RCD problem during test

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Dave.C

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Hi All

I am relatively new to the electrical scene, I have recently carried out some work on a new property ( less than 12 months). I finished off by carrying out a minor works cert, however upon testing the RCD on auto test  ( Megger 1552 using three lead method)  the tester kept reading TRIP, I assumed I must be doing something wrong so checked my leads were in the right hole and on the right terminal still nada, I moved to some sockets on the same RCD and used the plug and lead and got the same problem.

  I then moved to a ring on the other RCD and success, my tester was recently calibrated and works correctly on other circuits so my only conclusion was that the RCD is fritzed,  I have managed to get hold of the original EIC and it gives the readings.

 I have looked across tinterweb for info and cannot find any answers so have come to you all in the hope you can advise / reassure as it seems odd that the rcd should pack up in a short time.

many thanks in advance

Dave

 
Thanks for the replies

I just wanted to double check, as the house is brand spankin new, I assume the board must be under some sort of warranty so will advise her to go to builder / developer

Dave

 
Hi Dave, I assume it was tripping at 1/2x rated trip current and that it is a dual rcd board, did you do the test with all loads disconnected? There may already be earth leakage currents on that RCD side of the board, and adding 15mA could tip it over the edge. Best to check before going down the warranty route.

 
trp on 3 lead test is often a loose test lead, usually the neutral. it may also say that if it trips on 1/2 test, but it will never say trp if the RCD fails to trip when it should

did you try testing the RCD in isolation?

 
Canoeboy said:
Not sure where you got that, I found this manual http://docs-europe.electrocomponents.com/webdocs/07f1/0900766b807f1e4c.pdf where it says a two-wire test (or mains plug) is needed and that 0 and 180 tests are required for all values of trip currents.

I also noticed the 1 xI symbol in the display box, it shouldn't say trP at 1xI, as Andy pointed out.

Edit; does that mean that for type AC RCDs the 0 and 180 tests will have the same result using this megger?

 
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Canoe Boy

 That is my opinion, tripped before test could be completed although I think I need to do as Rob the Rich suggests and test whilst no loads are on.

As the same problem did not occur upstairs / on other RCD, there must be an issue with an appliance / load.

 I have found some other info and it states

If you get 'TRP' displayed on the Megger it either means you haven't connected probes correctly or there isn't an earth path. I speak from experience...

Jim


This was from a Screwfix forum, I know I had good probe connections so can only put it down to earth path, however, I had an R1+R2  reading of 0.39 from garage to main D/B, where the original cert shows 0.61 and Zs measured 0.22 vs 0.94 on cert which seems a huge difference.

I will go back to check with all loads disconnected

Thank you all for the pointers and advice

 
Andy

my apologies. 

 Yes the rcd had tripped on this test and the same alert is given during loop and pfc if you get the leads wrong. It will trip the rcd and show trp on the screen. 

     I have had it during loop and pfc but this the firs time on the auto rcd check. 

 
skip the vital details, get wrong answers....

unlikely to be the tester or any leads. most likely faulty RCD being too sensitive. id start with ramp test then in isolation

 
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