How would you explain how PME works?

securespark · 15 May 2007

The principle of carrying N/E combined into the property, then splitting it at the cut-out.

How would you explain how this works to beginners, especially the bit about how an RCD works in conjuction with TN-C-S. N/E together = RCD tripping......

Steve · 15 May 2007

Moreover, how would you explain why you cant carry the combined protective/neutral conductor throughout the installation?

securespark · 16 May 2007

That's the easy bit. Regulation forbids it!

JohnD · 16 May 2007

(it is done in some countries but would make RCDs unreliable as the paths are no longer independent)

Spark123 · 16 May 2007

You could start by saying that as a T system the supply is tied to earth at at least one point. N - exposed conductive parts are connected direcly to the earth point of the source, C- the neutral and earth are combined - S the neutral and earth separate out from each other (commonly at the distributers head), hence the TN-CS.
As the Neutral and Earth are combined along the distributers network, a break in this could lead to the ECPs in a premises becoming live. To reduce this risk the combined neutral and earth conductor is tied to earth at various positions along its length. This method is known as protective multiple earthing (PME).
It should be noted that for certain applications, a TN-CS is not allowed owing to the increased risk posed by an open protective conductor such as caravan sites or to boats.

Lectrician · 16 May 2007

Or petrol stations due to circulating network currents sparking on burried metal tanks in contact with mother earth.

Or swim pools / commercial shower rooms (not against regs, but against most suppliers conditions, and mentioned in GN and NIC). This is due to percieved shock due to voltage drop on the neutral due to currents in the network - this creates a PD between the ECP's and mother earth. A classic example of this is the outside tap of a house - the tap is connected to PME, and someone standing barefoot on soil could recieve a 'percieved shock' due to floating volts caused by large currents in the DNO network.

plugwash · 17 May 2007

securespark said:
especially the bit about how an RCD works in conjuction with TN-C-S. N/E together = RCD tripping......

NE together after RCD = RCD tripping happens on all earthing systems (with the exceptions of IT and earth missing) it just only happens intermittantly under PME.

The most dumbed down explanation i can come up with for rcds tripping on N-E faults is:

RCDs detect an imbalance between current out and current returning. If neutral connects to earth then this provides a path that only goes through the RCD one way. If conditions either within the installation or outside of it cause current to flow down this path then the RCD will trip.

but ultimately i think whatever you say is going to be gibberish to someone with no understanding of electricity

some diagrams could really help here. Show them how adding a N-E fault adds another possible path for current to flow back to the suppliers CNE core/the transfomer start point. You probablly want seperate diagrams for load on the RCD side and load on the non-rcd side and for both TN-S and TN-C-S.

Spark123 · 17 May 2007

TN-CS is slightly different to the other T supply types in that the potential between N-E at the suppliers service head is at the same potential.
An RCD works on the imballance principle, if a current is detected in the phase coil it must equal the current in the neutral coil. If the difference between the two equals greater than the trip current of the RCD (between 15mA & 30mA) the RCD will trip.
A neutral to earth short on a TN-CS system will not cause a trip if there is no current flowing in the system.
In a healthy system the current flowing in the system will flow through one side of the RCD and back through the other. If there is a N-E fault present on the system there will, in effect, be a parallel path around one side of the RCD. This parallel path may have a relatively large resistance/impedance in comparison to the RCD detection coils, though as it is in effect a resistance in parallel with it current will flow around it. If this current exceeds the trip current of the RCD (only a few mA) then it will trip.

plugwash · 17 May 2007

also for load on the non-rcd side there is a path in paralell with the neutral tail through the neutral side of the RCD, the faulty circuit and then down the main earth back to the head, this can also cause a trip if the current is high enough.

securespark · 18 May 2007

What I was gettig at was this:

Imagine you are not electrically-minded.

I have just told you:

a) an RCD looks for earth faults, of which N/E shorts is one.

b) a TN-C-S system has TN-C before the cut-out & TN-S after.

How do you explain how the RCD recognises the neutral and earth after the cut-out as separate, when they are combined in the cut-out?

plugwash · 18 May 2007

securespark said:
Imagine you are not electrically-minded.

If they are unable to grasp voltage and current at least in some abstract sense then i think you are wasting your time trying to explain RCD trips to them.

securespark · 18 May 2007

Well, I managed to get so far with them. They can at least understand the principle of

in/out = the same = no trip

in/out = difference equal to rating of device = trip

So I think I can get the idea through, given some clever descriptions & patience.

plugwash · 18 May 2007

once you've got them to grasp that i'd say its time to pull out some diagrams showing how in can end up different from out.

ColJack · 18 May 2007

Simplified operation of an RCD

power goes through the RCD on the live wire, and comes back on the neutral wire..

the rcd measures what goes out on the live wire and sees if it comes back on the neutral wire

if it doesn't all come back then there is a fault somewhere and it switches off..

( note, the word "power" is used as a generic term for non electrically minded people.. )

think of it like this.. ( bear with me, i'm getting abstract here... )

if there is a car park, and there is only one way in, and the guy on the gate counts the cars in and back out, he'll know how many spaces there are left for more cars..

if someone knocks down a barrier to get out without paying, and other drivers follow him, the poor guy counting will stop any more cars coming in because he think's the car park is full...

securespark · 18 May 2007

I think you're misunderstanding me!

It's not the operation of the RCD I'm struggling to explain, more how the RCD is able to see a PD between N & E, given they are combined in the cut-out...

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How would you explain how PME works?

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