RCBOs in question while they were there doing something else. They are type A. Now I realise something that I had done wrong. I can't believe that I had done this after advising others about changing the tester setting from AC to A when they had problems. ("Change it from sine wave to sine wave with pulsating DC. What do you mean "what do you mean"? OK. Change it from squiggly line to squiggly line on tits").

Mind you, it was the electrical laborer who was actually doing the Zs tests but that is no excuse as I should have realised. The tests were being done on AC setting. I can't believe I made such a stupid mistake. Unfortunately, I can't confirm that at the moment but there is still the server room and main switchboard with integral DBs to finish off and hopefully, it will be me that does them so I can test our whether this was the issue. I will post back on here if I can do that.