RCD test fault

[wayneward25]

Hi

I was testing 2 new rcd's yesterday on a split consumer unit.

They are bs-en type the first one tripped in 23ms and the 2nd tripped within 25ms but when i tested x5 i was receiving 1999ohm error?

I tested them at the consumer unit and still got the same error.

The test buttons work fine.

Are the rcds faulty?

 

[RF Lighting]

Did you test the RCD's with nothing connected to the load side?

 

[wayneward25]

they were still connected but I turned the mcb's off

 

[JohnW2]

They are bs-en type the first one tripped in 23ms and the 2nd tripped within 25ms but when i tested x5 i was receiving 1999ohm error? ... I tested them at the consumer unit and still got the same error.

Is your meter not simply telling you that the Zs (or Zdb/Ze, when testing at the CU) is too high to undertake a x5 (150mA) test? AIUI, my Fluke MFT refuses to do an RCD test if a pre-test indicates that the test current would result in a fault voltage >50V (i.e. if Zs is greater than about 333Ω for a 150mA test, 1667Ω for a 30mA test).

Is this a TT installation? What is the Ze?

Kind Regards, John

 

[wayneward25]

Its a PME with a ZE of 0.07ohms

 

[JohnW2]

Its a PME with a ZE of 0.07ohms

Fair enough. Do I take it that you get a suitably very low Zs or Zdb when you measure in the circuit or at CU (i.e. confirming that the MET is actually connected to the CU!!). If so, I don't know. What meter are you using - my experience is primarily of Flukes, and they don't produce the "1999 ohm error" you mention, they just don't do the RCD test if the loop impedance is too high.

Kind Regards, John

 

[wayneward25]

Ive got a zs on a 6mm cooker radial circuit of 0.27

Immersion heater circuit with 2.5mm radial of 0.15

2.5mm ring circuit of 0.40

Im using a telarus unitest 0100 multi function tester

 

[JohnW2]

Ive got a zs on a 6mm cooker radial circuit of 0.27 ... Immersion heater circuit with 2.5mm radial of 0.15 ... 2.5mm ring circuit of 0.40

Fair enough - that's obviously all very reasonable.

... Im using a telarus unitest 0100 multi function tester

Are you sure that you were getting a ">1999ohm" error? According to your meter's manual, that should (not unreasonably) only arise with loop impedance or 'low resistance' ('continuity') measurements, not RCD ones. If the error you were getting was, in fact, ">1999ms", then according to the manual, that means:

Trip time exceeds 1999 ms or RCD has not tripped (only at nominal fault current I∆N = 5 x 30 mA)

... which really would make no sense, given that your x1 test gave satisfactory results.

I'm therefore running out of ideas. Is it possible that your meter is misbehaving (e.g. because of failing batteries)? Could you try x5 measurements on some other RCDs?

Kind Regards, John

 

[wayneward25]

Yea sorry it just read 1999 which i just presume to be ohms because thats the reading i get when i do my insulation resistance tests. I had a faulty rcd before that didnt trip and it came up with an error exceeding 500ms maximum limit. so thats why i wasnt sure with this error

 

[wayneward25]

my meter was only calibrated and tested last week so i would hope itl is in working order

 

[JohnW2]

Yea sorry it just read 1999 which i just presume to be ohms because thats the reading i get when i do my insulation resistance tests. I had a faulty rcd before that didnt trip and it came up with an error exceeding 500ms maximum limit. so thats why i wasnt sure with this error

Fair enough. As I said, per the manual that error means that it didn't trip (at x5) within 1999 ms - which makes absolutely no sense if you were getting tripping in 23/25ms with x1. That's why I'm suspicious of your meter. As I said, can you try x5 tests on other RCDs?

Kind Regards, John

 

[wayneward25]

Ive just done a x5 test on another house im at with the same tester and it was perfect 12ms ?

 

[JohnW2]

my meter was only calibrated and tested last week so i would hope itl is in working order

Very odd. As I've said, it makes no sense that an RCD should trip in 23/25ms with a 30mA test current yet not trip in 1999ms with 150mA.

If it were me, and despite the recent calibration, I would want to see it giving sensible x5 readings on some other RCDs.

Kind Regards, John

 

[JohnW2]

Ive just done a x5 test on another house im at with the same tester and it was perfect 12ms ?

Hmmm - I think I will have to give up! If you continue to get satisfactory x5 results on other RCDs but the original ones persist in giving this error, I suppose you would have to 'presume' that both those original RCDs were suffering from extremely bizarre fault, which resulted at them tripping with low current imbalances but not with high imbalances - even though that sounds very far-fetched to me.

I suppose you could at least temporarily swap one of those new RCDs for a different one (perhaps a different make, in case there is a 'batch problem') and see if that makes any difference.

Maybe we're both missing something obvious, but I can't think what. Did you try testing the RCDs at the CU without (literally) anything connected to their 'output' side?

Kind Regards, John

 

[wayneward25]

I believe i may have found the issue.

I changed my test leads for another pair and it wasnt working just i got the 1999ms reading yet 2 minutes ago with the different leads it tripped fine.