trouble with newly installed MK c/u

[jenions]

recently changed an old fuseboard to a new MK consumer unit with 2rcbo's and then RCD with 5 mcb's on it, now i IR all the circuits and all came out at <299Mohm but since day 1 the rcd has tripped whenever a high load is switched on? i tried the up and down ring and the fridge radial on 1 of the rcbo's (40A) and they worked fine but if i put them back on the rcd side they trip. anybody got any ideas? also the test button on the rcd isnt working, now this is the 2nd rcd ive tried as i changed the 1st 1 just incase that was faulty.

 

[topquark]

Assuming that the RCD is OK, there's something wrong with the way the CU is wired if the test button isn't working (pressing it introduces a neutral -> earth fault at the RCD). That's what I'd be fixing before looking elsewhere. Check through all of the CU cabling connections (particularly the neutral and earth).

I assume that the IR readings where >299M Ohms

 

[alanl]

"the test button isn't working (pressing it introduces a neutral -> earth fault at the RCD)."

Are you sure about that?
I always thought it was a purely mechanical test.
Alan.

 

[topquark]

"the test button isn't working (pressing it introduces a neutral -> earth fault at the RCD)."

Are you sure about that?
I always thought it was a purely mechanical test.
Alan.

Positive, thanks. What would be the point in testing quarterly an electronic device via mechanical means ?

 

[topquark]

Here's a little quote from the MK datasheet:

Where connection of a neutral supply is not
possible, then an alternative method of
ensuring that the test button is operational is
to fit a suitable power resistor (rating 4W)
between the load side neutral pole and a
phase pole not associated with the normal
test button operation

 

[nickblake]

you could have a fault in a socket or an intermitant N to E fault it could be on any circuit not just the heavy load circuit as all the N's and E's are joined together on there bus bars they are common so although on low loads it doesnt trip on the higher it could increase the loading around the fault although just a tad but enough causing the RCD to trip ,oh and watch the main switches they have a habit of flipping on

 

[Knobhead]

The clue may be in the OP, “even the test button doesn’t work”. Change the RCD. The test button should work irrespective of any load.

 

[topquark]

The clue may be in the OP, “even the test button doesn’t work”. Change the RCD. The test button should work irrespective of any load.

That's a good point, so it must be phase->earth when the test button is pressed, oops!

 

[ezzzekiel]

i suspect a n-e fault somewhere, had one recently where rcd wouldnt trip and zs testing on ring failed with a trp message, found a low n-e fault to an outside light.

 

[platty1005]

The test button does NOT introduce a L - E fault on an RCD
The test button is connected to the Line conductor on the load side of the RCD this is connected via an internal resistor to the feed neutral. Pressing the test button allows current to pass through the RCD in the line conductor through the test resistor and back to neutral on the feed side so the RCD sees a difference in line and neutral current inside the RCD and trips out. If the test button is not working check supply polarity is correct L-N if yes Take the RCD out and change it again ?

 

[nickblake]

i suspect a n-e fault somewhere, had one recently where rcd wouldnt trip and zs testing on ring failed with a trp message, found a low n-e fault to an outside light.

agree with you check the outside lights dont have water in

 

[Knobhead]

Where the hell do outside lights come in to it!

For gods sake it’s not difficult if the RCD fails to trip on test, the faults in the RCD.

View attachment 7289

 

[jenions]

no outside lights apart from 1 on a plugtop and that isnt plugged in, can see this being 1 of those jobs

 

[jenions]

think is this is the 2nd rcd ive tried surely 2 in a row arent broke

 

[Knobhead]

No wonder I need tablets for my nerves!

 

[platty1005]

Good drawing Tony
Bottom line There are 2 faults

1) The RCD wont' trip on test button operation - check supply to RCD - Test RCD on a known good installation (own house) If RCD wired correctly it should trip SEE Tony's post
If not tripping change it again

2) The RCD trips when loads are switched on
but not when no loads connected - appliance/load problems

PS long distance fault finding is proving quite difficult

 

[mickys86]

Where the hell do outside lights come in to it!

For gods sake it&rsquo;s not difficult if the RCD fails to trip on test, the faults in the RCD.

View attachment 7289

Or the wiring beforehand

 

[sparkinlee]

I agree with Tony and others... if the test button is not tripping the RCD, that needs sorting before any other fault finding, and the test button not working would be caused either by a faulty RCD unit, or incorrect wiring configuration within the CU (but I can't think what!).

 

[Knobhead]

I’ve just re-read this thread.

I’m sorry but for professional electricians some of you should sign yourselves as “Hop-a-long Cassidy”. You condemn cowboys, but then give advice that is so off target it’s a wonder you’ve got two feet left!

A RCCD is a simple thing to figure out, if what’s going in doesn’t equal what’s going out then you’ve got a down stream fault and it trips. If it doesn’t trip on the test button then the RCCD is faulty (unless you’ve used a 4 pole 440V unit).

Sort your testing in to logical order, it’s the only way to fault find.

Sorry but people rely on you to get things right, I wouldn’t be impressed with someone constantly on their I-Pad asking advice.

Don’t get me wrong, I’ll help anyone, but you have to have the basics. Not just being able to quote regulations ad nauseum.

 

[RomRobi]

I quite agree with Tony and patty1005. I've had similar problems in the past. The RCD detects an imbalance between the line and neutral on the load side and pressing the test button simulated this imbalance to cause RCD to trip. I guess a few guys were right too, in carrying certain test and in particular trying the RCD in a complete different CCU to check its functionality.

 

[nickblake]

Where the hell do outside lights come in to it!

For gods sake it’s not difficult if the RCD fails to trip on test, the faults in the RCD.

View attachment 7289

Sorry Tony , but i have come across this before RCD trips on high loads brand new install ,the fault was caused by an incorrectly fitted lid on an outside light ,the light was half full of water ,replaced the fitting fault gone , this was the reason i mentioned outside lights