RCD, no trip

Discussion in 'Electricians' Talk' started by seneca2, Jun 4, 2009.

  1. seneca2

    seneca2 New Member

    Hi all, working in a house where a new kitchen is being fitted, have added and moved some sockets, c/unit to change next week. It's a TT system, earth was apparently by water main until changed to plasic a few years ago, since when no earth at all! Old Wylex 3036 board, looks like there was an old elcb at one time which has been replaced with a Henley block! (get's worse don't it). I've put a rod in, got around 150 ohms. Now to the point, in a fit of concience I decided to temporarily install a front-end 30m/a rcd. Strange thing is, the rcd won't trip unless the load is removed, I know this is how it should be tested anyway but it's a bit puzzling that it won't trip at all isn't it?. (tried another rcd) To re-cap, if I test at the rcd output with no load it trips well whithin correct times, if I now switch the main switch on it won't trip either at the rcd terminals or at a socket outlet. Zs at a socket seems ok at around 150 ohms, any ideas anyone? Makes me wonder what i'll find when I change the c/unit next week. Sorry for length of post but I wanted to explain it fully,
  2. leitzz

    leitzz New Member

    Neutral Earth fault in the installation I guess

  3. seneca2

    seneca2 New Member

    That would make it trip all the time leitzz.
  4. JP.

    JP. Screwfix Select

    Could it be something to do with the 150 ohms?..presumably the trip tester is using earth neutral to try and actuate RCD but at 150 ohms this maybe will not happen. A live to E will maybe trip the RCD under load?
  5. seneca2

    seneca2 New Member

    JP, well I suppose it might be. I do quite a lot of TT jobs & I usually manage to get the rod well below 100 ohms but in this case I just couldn't get it that low. In theory a 30 m/a rcd should work ok with a Zs up to 1667 ohms though!
  6. e30mark

    e30mark Member

    My money is on a neutral / earth fault in an unenergised circuit. ie immersion heater etc.
  7. seneca2

    seneca2 New Member

    That would make it trip Mark, not stop it.
  8. JP.

    JP. Screwfix Select

    See your point Sen..good luck M8..:)
  9. leitzz

    leitzz New Member

    Its a TT system with a high Ra - you have a Neutral to Earth fault.

    There is some level of earth leakage but this is below the threshhold ( 20mA ish) of the 30mA RCD.

    The RCD does not trip with an RCD test as the RCD test current is going around the phase/neutral loop instead of the phase earth loop.

    To test the rcd with the fault present, connect phase and neutral as normal, but connect the earth lead of the tester to the incoming neutral on your rcd. The RCD should now operate even though the installation is "on load"

  10. Buzx1313

    Buzx1313 New Member

    Neutral earth fault deffo...

    will not trip on a TT supply dead short between Earth And Neutral...

    Get your insulation tester out..
  11. propper spark

    propper spark Member

    There must be something preventing it from creating an imbalance to the rcd and as a result it won't trip. Disconnect all neutrals and test again putting each one in until you find the circuit preventing it from tripping. That all sounds a bit back to front to me as like said a neutral earth fault will make it trip all the time. I’ve never had that one myself but you need to resole it as a TT installation without proper rcd protection can be dangerous.
  12. wally2

    wally2 New Member

    Hi seneca, seems strange, as you said, it,s unlikely to be e/n fault as I reckon it would have the opposite outcome. I remember seeing somthing in the Crabtree accessories book & have just found it. Might not be any help but i mention what it says.
    Fault current sensitivity.
    semi conductor devices are now incorporated in equipment used throughout industry, commerce & in the home. Typically, the purpose of these semiconductor devices is for vdus,printers,washing machines etc. In the event of an earth fault the presence of semiconductors may result in the normal ac waveform being replaced by a non sinusoidal fault current. In some cases the waveform may be rectified or chopped. These waveforms are said to contain a pulsating dc component which can eighter partially desentisise or totally disable a standard typeac rcd.

    There seem to be 2 types of rcd, type A & type AC, one allows for this type of appliance & the other does not I wonder if you unplugged all the items on the sockets, it may be one appliance with the above combination. Might not be any help to you but its the best I can do.
  13. sinewave

    sinewave Screwfix Select

    As stated, tis a dead N/E short on one of the installations circuits.

    Insualation test the individual circuits or remove the Neuts one by one at the board to find the offending circ.
  14. sinewave

    sinewave Screwfix Select

    Temporary linking the MET to the DNO neutral would prove it as the RCD would trip instantly and stay tripped until the offending circuit is moved 'off line'
  15. seneca2

    seneca2 New Member

    So why is it not tripping all the time then sine?
  16. sinewave

    sinewave Screwfix Select

    Because when you test with your RCD tester at say a socket outlet the tester is dumping a controlled test current down on to the cpc.

    As the cpc is shorted to the neutral inside the installation then the test current takes the path of least resistance, which in this case would be back down the shorted Neutral and not the 150 Ohm cpc and being as the RCD will still see it's 'balance' then it won't trip.
  17. seneca2

    seneca2 New Member

    Thanks for all the replies so far, I will be doing a full test when I change the c/unit next week. This present siuation only arose because I was uneasy about having done some of the work there and there not being any rcd protection, although the place has had no rcd (or earth for that matter!)for years it would be sod's law for something nasty to happen before I get back there, that's why I decided to put a temp front-end rcd in. (that'll teach me to be conciencious won't it!)
  18. Slap Dash Harry

    Slap Dash Harry New Member

    This one is quite interesting and not one iv've come across before. On first reading people saying N-E fault i thought the same as seneca that the RCD would simply not stay in. But i can see the logic in what the others are saying. Say you have 150ohm Ze on your rod then if there is a dead N-E short then you have approx 1.5A earth fault current, more than enough to operate any RCD, however as already stated it taked the path of least resistance though the neutral and none is dumped to earth. Hence RCD stays in. Also when you do your test any current is also sent past the N-E short and all goes back down the nuetral, hence the RCD senses no imbalance. On a PME set up the N-E loop impedences are so close that the fault current would be almost shared which would cause an imbalance. Interesting one that!!
  19. Slap Dash Harry

    Slap Dash Harry New Member

    The last bit i meant the PEFC and PSSC are almost same and would share fault current.
  20. leitzz

    leitzz New Member

    Because the water main is plastic (and you don't mention gas) - you have a neutral to earth fault in the installation which is likely to be a far lower resistnce than your earth electreode (150 Ohms I think you said). The majority of the current from your rcd tester will take this route, back through the neutral of the rcd, rather than the higher resistance path via the earth rod, resulting in the rcd appearing to be faulty by failing to trip.


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