Anything (unbalanced across phases) which may be connected to the circuit's cable in the future.
What I'm really saying is very simply expressed as "if there in an N conductor in the circuit's cable, then connect it to the RCD".
This Simple RCD Error Could Be Deadly!
- Thread starter ericmark
- Start date
What I'm really saying is very simply expressed as "if there in an N conductor in the circuit's cable, then connect it to the RCD".
Yes of course
The only thing I see wrong anywhere in this thread - othe than possibly Erics #4 - is if neutral is not provided to the RCD it will never trip.as the tripping solenoid relies on N & L1. I've always argued it should be between L1 & L2
Edit: Oops sorry wrong type of RCD selected for explanation
Edit2: the circuits on the front of RCD's indicates the solenoid is only energised by the current transformer; my experience here may be way out of date and too limited but it is certainly the case that early RCD's the solenoid was activated by 230V and required 'N' to be present to energise the solenoid, even if not used after the RCD
Edit: Oops sorry wrong type of RCD selected for explanation
Edit2: the circuits on the front of RCD's indicates the solenoid is only energised by the current transformer; my experience here may be way out of date and too limited but it is certainly the case that early RCD's the solenoid was activated by 230V and required 'N' to be present to energise the solenoid, even if not used after the RCD
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As I see/understand it, it depends upon the load(s) - if the currents in the three phases are not perfectly 'balanced' (i.e. their vector sum is not zero), then it will surely always trip if the N does not also go through the RCD, won't it?
That would be a new one on me! It's always been my understanding that all the 'live' conductors (3 phase and one N) go through the RCD and if the vector sum (of all four) (i.e. the 'residual current') is not zero, then the device will trip. Of course, if the three phases are perfectly balanced (vector sum zero) then the neutral current will also be zero, so there would be no residual current detected (hence no trip) whether or not the neutral was connected.
If the circuit is loaded, there will surely always be a net current (hence detected as a 'residual current', causing a trip) between any two phases (or between any one phase and N), won't there?
If there is a neutral and any current flowing through it (but not through the RCD) the RCD will trip. Otherwise it won‘t, with or without a neutral.
Absolutely the only thing that relies on a neutral connection to function is the test button on most RCDs but there are still some that have the test button wired between L1 and L3.
Absolutely the only thing that relies on a neutral connection to function is the test button on most RCDs but there are still some that have the test button wired between L1 and L3.
That's essentially what I wrote, isn't it?
As I've said, that strong assertion is surely only correct IF there is a perfectly balanced 3-phase load (i.e. zero vector sum of the L1, L2 & L3 currents). In any other situation (e.g. if there were any single-phase loads fed by one or more of the phases), in the absence of a neutral connection the device would probably be permanently tripped, wouldn't it?
Yes correct and has historically been used to detect and shut down unbalanced phases.
Yes but my comment was based on the days when the solenoid was a 240V coil and effectively connected between L1 & N to trip.
Well yes the whole point about an RCD is to detect any imbalance across all of the lines running through it. But my comment was based on the days when the solenoid was a 240V coil and effectively connected between L1 & N to trip whereas I believe it should have been on 2 phases.
My observations of this situation started in 1996 on a low current RCD (say 25A) with 2 poles run in parallel to use on a higher current on single phase. This was as the main switch in a metal Wylex fuse box used for an NEC exhibition stand running >100A and overheated, melting insulation of the wires traversing the ferrite core and taking out the 125A supply fuse. After a hasty repair (sans RCD) I pulled it apart to investigate.
Subsequently I discovered a 4 pole RCD with only 3 used in a water booster pump did not trip when an intermittant earth fault developed in the motor winding and adding a neutral to the supply side (actually I used earth as there was no neutral available) allowed it to function correctly.
Later still I changed some 4 pole RCDs to 3 pole which didn't have the same issue.
That's fair enough, so long as all the loads are balanced 3-phase ones. Even with true 3-phase machines, is it not sometimes the case that just one of the phases is used to power 'control circuitry' or suchlike?
Fair enough, but I'm not familiar with the situation "in those days", and nor do I fully understand what you are describing.
However, I think that everything I've written is true in terms of "the current days", isn't it?
I believe so.
Additionally I have never looked at why some RCDs and RCBOs have earth wires. But I feel a Mr Google coming on.
Thanks.
I'm not sure that I've ever seen a 'RCCB' with an earth wire, but it was certainly fashionable with the early RCBOs, and is still the case with a good few.
I'm pretty sure that you will find that the reason for these 'functional earths' was to ensure that there was power (derived from L-E) available to run the electronics of the device (essential for functioning) even in the event of a 'lost neutral'. However, I think that many manufacturers decided that it would be extraordinarily improbable that there would ever be a 'lost neutral' at the very same time that a fault requiring RCD operation arose, and hence decided that the functional earth was not really needed.
The same argument could presumably be produced in relation to RCCBs (all of which I think rely on electronics these days) but, as above, I don't recall having ever seen one with a 'functional earth'.
Functional earth so it still works if the neutral is lost, I think.
Although I've never seen a pink (since it changed to pink) fly lead only white.
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