RCD problems

[jrogers]

My first post here, though I have used the site many times to get answers on a few things.
I fitted a 10 way dual RCD board a few days ago in a domestic property. Everything was going absolutely fine until I reached the RCD tests. Tested the nearest socket to the board first just to make sure it worked, and the RCD didn't trip (on 1x). I tried on 5x and still nothing. Tried the manual test and it worked fine. I tried on another circuit on that RCD and it just won't trip. I then tried the other RCD and, while the readings were absolutely fine, BOTH RCDs tripped.
My first thought was incorrectly wired neutrals somewhere in the board, but everything appears to be correct. NO circuits on RCD1 will make the RCD trip, but ANY circuit on RCD2 will trip both RCDs. I later found a tripping problem on the kitchen lighting which originally tripped just RCD2 (the correctly functioning one) but then started tripping both for some reason. I sorted that problem out (was a borrowed neutral), but this didn't fix the RCD problems.
Anyone got any ideas on this? I'm stuck and it comes on a bad week where I've got a reading of over 110ohms for Ze!

 

[malcolmsanford]

110 ohm Ze .....TT?

Swop the RCDs over

 

[IQ Electrical]

Test each RCD in isolation ie. nothing connected on the outgoing side (remove link to neutral bar).

At least this will rule out a failed RCD (more common than you might imagine).

Have you done IR testing on each circuit or a global test?

 

[jrogers]

No actually the 110ohm Ze was on a TN-C-S.. Had to get Western Power out who apparently found the property wasn't earthed at all so they quickly got to work digging up the pavement outside the house.
A faulty RCD is all I could logically put it down to, but didn't have time to check this on the day as we got called out to another property. The property is empty for a while anyway so nobody will be using the power. I'm actually just the apprentice but I'm very interested in finding out what this problem is! I/we will change the potentially faulty RCD (or both even) in the next day or 2 and see if that solves anything. Did IR on all circuits separately and at first 2 were a little dodgy but these were resolved by finding the annoyingly elusive loads.
I don't think my boss would see it this way, but I like getting problems like this because I see it as good experience.

 

[IQ Electrical]

No actually the 110ohm Ze was on a TN-C-S.. Had to get Western Power out who apparently found the property wasn't earthed at all so they quickly got to work digging up the pavement outside the house.
A faulty RCD is all I could logically put it down to, but didn't have time to check this on the day as we got called out to another property. The property is empty for a while anyway so nobody will be using the power. I'm actually just the apprentice but I'm very interested in finding out what this problem is! I/we will change the potentially faulty RCD (or both even) in the next day or 2 and see if that solves anything. Did IR on all circuits separately and at first 2 were a little dodgy but these were resolved by finding the annoyingly elusive loads.
I don't think my boss would see it this way, but I like getting problems like this because I see it as good experience.

It's great experience, be methodical and rule out the obvious first (failed devices etc.) it saves a great deal of time in the long run!

 

[malcolmsanford]

Was it a bad neutral on the outside system.

Ohhh I can see your the type of apprentice I'd hate ...............

No seriously good for you son, keep learning

 

[Chr!s]

Well if you have a pme link, and an earth of 110 ohms your earth will be at a similar potential to the neutral and possibly the line dependent upon load, i suspect this may be what is preventing you from testing.

Your also going to have a large volt dop.

make the installation safe and wait for dno

 

[IQ Electrical]

Well if you have a pme link, and an earth of 110 ohms your earth will be at a similar potential to the neutral, i suspect this may be what is preventing you from testing.

Your also going to have a large volt dop.

make the installation safe and wait for dno

Oh, I assumed the PME Ze problem was on a different contract?

 

[Chr!s]

Oh, I assumed the PME Ze problem was on a different contract?

Oh, maybe its me who assumed incorrectly ?

 

[IQ Electrical]

jrogers, correct our assumptions please!

 

[jrogers]

Sorry for the delay guys! Yes, the 110ohm Ze was a different job to the RCD problems. Both were consumer unit changes, but different properties. Apologies for the confusion caused
The RCD problems were in a property that seemingly has no other major problems (since I corrected the borrowed neutral in the kitchen lighting anyway). Bonding is present, Ze is fine, R1+R2 all fine, r1, rn, r2 (where applicable) all fine, Zs fine and IR all fine, if not perfect. It is a fairly old property located probably less than 50 yards from the river and from the metal boxes in some rooms, damp problems may exist. Everything tested fine though apart from the RCDs.

 

[malcolmsanford]

Sorry for the delay guys! Yes, the 110ohm Ze was a different job to the RCD problems. Both were consumer unit changes, but different properties. Apologies for the confusion caused
The RCD problems were in a property that seemingly has no other major problems (since I corrected the borrowed neutral in the kitchen lighting anyway). Bonding is present, Ze is fine, R1+R2 all fine, r1, rn, r2 (where applicable) all fine, Zs fine and IR all fine, if not perfect. It is a fairly old property located probably less than 50 yards from the river and from the metal boxes in some rooms, damp problems may exist. Everything tested fine though apart from the RCDs.

Proper terminology ...............careful son your starting to impress

 

[telectrix]

obviously, this guy's not a 5 day wonder. seems to have his head screwed on the right way round

 

[jrogers]

Thanks for the kind comments guys. I am trying to learn as best I can to do everything properly and get a decent understand of not only what, but also why I do things. I've actually only been doing this less than 2 years; last year I paid to do the level 2 certificate by myself with no practical experience, and in september I found an employer to endorse me as an apprentice to get me through my level 3 and complete the NVQ. I only work 2 days a week, but I am learning so much more in those 2 days than I will ever learn at college I think.
I'll let you all know what the problem was when I find out, hopefully I'll be back to look it over in more detail later in the week. I guess my first steps will be to check continuity between the N1 and N2 bars, and if that's not the problem, check for a faulty RCD.

 

[malcolmsanford]

Well you seem to be working at it son and I hopeyou will get a full start with your boss ...................As long as you listen to IQ and Chr!s you won't go far wrong, as the other old git Telextric and me ............well we just sit in the corner remonising.