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Posted By Topic: RCCB wiring

Apr 03 2019 17:56

I'm an apprentice and was taught that to wire RCCBs correctly current needed to flow in opposite directions through the RCCB ie if a RCCB were feeding MCBs with feedcomb from the bottom the MCB neutrals would be terminated in a bar fed from the bottom neutral terminal of the RCCB. On a job today I noticed a SB with the MCB neutrals in a bar connected to the top neutral terminal of the RCCB ie current flowing from the top through both neutral and phase of the RCCB. Is there any functional difference?

Apr 03 2019 19:08

Standard RCD diagrams printed on RCDs show supply from the top, both phase and neutral. Most will still work if the supply is from the bottom. RCD’s work on an imbalance of current in phase and neutral so technically top feed or bottom feed works. I would not supply one pole to the top and the other from the bottom mainly so that one can be clear that either the top or the bottom is the supply, not both

Apr 03 2019 19:17

Most RCDs should say in the packaging or on them whether they can be fed in either direction, or whether one side has to be the load side.

Some also have to have the neutral on a particular side; others don't care. This will also be marked.

As long as the supply phase and neutral are on one side, and the load phase and neutral on the other, what happens with the MCBs doesn't matter.


Apr 03 2019 20:10

But any RCD marked ön the RCD with "Line" and "load" must be followed. In some cases if incorrect line and load connections are used the RCD internal circuits will be destroyed when is power is applied.

And the test button circuit will also be destroyed if the test button is pressed.

For all RCDs it is good practice to use the top and bottom terminals as matched pairs of terminals.

Apr 04 2019 08:26

The truly worrying thing is the statement 'I was taught..."; and the question must be asked: by whom?

If by your employer or a co-worker tradesperson; they're simply wrong, but you've now been given the correct info and little lasting harm done. Two lessons learned; 1 this specific matter, and the other the more general one not to blindly accept everything you're told. On which, congratulations for asking the question.

But the other possibility is that some f-wit is spreading this sort of stupidity formally through off-job training to many trainees. In which case, it needs to be tracked down, and a stop put to it.

Apr 04 2019 19:18

Right so to clarify my original description in this image the RCCB is fed phase from the top while the main neutral is in the bottom. It is my understanding the currents need to be equal and in opposite direction through the RCCB but here they are in the same direction.

Apr 04 2019 19:55

Well if wiring them in opposite directions is stupid, then I'm stupid too. Good luck deciphering the wordless European type symbol diagrams - I'm not that clever. A few months ago I phoned the distributor of a brand (NHP I think) that was tripping with no fault on my freshly wired DB. They confirmed it had to be opposite directions - I'd made a mistake and had P & N flowing the same direction. On some brands it does not matter. So now unless it is clearly labelled with words such as Line, Load and P & N I phone for advice or buy another brand.

Apr 04 2019 22:17

JamesB wiring the way you have shown is incorrect and the test button will not work.

Those RCDs must be wired with the line N & A on one side, and the load N & A on the other side. Doesn't matter top or bottom, but they must be on the same side.

There's not such thing as "equal and opposite" direction for the wiring, and whoever told you that is an idiot quite frankly.


Apr 04 2019 22:20

Also, wired the way you have shown will burn out the test button resistor if you hold the button down, because it doesn't trip immediately when pressed.

The resistor is only rated for a very short duration.

Apr 05 2019 09:43

Not wanting to be overly insulting; but if the cap fits...

Think back to 1st-year a.c. theory. There is NO "flow" of current as such; the current reverses 100 times a second. Therefore for a single-phase circuit if supply A & N are connected at one end, and load A & N are connected at the other end, then at any given moment the instantaneous currents are equal & opposite. Whereas with one side reversed, the instantaneous currents will be in same direction.

But even that is mostly a red herring, what really puts the nail in the coffin of this stupid "equal & opposite" idea that currents must be somehow "opposite" is a 3-phase circuit, with 3 As + N. There's NO way any of the currents will be in direct opposition to any of the other three, the As will all be offset by the phase angle, and (for a balanced load) the N current will be zero.

The principle that RCDs work on is that the sum of all the currents in live conductors of an a.c. circuit is zero. The associated magnetic fields also balance to nett zero (or very close). Any imbalance means some current is finding another path; and creates a small magnetic flux that is picked up by a sensing circuit and used to create a trigger impulse to operate the RCD.

I do not believe there is ANY brand of RCD available on NZ market (ie Approved for supply in NZ as required by ESR 84) that should have supply A & N at opposite ends. There's NO way an RCD requiring Line a one end and Line N the other is going to get the Approval; and - without checking - I'm prepared to baldly state that all the relevant IEC Standards they are built to will forbid that sort of configuration.
ALL of them either have one end marked as "line" and the other as "load", else either end can be line.

Look at that diagram in DougP's post (which coincidentally is for same brand as JamesB); and note the test circuit connections. Assume Line connections at top. The connection to active is clearly from the terminal labelled "1", so that when the RCD operates by pressing the button the supply to the test circuit is removed (because it's on load side of TWO contacts - the Active load contact and a test circuit contact). This prevents the test circuit from being burnt out if the button is held down.

Now consider reversing connection so line is at bottom, with line active to terminal "2". Test circuit still protected from burning out.

Also with this configuration, regardless which end is line; there's no way that pressing the button can liven the load side N conductor. With some alternative configs; that could happen (but the RCD would NOT comply with AS/NZS 3190; which specifically forbids it).

Now consider this RCD connected as suggested by JamesB; with Line A on "2" and Load N beside it: every press of the button livens the outgoing N; even after the RCD has operated (until, as DougP said, the resistor burns out because the button was held in too long). That's stupidly dangerous. Also non-compliant with "3000", because the RCD does NOT "disconnect" (ie isolate) the circuit when operated.

Clearly the person mazdaman spoke to was just as dangerously ignorant as whoever taught JamesB. And I'll guarantee the MIs for the product say otherwise.

Over last 18 - 24 months there's been a huge panic in Oz, triggered by discovery of multiple sites where RCDs have been incorrectly installed, and also not properly tested. Large numbers of RCDs in existing RCDs have been found not to operate; either because the test circuit has burned out, or because they were so badly installed that even when they operated the circuit wasn't isolated. It's been discovered following investigations of fatalities. And as a result Oz is considering joining us and requiring RCD tests every time, instead of only if supply is available at time of installation.

And I'd suggest mazdaman's "tripping with no fault" was in fact just the common "tripping-for-fault combined with ignorance-of-fault-finding-for-RCDs"; with an outside chance of "faulty RCD".

Bottom line:
If you connect with Line A & N at opposite ends, the installation will NOT comply with "3000". It will also be contrary to MIs.


Apr 05 2019 15:30

Hopefully those who connected at opposite ends go back and correct their installations.

Apr 05 2019 17:12

In the interests of continued nitpicking, there are some RCBOs with the line A&N at opposite ends; those intended for mounting to comb bar/pan assemblies, with flying neutral leads.

It is very hard to screw them up though, as they are well labelled etc. And mostly not used as only one manufacturer (NHP/GE) seems to make them with a switched neutral.

Apr 05 2019 19:00

Alright guys to clarify even more before the weekend. If you can actually see the image you will see a physical representation of 'equal and opposite currents' as well as literally those words to describe the operation of an RCD under normal conditions. My concerns arose because the circuit shown in the switchboard photo has current 'flowing' in the 'same' direction through the RCD as per the terminology of "Electrical Wiring Practice AS/NZ 3000 2007 Volume 2 7th edition" After raising my concern with my supervisor and being told that it was wired correctly I came here to confirm my suspicion before going further. You have confirmed this suspicion and I will have to get someone to rewire it before the owners move in next friday. You can thank me on Monday have a nice weekend

Apr 05 2019 20:23

James, that's a good illustration (current imbalance through a toroid in the RCD).

Din-rail mounted RCDs are generally forgiving if you transpose a main live in and a main neutral to the rcd neutral output - ie the rcd may not work or won't reset.

A PDL 691RCD or a 695RCD isn't forgiving and will usually go bang with a bit of smoke. I've found out the hard way at least twice.

Apr 06 2019 01:02

As i read this i am scratching my head and wondering why anyone would ask a sales muppet about the wiring of any electrical product.
Most every RCD comes with a diagram printed on the side and usually there is one in or on the box.

I have never seen an RCD that has the Line Phase and N connected at opposite ends.

Apr 06 2019 18:02

Don’t run down sales muppets, some of them are electricians who have come in out of the cold. Do need to make sure you are asking one who can tell the diff between RCD and a RCBO or a themostat and a thermistor, whatever the subject of interest may be