Adding another RCD in series with existing RCD ?

[GettingOlder]

Hi el-nerds, I would like to receive advice about RCDs...

My (well, I am a farang, so not completely true...) house has a 40A max, 30mA RCD of ABB F202 type installed after the Main breaker (50A), but 6 smaller MCBs of 16 or 20A are installed in parallel with that RCD.

As far as I can deduct from various info this installation has the following problems:

1) The loads connected to the RCD breaker is only current overload protected by the Main incomer breaker. However not that much difference between 40 and 50A, so perhaps acceptable... But can I risk the RCD will not operate as being overloaded as breaker is 50A while itself rated 40A?

2) All loads/appliances connected to the small MCBs are not current leakage protected.

So I consider installing another RCD unit in front of the Main breaker, thus being the entry point to the house. First I thought replacing the existing 50A Main incomer breaker with a ABB DS unit, having both overload and leakage protection, but those seems limited to 40A only. So guess the solution will be another F202 unit rated for minimum 50A (probably 63A which seems to be a standard) and 30 mA. That would mean the 2 RCDs are connected in series - at least for some of the load. Does anybody see a problem with that?

Other suggestions how to modify the installation are also appreciated, thanks.

[RichCor]

A picture would be worth a thousand words.

Your description of a device being installed in Parallel doesn't make sense, unless it's for overload/lightning protection.

Pictures of your current setup please.

[Crossy]

Yeah ^^^, I think some photos would be useful, if the devices really are in parallel you have some serious issues.

It's quite possible that what you have is just fine and you are misinterpreting how it's installed, or it's exactly how you describe ...

[RichCor]

Most circuits are placed in Series, then Branch.

Having breakers in a Consumer Unit next to each other is an example of a Branch off the common Live/Load Buss Bar.

Yeah, they might be right next to each other, but they don't act as if they were Two 1.5v Batteries placed in series for 3v, or placed in parallel for 1.5v and recombined to provide increased Wattage.

...

For safety, many people install a single Safe-T-Cut rcd-type device before the CU to provide whole-house protection from itinerant L/N balance mismatch (current leak) events that might otherwise lead to electrical shock or electrocution.

But if you are running appliances that do 'odd' things with the power, or some of your branch circuits are in 'wet zones', you may find it nuisance trips the whole-house Safe-T-Cut. To reduce the occasions that the whole house goes without power you can replacing the suspected individual branch miniature circuit breaker (MCB) with a dedicated residual current breaker device (RCBO) that detect both an imbalance in the live and neutral currents AND current overload. Sometimes this isolates the issue, sometimes all the RCDs get tripped and your still sitting in the dark and another approach need to be devised to provide safety and continued operation of non-affected branch circuits.

What is the difference between MCB, MCCB, ELCB, and RCCB

Electrical Engineering Portal

[GettingOlder]

Yes, I agree that photos tells a lot...

Here is the wall box seen from all sides. Although the wiring is probably not that clear on photos (should I have removed the protection conduit at box underside?) I am now in no doubt that the 16/20A MCBs are not serially connected with the 40A RCD.

Because today the RCD tripped (for the first time BTW) when I was detaching some junction boxes from the boundary wall which is being painted. And while tripped (and no power to gate and other things) the TV and several lamps were still working.

So would appreciate suggestions what to do, thanks :-)

[RichCor]

Can I assume the color-swapped Black/White on the master breaker are MAINS INCOMER?

Where do the Black/White wired connected to the top of the RCD run?

Looks OK to me

Though I don't understand the odd color swaps on the MAINS and 5th breaker (other than to quickly fix a L/N swap) ...well, and the lack of third-wire grounds.

[Crossy]

Is the incoming mains the correct way round? The right hand terminal (white wire) should be live, check with a neon screwdriver.

Looks like that RCD is protecting only one circuit (water heater perhaps).

Our OP is correct in asserting that the 40A RCD is only overload protected by the incomer, not good, but in reality it will work just fine.

One could add a front-end RCD/RCBO (Safe-T-Cut) or with a little re-wiring convert the existing RCD into a whole-house arrangement. Even if one chose to replace the 40A unit with a 63A one it would still be cheaper than the somewhat over-priced Safe-T-Cut units.

And add a ground although there don't appear to be (m)any grounded circuits.

[RichCor]

As Crossy reiterates, only one circuit is currently being protected by the RCD in your CU. So, yes, that circuit is only being overload protected by the incomer.

The ABB RCD are normally wired LINE IN at the top, LOAD OUT through the bottom and feeding multiple neighboring branch MCB (as shown in this ThaiVisa image):

While the ABB RCD can be wired to protect a single circuit, as it's currently wired, it could also be wired to protect the whole house (Single Load Board), or protect a portion of MCBs (split load board - if the CU provides for isolating neutrals). The follow-on MCBs then are rated to the max circuit load or wire load capacity, whichever is less.

(image borrowed from dyidoctor.org.uk Consumer Unit Installation - Consumer Units Explained)

[RichCor]

I am now in no doubt that the 16/20A MCBs are not serially connected with the 40A RCD.

Because today the RCD tripped (for the first time BTW) when I was detaching some junction boxes from the boundary wall which is being painted. And while tripped (and no power to gate and other things) the TV and several lamps were still working.

Ideally, an RCD should only 'trip' when an imbalance is sensed only on the 'load' of the circuit(s) feed through it.

They'll sometimes 'trip' for other reasons, like vibration or borrowed neutral. Even sympathetic tripping.

Though, given your description above... if the RCD was the only thing that 'tripped' (none other other MCBs opened) and the powered 'gate' and probably the 'boundary wall junction boxes' went dark, then are they powered through the RCD???

[bankruatsteve]

I am now in no doubt that the 16/20A MCBs are not serially connected with the 40A RCD.

Because today the RCD tripped (for the first time BTW) when I was detaching some junction boxes from the boundary wall which is being painted. And while tripped (and no power to gate and other things) the TV and several lamps were still working.

Ideally, an RCD should only 'trip' when an imbalance is sensed only on the 'load' of the circuit(s) feed through it.

They'll sometimes 'trip' for other reasons, like vibration or borrowed neutral. Even sympathetic tripping.

Though, given your description above... if the RCD was the only thing that 'tripped' (none other other MCBs opened) and the powered 'gate' and probably the 'boundary wall junction boxes' went dark, then are they powered through the RCD???

It seems obvious that your RCD is only protecting some circuits - not all. RCD will trip with any current imbalance between L and N. That usually is ground fault to L but could also be ground fault to N. RCD is NOT over-current protection so there is no correlation to normal MCB's.

[GettingOlder]

Thanks for responses - and sorry for replying late (been out of town etc).

Yes, clearly the RCD is protecting only the consumers wired up directly to that circuit being the white and black wire leaving at top of it. While the 6 other MCBs are fed directly from the Main 50A breaker, and thus NOT leakage protected. Clearly an unsustainable situation !!

My plan to protect the 6 MCBs WAS: Add another RCD and install it between the 50A Main breaker and the 6 MCBs. Thus split the current after passing the Main breaker into the 2 RCDs being in parallel. And current from new RCD feeding the 6 MCBs in parallel (using existing cupper bar) while existing RCD carried on protecting its own circuit.

THEN I wondered: Why does the house have this strange configuration in the first place? Maybe some hidden problems? So I decided to test a bit... I disconnected the 6 MCBs from the Main breaker, and instead fed them via the existing RCD, see photo. So having in series 1) Man breaker, 2) RCD, 3) the 6 MCBs + the RCDs existing own consumer circuit. I did not change any Neutral wires...

By doing that the RCD trips for each single MCB coupled in. RCD can still stay activated with its own circuit but whichever MCB I try to add on, the RCD trips instantaneously. I also disconnected the original black/white circuit at RCD outlet, for RCD only to protect the 6 MCBs (equal to the foreseen task of a new RCD) but the RCD still tripped when activating any one of the MCBs.

The RCD type is the standard "AC" model, which I however saw in ABB catalogue is not recommended for Aircons with inverters and all kinds of sophisticated electronics... They apparently offer a more sophisticated A-APR model which is better suited for inverters but could not find any for sale in the outlets around here (south Pattaya).

Anybody who can advice on this? Has it to do with "borrowed Neutral" and what to do about it...

Thanks in advance.

[Forkinhades]

Not borrowed neutrals, but simply it's crossed neutrals.

The rcd is a simple monitoring device, which measures current flowing in the phase (live, hot) and neutral, if the imbalance is more than nominal current, (30 ma) the rcd will open.

By adding the other MCBS onto the existing rcd, you now have imbalance of current on the neutral.

Best solution is FRONT END safety-cut. Which is exactly what states, by having the Safety-cut at the front of all your circuits, you also need to ditch existing device in your current configuration.

EDIT:

IF you feed your MCBS from the load side of the rcd, instead of the supply side it will work.

[Crossy]

If you didn't move any neutrals the RCD will trip all the time.

Both live and neutral currents must pass through the RCD, it measures the difference so it knows when to trip.

[RichCor]

To add to what both Forkinhades and Crossy just posted, the Neutral Bus Bar needs to be isolated so that it is ONLY FEED by the Neutral coming through the RCD.

Your Neutrals (MCB Load returns) are currently bypassing the RCD, therefore the RCD opens when encountering the mismatch L/N load.

You need to fully understand how the RCD functions, and how it needs to be wired into your CU. A wiring mistake (similar to what is currently in place) can kill you or cause major damage. Also, the lack of properly colored wiring isn't helping matters AT ALL.

The Neutral from the Main Breaker needs to be feed ONLY though the RCD, then the RCD Neutral output passed through to the Neutral Bus Bar. This way your L/N LOAD usage remains balanced as far as the single RCD is concerned.

Edited November 22, 2015 by RichCor

[Forkinhades]

Need clearer picture of neutral connections.

[RichCor]

Need clearer picture of neutral connections.

Neutrals are being feed directly from the 50A main breaker (see WHITE Wire in photo, running from bottom of main breaker to Neutral Bus Bar).

[Forkinhades]

Well spotted RichCor!

[GettingOlder]

Thank you again for good advices - I now think I fully understand the RCD function.

So now I

1) disconnected the Neutral wire going from Main 50A breaker outlet to Neutral Bus bar, and moved that wire to connect between RCD outlet and Bus bar.

2) disconnected the Live feeding of the 6 MCBs from the Main breaker and instead feed them from the RCD outlet.

Now no RCD tripping !

Then I wondered if this existing 40A RCD actually is sufficient to pull most of the appliances in the house, as in that case no need to install another one in parallel with it. So I had the oven, the electric cooker hood, microwave, hot water maker, vacuum cleaner and 1 aircon running at same time which should be in neighborhood of 10 kW. No fall out of either Main breaker or individual MCBs and no RCD tripping although I dont know how the RCD should behave if pulling more than the rated 40A through it.

So now this is my configuration, and I even save those 1700 bht for another RCD. The proper grounding is still pending but will be looked at at a later stage... A lot of other things to attend to in this newly bought 5 years old house - as unfortunately Thai built.

Thank you all

[Forkinhades]

The RCD will handle about 20% more than the nominal value for short periods.