Current flow and RCD...

[the3coopers]

Originally Posted by mAJORD

I'm not an electrician. but I'm fairly certain you have loads / outlets connected in parallel in a typical installation

We are saying the same thing, it's just semantics. Yes there may be parallel connections (GPOs) and on a single-phase RCD-protected circuit they all share the same Active AND Neutral. Hence a imbalance at any point on the entire circuit will trip the RCD.

hence I used the term 'sharing' a neutral (and an active) back to the breakers, then RCD

When I commented about "shared Neutral" I meant the situation where a certain neutral was shared between 2 or more separate Actives (across different sub-circuits). So, as far as the RCD is concerned, there is no sharing of the Neutral between circuits. And in such a circuit there cannot be, or the imbalance will make the RCD trip.

Originally Posted by mAJORD

So if you short Neutral to Earth on a power outlet with no load, it will still trip the RCD if there's other loads on the same 'circuit' as "return" current will flow through neutral and Earth, ~halving the current in the neutral wire in respect to Active.

Yes, that's a good point. I was only considering the final sub-circuits "as a whole", which is how the RCD will see the situation. Since each power outlet is probably not on a dedicated RCD, then there will be interaction... so a E to N short at any point on the circuit will cause the RCD to trip if there is any current flowing on that WHOLE circuit.

[Silhorn]

Thanks everyone for your replies.
I understand now.

[bergerac]

In regards to 4. you will trip the RCD if you have an older setup where multiple circuit breakers share one RCD and you have isolated one of those circuit breakers. The single neutral from the RCD is common to those circuits in that case and shorting the neutral and earth on the isolated circuit will cause current to flow through the neutral from the other circuits on that RCD to earth.

[Big Steveo]

Originally Posted by Silhorn

Hello,
Event 1:
If a short between the active and neutral is made, the current will flow from the active to the neutral and then through to the neutral bar (which is linked to the earth bar) and back to the street to the transformer where it enters the earth.
In this case, the RCD will not trip because the current going in still equals current going out but the circuit breaker will trip because of over current in the active.

That's correct. But these days a combination circuit breaker/RCD are commonly used. That is my preferred method of circuit protection

[RussellK]

Originally Posted by Big Steveo

That's correct. But these days a combination circuit breaker/RCD are commonly used. That is my preferred method of circuit protection

Except with RCBO's you usually can't tell if it tripped due to over-current or earth leakage, which can make diagnosis a bit harder than when they're separate devices.

[Symon]

Originally Posted by RobRoySyd

Agreed however exactly what happens in the real world with an E to N fault is hard to predict. It can be compounded by earth to ground connections on the load side.

Agree. As you well know the real world throws all sorts of funny situations at you

[maddhatter]

4 is not entirely correct - shorting n and e will create a secondary men and depending on the resistance of the main men you will may get an imbalance which may trigger the rcd.

[paulvk]

There is a very simple reason why they trip when the neutral goes to earth and that is induced current if it is great enough the unit will trip.

Now for a quiz What was the first device called that did this job and where did it first get extensive use?

[BeanerSA]

ELCB? Is that what you meant. Core Balance Device?

[the3coopers]

Originally Posted by paulvk

Now for a quiz What was the first device called that did this job and where did it first get extensive use?

The first device with a decent senstitivity was the MagAmp, which used an amplifier to allow detection of currents small enough to not cause "death". Earlier traditional core balance device that only tripped at like 10A or so, which was not really a safety device (for humans at least). These were first used in mining in South Africa.

[Symon]

Originally Posted by maddhatter

4 is not entirely correct - shorting n and e will create a secondary men and depending on the resistance of the main men you will may get an imbalance which may trigger the rcd.

If there was no load connected you can short N-E and it will not trip, and the MEN resistance is irrelevant in that scenario. If you had loads on the circuit (such that you had A-N current flow) then yes it would trip.

Originally Posted by paulvk

There is a very simple reason why they trip when the neutral goes to earth and that is induced current if it is great enough the unit will trip.

Now for a quiz What was the first device called that did this job and where did it first get extensive use?

That's one of those 'real world' scenarios that can happen, but doesn't really fall into the context of the OP's questions.

[paulvk]

Yes the Core Balance relay was the device it came out of development in telegraph systems but saw extensive use in teleprinter machines it detects an imbalance in curent through its windings just some history fo you.
Now when you short the earth to neutral you create a loop from the point where the neutral is shorted to earth in the meter box through the neutral wire to the short through the earth wire back to the meter box this loop will pick up induced current from a number of sources ,radio and tv transmitters and the mains wiring it passes near this causes current flow in the loop if it is large enough the device will trip, now in the case the neutral goes to another earthed object a person say then this current will flow through the ground itself back to the earth in the meter box which will normally mean a larger amount of induced current.

[Dice]

Originally Posted by paulvk

if it is large enough the device will trip

Yup. Still an aberrant condition rather than the norm.