what would the line voltage be for the rcd to operate?
measring equipment is picking up no funny voltages which you would expect with a loose supply neutral! is it possible the 300ma rcd main switch is operating before any fluctuating voltages. Also the neutral current is not monitored with the equipment. thanks in advance

If the neutral is not monitored by the RCD then any imbalance of the phases will cause it to trip.

i know it only takes 300ma of imbalance to trip the rcd so at this point what would the line voltage likely to be? what im asking is , is the rcd operating before any unusual voltages can be recorded on the voltage monitoring equipment print out?

Why do you suspect a loose netural ??

Is the supply TT or has it just got an RCD for additional protection (farm etc) ?

Whats the loop resistance look like P-N ??


Stu

Edit: because I didn't read the question properly...

yes its tt, what i need to know is if the fault current was 300ma what would the line voltage be? i think its a floating neutral on the supply which would cause spikes in the voltage normally but would an rcd main switch cut out before any unusual readings could be obtained on a voltage recorder

I'm not sure what the voltage would look like under such conditions. Obviously the PN voltage would change and if you've got 3 phase loads connected it would increase rather than drop.

I don't suppose you can put your voltage recorder between neutral and "true earth" can you ? as it might give a better clue as to what the neutral volts are doing.

Whether the RCD would trip before the voltage recorder sees it, I suspect it might but couldn't be certain. If you think about it, the RCD is likely to go in 40ms or so, but the voltage recorder may well miss the change just prior to the rcd operating as it's then seen the total loss of supply which it's going to be more interested in.

Have you tried talking to the local DNO ??, around my parts they will normally respond quite well to such reports.


Stu

yes thats it they are toatally dimissing it.they say no funny readings so no problem but im trying to say the rcd is stoppiong funny voltage readings being recorded if it was just a main switch they would record the fluctuation in the voltages its as if the rcd is letting them get off scott free

Last time I had a DNO voltage recorder looking at a problem, it was connected to the cutout so before the mainswitch/rcd.

Have you tried making the fault more apparent by putting lots of load on ?. It would have to be single phase loads though for obvious reasons and ideally only on one phase to maximise neutral current.

Sometimes things then get nicely warm and prove the point.

Stu

basically i know 100% its a loose neutral on the supply overhead line, but there defence is that there are no funny readings but i want want to dismiss there readings because i believe the rcd is cutting out before readings can be obtained.

You're ahead of me then. Never had the same problem as my local DNO's guys are pretty good as I've already said... Alan Capon is the man then if he's about..

I do know how frustrating these things can be though, trouble is it's always your phone that the customer rings.


Stu

exactly dont really blame him we checked our installation twice over but all our results were perfect , dont want to say to much just hoping to dismiss there readings they claim are ok which i dont disagree with, but as i say its the rcd main switch that is saving them. if it was an ordinary main switch the voltages would be all over the place. i was hoping the higly intelligent people would tell me the voltages, i believe the voltages only need go as low as 229volts which could trip the rcd working it simply on the ohms law theory

i agree with slittle, put a high single phase load on, if, as likely its a high res on the o/h, it will show up.

I think you're going to have to explain your thinking a bit on these points. Drawing 300mA isn't going to make any significant difference to the supply voltage. What is the "resistance" you've got in mind when applying Ohm's Law? Also bear in mind that a public supply can be as low as 216.2V and still be within acceptable tolerances.

What's the installation's insulation resistance looking like?

- Andy.

the connection has been changed on the overhead supply and since that day the rcd has never tripped out. but im not that clever to work out how a loose supply neutral pme connection could cause the site rcd to operate , i hope im explaining it ok, or has anyone come across a similar problem as i say since a new connection was made there has never been a problem .

I presume it's not the "active" type that automatically open on power loss (I've only seen them on plug-in RCDs and socket RCDs, but you never know what they'll come up with next).

Some RCDs do trip out when N is lost but L and PE remain (as a safety feature as without N there's no power to drive the electronics to detect/amplify the residual current), but as far as I know it's only some of the types with an earth fly-lead that can do this - i.e. more common with RCBOs than plain RCCBs.

Arcing has been associated with RCDs tripping - both upstream and downstream from the device.

Other thought is a fault with significant impedance between N and PE within the installation (or connected equipment) - with N close to 0V only a small amount of current is lost through the fault, if N increases substantially w.r.t. earth, then more current will pass across the fault - perhaps enough for the RCD to notice.

All guesses, I'm afraid.

- Andy.

thanks very frustrating , i need to work out the theory why a bad neutral on the supply side would cause the rcd to trip.the site has been tested twice throughout with great results so am 100% happy its not on the site