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.

 

[ezzzekiel]

try removing all the neutrals from the bar and test the rcd alone see if it tests out ok.

 

[vernam616]

Did you test for an earth?

seems like a odd one, one of them " have to be there ones " haha

Good luck, try the above suggestion though

 

[egginyourface]

seen something similar once before, could be reverse polarity somewhere.

everytime customer used hoover rcd would trip. located mixed connections in thermostat which plumber had connected

 

[vernam616]

Good call "egginyourface", try removing circuits one by one and trying it, just to pinpoint

 

[telectrix]

have several times found where customer has replaced a socket face and has reversed N/E. as soon as a load is applied, the RCD will trip.

 

[egginyourface]

easier way would be to interconnect live and earth and do r1 + r2 test at board and locate fault like that. you'll soon find where its been connected wrong

 

[jenions]

annoying thing is, the rcd dont trip if you plug small loads in

 

[vernam616]

Go round with a martindale before removing all the socket fronts, to check for polarity

 

[platty1005]

it may sound obvious but make sure the neutrals for the circuits connected to the RCD are indeed connected to the RCD it sounds as if a neutral is in the wrong place in the CU.
Test button should work if L and N are connected correctly check polarity at the incoming main switch on the CU. If LN ok and test button not working take it back and exchange RCD as faulty.
What type of earthing system is it?
What was the Ze ?
Did you carry out a full PIR and schedule of tests before changing the CU?

 

[Octopus]

Go round with a martindale before removing all the socket fronts, to check for polarity

But don't forget that a Martindale can't "see" a N to E reversal

 

[Mark_Burgess]

Did you notice anything strange about those boards?

Like the main switch has to be levered into the off position but flicks on!!

I had a similar problem last night, it was the fish pond pump.

 

[Guest123]

You say the RCD only trips under 'high loads', is it just one particular item??? was there an RCD present beforehand???

 

[egginyourface]

if its a borrowed neutral both rcds would trip???

 

[Guest123]

if its a borrowed neutral both rcds would trip???

I read it that he has used a 16th ed style split load CU with 1 x 100A main switch hence the RCBO's & 1 x 30mA RCD with the MCB's on???

 

[vernam616]

If tripping under heavy load , id very much doubt its N-E reversed

 

[jenions]

no rcd before hand jus old wylex fuse board, got a borrowed neutral but sorted that at the moment by putting them in same rcbo till i can run a new cable. i tried numerous high loads such as tv, fridge, 3kw heater even me sds drill but all tripped it. i got a low ze cant remember reading i wrote it down and its with all me gear on a job, its just thrown me as this is the second rcd ive tried which makes me think its something on the installation.

 

[platty1005]

When you carried out IR test I assume all loads were disconnected. It seems that there may be a leakage problems on the loads rather than fixed wiring, the old CU would not detect earth leakage on loads. PAT testing of the appliances that trip the RCD might help. If you cannot isolate one load as the cause it may be a cumulative effect IE 15mA leakage on more than one load.

 

[jenions]

but surely not every "high load" has a leakage problem? everything works on an rcbo but whack the rcd in instead and it trips

 

[egginyourface]

how many socket circuits are there?have you tried rcbos on every circuit??

 

[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