RCD protection on three phase/single phase.

[Mrdjc]

Another theoretical question I'm wondering about.

Say you have a CU with 3 phase incommer.
Off that consumer unit you wish to run:

One 3 phase MCB for a 32A 5 pin plug
Three single phase MCB's for 16A 3 pin plugs Each on a seperate phase.

How does the RCD work in that case?

Does the 3 Phase RCD simply work on the principle of:
L1 + L2 + L3 = N ?

Does it matter if there is a higher load on one phase than another?
Will the RCD trip if L1 is 20A, L2 is 32A and L3 is 20A?
Or will it be fine as long as what is going in is measured going out again over neutral?

How do the ratings work for it, say its rated for 40A, is that 40A per phase maximum?

 

[RF Lighting]

It works similar to a SP RCD, but if the phase currents are equal there will be no neutral current. As long as the sum of the difference in phase currents = neutral current the RCD is ok.

You could pull 40A on L1 and leave L2 and L3 unloaded and the RCD would be happy.

Yes a 40A RCD can carry a maximum of 40A per phase.

 

[Adam_151]

You would normally use ceeform outlets with integral RCDs if rcd protection was necessary. RCD incommers are best avoided in commericial industrial without very good reason

 

[Mrdjc]

That's very interesting, exactly why I asked the question as it was confusing me to high heaven.

One more thing though, 40A Rating per phase at which voltage, presumably phase to neutral voltage? Or Phase to phase?
Thank you!

 

[Taylortwocities]

YRCD incommers are best avoided in commericial industrial without very good reason

Very good point.

In fact, RCD incomers should be avoided if at all practical.

OK for a garage - with just a couple of sockets and lights, but (IMO) a potential liability even in a house.

 

[RF Lighting]

That's very interesting, exactly why I asked the question as it was confusing me to high heaven.

One more thing though, 40A Rating per phase at which voltage, presumably phase to neutral voltage? Or Phase to phase?
Thank you!

The rating will be for line to earth voltage.

I'll third what the others said. Do not use RCD incommers.

 

[Mrdjc]

And the reason for not using them?

It would be a garage installation, with the 3 phase almost never used, and when its used its to power a 7KW immersion pump for emptying a well, maybe once every two years.

The normal 230 outlets would be used quite reguarly for tools as well as lawnmower, electric strimmer, hedge trimmers, immersion pump for watering garden etc.

It's just one section of the house wiring,it has its own supply from the master CU next to the electricity meter to its own CU located in the garage with RCD protection only for the circuits within the CU in the garage.

There's no issues with "accidental" tripping there.. If someone hacks through a cable with the lawnmower at least it will be safe.

 

[matt1e]

That's very interesting, exactly why I asked the question as it was confusing me to high heaven.

One more thing though, 40A Rating per phase at which voltage, presumably phase to neutral voltage? Or Phase to phase?
Thank you!

the voltage is irrelevant, it's current rating is the max current the RCD can handle on each phase while operating normally (untripped)
The RCD will of course have a voltage rating as well.

Matt

 

[ericmark]

A consumer unit is a type tested distribution unit not sure if technically a consumer unit is the right term when used with three phase? This is because they can be configured in so many ways.

Using RCBO's which are in fact linked MCB and RCD on the output to three phase sockets was common where I worked however it did take up 6 ways with neutral it would be 8 so I can see the point in using a common RCD on incomer.

We often had multi-RCD's protecting circuits each one 3 times higher rated than the last and with moulded breakers also increased time delay on each. So using a 100mA RCD on the incomer was common.

Neutral - Earth faults were a real pain where the neutral was not switched as it often mean we needed to open the distribution unit and isolate all in order to find which circuit was at fault. Of course with plugs not an issue.

The reason for 100mA RCD on incomer was our supply was from a main distribution unit which was not ours and this had a RCD set at 60 seconds and 1 Amp so we wanted our RCD to trip first. However when some one knocked in a nail to hang their coat on through the cable in a Portacabin it took all 4 RCD's out 30ma, 100ma, 300ma and 1A.

With larger RCD's they came in three parts. The moulded breaker had a release coil which was connected to a control panel which in turn connected to the current transformer so in theroy one could have some outputs RCD protected and some not from the same moulded breaker. Instead of tripping breaker it could of course work a warning lamp with so many ways to configure one could of course make mistakes. This is why they are not allowed to be accessed by non trained personal.

It does seem strange that the whole idea of a type tested distribution unit was so in domestic situations the owner could not configure it wrong. Then Part P says not DIY!

 

[Spark123]

It works similar to a SP RCD, but if the phase currents are equal there will be no neutral current. As long as the sum of the difference in phase currents = neutral current the RCD is ok.

It isn't quite as easy as adding the differences in phase currents, I think they need to be added as phasors?

 

[bernardgreen]

There's no issues with "accidental" tripping there.. If someone hacks through a cable with the lawnmower at least it will be safe.

It will not be safe, the cut end will only be slightly less dangerous.

If you cut the cable without any connection to ground the RCD will not operate. The MCB might operate on overcurrent but only if there is a short circuit between Live and Neutral when the cable is cut.

If after the cable is cut there is a path to ground from the cut Live conductor ( such as through the body of the person who picks up the cut cable ) then the RCD is likely but not certain to trip.


An RCD on a three phase supply incomer is not a good idea as a fault on a single phase circuit will open all phases turning off circuits on the other phases which are not faulty

For the pump supply fit a 3 phase RCD that supplies only the pump.

 

[bernardgreen]

It isn't quite as easy as adding the differences in phase currents, I think they need to be added as phasors?

If there is no connection to earth or ground then the algebraic sum of the four currents in the three phases and the neutral is zero at any and all moments in time.

 

[SimonH2]

It isn't quite as easy as adding the differences in phase currents, I think they need to be added as phasors?

Correct, you need to do a vector (magnitude and phase angle) addition of the three phases and neutral - measuring all the currents in the same direction (ie don't measure neutral in the opposite direction to the phases). If it's a balanced load then In is 0, the three phase currents are spaced 120˚ apart and thus add to zero.
For a single phase load, the other phase currents are 0, and the neutral current is 180˚ out from the phase current and thus cancels it.

That should be true at any point in time if there are no earth leakage currents.

 

[bernardgreen]

Correct, you need to do a vector (magnitude and phase angle) addition of the three phases and neutral

And then when the calculation is made the effects of phase angles cancel each other out and can be dropped from the equation. The exception is when there is significant capacitive or inductive components involved and phase delay complicates the calculation. But even then it comes down to " no path to earth means no lost current " so what ever goes in on a phase has to come out spread among the other two phases and the neutral.