So, I'll start again. A neutral / earth short circuit will not cause an RCD to trip. If a fault or high impedance earth loop is used, then the neutral becomes higher in potential than it should be, so it isn't at neutral potential any more because neutral potential is the same as earth potential. What has happened is, the conductor that is used as the neutral bond has now risen to phase potential because earth hasn't removed the residual current from it. The neutral is now a phase. If the phase then leaks to proper earth potential through a lower impedance path, then an RCD is designed to detect this leak and trip open. Again. Neutral and earth are at the same potential, so a straight short across them will do nothing. If an induce current of any kind lifts the neutral conductor to phase potential, then it no longer is at neutral potential. This can be caused, as you have demonstrated yourself above, when the earth bonding does not remove the residual current properly and causes what should be the neutral conductor to rise in potential energy, and so it becomes more to phase potential. Now create a short circuit across the neutral conductor which is now sitting well above earth potential and you cause an imbalance between phase potential and earth.
An RCD will not trip open with a fault between neutral potential and earth potential, because both neutral potential and earth potential have exactly the same potential. An RCD will only trip open circuit if it detects an imbalance between phase potential and earth potential.
Exactly so. Congratulations: you've managed to say in 5 lines what usually takes me several screenfuls - and just took Andy W nearly as long, but with more cases more thoroughly considered - on the frequent occasions that this FAQ comes up!
It does, demonstrably, and without any further faults.
Your mental model of what an RCD does is all at sea, I'm afraid. Since it's a CURRENT BALANCE device, it's easiest to see what happens in an N-to-E fault in terms of CURRENT.
By "it's a CURRENT BALANCE" device, what I mean is this: it compares how much current goes 'out' from its L or 'phase' terminal, to how much comes 'back' to its other, N terminal (both these on its load side). While wot-comes-back is within 30mA of wot-goes-out, it's happy; if the imbalance exceeds 30mA, it trips. This contributes to safety, since that
30mA of current is going somewhere it oughtn't to - just maybe through a person; 30mA is chosen as it's around the current needed, given typical body resistance, to be approaching Serious Consequences.
When there's an N-to-E short, there's a second current return path created: the nature of this return path and the direction of the imbalance vary according to the source of the 'outgoing' current and the earthing arrangements.
In the case most relevant to the OP: the RCD is supplying the load (here a washing machine) with the 'outbound' current off its L terminal, and wants to see it all come back to its N terminal. With no N-E fault, this is just what it gets. (Indeed, if the E for the circuit/socket/plug supplying the washing machine becomes disconnected, the RCD is *still* in balance, and doesn't trip.) Now, if an N-to-E short happens on the circuit supplying the washing machine - or inside the washing machine itself - all the current which went 'out' to the washing machine now has a choice of routes 'back': the 'original', intended route, back through the RCD's N terminal (and back along the relevant CU's N busbar, the black meter tail, and back to the substation/transformer where the supply's L side comes from too), but also a new route through the 'E' conductors (the circuit-protective-conductor in the T&E cable supplying the washing m/c), crucially BYPASSING the RCD's N terminal, back through the mess of E wiring around the CU, and finding its way back to the substation and points closer to the installation through the many paths between the installation's main E terminal and the supply's N. (In the case of a PME (TN-C-S) installation, there's a local low-resistance path right at the supply entry point; in the case of a TN-S installation the supply E and supply N may not be in good metallic contact until we're back at the substation; in the case of a TT installation there's only the 'mass of earth' path back to the originating N.)
So, how much of the load current coming out of the RCD's L terminal 'chooses' the 'balancing' path back to the RCD's N terminal, and how much goes the 'wrong', 'imbalancing' way? It depends completely on the relative resistance (impedance, if you want to get picky) of those two paths. For a PME installation, as outlined above, the 'imbalance' path is all within the house, and is almost as low as the 'balancing' path - a little higher, since CPCs are usually a bit thinner than N conductors; so for a 10A load you might find 7A going back the 'right' way and 3A going the 'wrong' way. The RCD will certainly trip in this case! The TN-S case isn't that different, except that the nearest link between the installation's E and the supplier's N isn't within the house, but might be as far back as the substation/transformer: but both paths are low-resistance and metallic, so again you can expect a substantial proportion of the load current to 'choose' the 'imbalance' path, and again the RCD will trip. At the other extreme, for a TT installation, with a relatively high earth electrode resistance, the 'imbalance' path could be at least 100 times higher resistance than the 'balancing' one, so an on-load N-to-cpc fault isn't nearly so likely to cause a trip.
The analysis is similar for the related FAQ case of 'I was working on a ring circuit I'd turned off the MCB for, touched its N to its E and all the RCD-protected circuits in my house went dead'. Here the just-created N-to-E short provides an alternative path bypassing the RCD's N terminal for all the loads in the *OTHER* circuits the RCD is supplying, since the N in the 'isolated' ring isn't isolated, but connected to all the other N's through the consumer-unit busbar. Andy W's essay covers those cases in detail, so I won't go on any more.
Whether this is enough to convince you that your 'neutral potential' model is misleading, I can't tell. I assert again that a 'pure' current-balance RCD doesn't give a monkey's about the relative potential of L, N, and the installation's E (to which it's not directly connected), though obviously you need to have an idea of their potentials to see whether any current will flow among them. (Old-style voltage-operated earth-leakage breakers are different, and some 'modern' electronic RCDs have further refinements beyond the pure 'current balance' idea; but that's another discussion). I hope it at least gives you some pause for thought, and might even inspire you to do a (safe ;-) bench experiment with a small load, a current-balance RCD, and selected-resistance N-to-E faults ;-)
No it doesn't. Neutral and earth points are taken from the place at earth mass, so how can a short across neutral and earth cause an imbalance between them, when they are already short circuited together at the point where they're both connected to earth mass. It can only happen when one of the conductors of either earth or neutral has risen or dropped below the potential at which they where both connected to earth.
Just to chuck my 2 penny worth in Stefek has this right and Roger has put the same thing the same way.
All you need to remember is the FUNDAMENTAL thing about a RCD is that it compares the current IN to the current OUT if they are unbalanced by more that the rated trip current nominally 30 or 100 ma then the device will trip.
Remember the word !!CURRENT!! flow thats the important part.
It matters not whether the RCD is supplying a single table light or a shower and cooker on the go together. As long as the CURRENT IN matches the current OUT then all will be well. Offer the electricity an alternative path and this balance is upset and off the trip will go.
That is all there is to it.
Let me try to put something else in perspective.
Imagine an RCD connected to the incoming supply. Nothing is connected to the output of it at all.
Now take a wire thats connected to a good earth like a stake in the ground outside in very damp soil and after its been raining. Touch this to the live and the trip will immediately fire off.
Now do the same with the neutral output side of the trip and it won't trip. So why does this happen?. Remember CURRENT flow. In the first instance from the live to your earthstick current is flowing from the incoming live line, through the RCD and then via your stick and then through the earth back to the substation where one side of the supply the neutral is connected to earth so you can see that current flows through the live side of the RCD back through the earth connection to the supplying transformer. It is NOT going back through the RCD so a current imbalance has taken place.
The important thing is that there was an !!imbalance!! because more was going OUT of the RCD than what was coming BACK through it!.
However when you touched the earth stick to the neutral NO CURRENT was flowing through the live side, so no current was or could have been flowing back!.
Now suppose you have a one bar electric fire connected to the output terminals of the RCD. Normally what is going out is going back through the RCD, so all is well.
Now connect that earth stick to the neutral again and it will trip. Reason is that the CURRENT has an alternative way back to the earthed side at the substation, some current is flowing through the RCD back to the substation , but some is now taking the alternative route home via the earth stick.
So an imbalance is caused, their being current flowing OUT through the RCD but it has an alternative route back and if its going back and its NOT through the RCD so its not the same as what's coming IN, then off will go the trip.
Simple as that.
So thats why sometimes when you have an Earth to Neutral fault, it doesn't show up until some "current" is flowing through the RCD through the alternative path back to the substation, as until their is some current flowing "through" the RCD, then it won't trip. Course that only needs to be more then the rated tripping current but as long as thats exceeded then off it will go, but it CAN'T trip unless there is some current actually flowing through it in order to exceed this rated trip imbalance level...
Amongst your lengthy explanation which is a fair approximation to real life you missed the point that in many installations a N-E voltage exists. (I'm not entirely happy about describing an AC current as flowing "from" the live but lets avoid pedantry.) O.K. its often "small" but present nevertheless. A short between N E wiries will thus have a resultant current which is not being cancelled out in the current balance detector and thus the device will trip. Depending on the N E voltage and the rating of the RCD this can require a full short or it can sometimes be sufficient to get your body parts (fingers etc) across the N E wires.
Sigh. My last word follows, you're welcome to have the real last word.
If you persist in the mistaken belief that 'neutral and earth points' are in all non-fault situations already 'shorted together' at zero resistance, I'm sure there's nothing I can say which will shake your belief. The fact that the belief is wrong, will mislead you in doing fault-finding on RCD trips (and propagating the belief would mislead others), doesn't take into account the finite resistance of copper conductors, and that in the case of current-balance RCDs the *location* of any such 'final' bonding of N to E is crucial in understanding how the current flow divides up between a through-the-RCD path and an around-the-RCD path - all of that is is just sad; at least until such time as parts of that belief affect the safety of others.
I don't doubt that the bulk, and quite possibly all, of the work you do both personally and professionally in the electrical sphere is both safe and competent. If part of your mental model about how it all works is interestingly non-standard, it doesn't matter that much. Hell, one could go through an entire career in electrical installation and design work believing that current was carried by little green worms (which don't like wriggling through narrow wires), or indeed that it was the mass movement of electrons themselves, without doing anything dangerous...
Its because the supply transformer has one side connected to earth which forms the neutral line..
What you need to consider is that the mains is tied to EARTH when it comes out of the substation transformer. There is a drawing of this on the TLC website.
Consider the circuit as follows. Say we have a mains transformer up a power pole say 200 metres away from the subscribers premises. Now trace the circuit from lets say the live side of that supplier transformer. It then goes along the live line to the supply fuse through the meter then through the RCD then through the fuse board isolator switches etc then if the "earth stick" was connected to the live output side of the RCD the circuit would continue back through earth to the earth plate at the substation so there would be a "load" placed across the supplying transformer, and hence current would be flowing through the live conductor of the RCD but not back through the neutral side of the RCD so the RCD will now be unbalanced and thus trip.
Now consider the same circuit from the earthed side of the supply transformer trace it along the subs premises through the isolator switch and then out through the neutral side of the RCD. Now consider the same earth stick connected this time to the Neutral conductor this will provide a path through earth back to the supplying transformer.
No current will flow and you should be able to see that no current will flow 'cos we've "connected" one side of the supply transformer back to itself so no current is flowing save a "very" small amount of induced leakage!..
The problem arises with the neutral short to earth when the current that comes in on the live line now has another path back to the earth plate at the supplying transformer as well as the one, and only one, it should have through the neutral side of the RCD.
So suppose we have 10 amps flowing through the live side of the RCD we don't have the full 10 amps flowing back through the neutral as some of its being diverted back through the neutral-to-earth short of the consumer side, or protected side of the RCD, so as long as that current is greater than the 30 milliamps difference that the RCD is set to, then off goes the RCD.
Errr, in fact it might. There is a very good chance that a potential difference will exist between your earth stake and the neutral connected to the incoming side of the RCD. Hence current can flow.
This is true.
No, there could be current flowing back - being sourced from your earth stake.
true...
Yup, so long as you don't fall for the BigWallop assumption that E and N are going to be at the same potential. The only setup where that is likely is on a PME system, and then only where there are no circuits turned on at all.
You don't have to. Wire a plug with two wires, one to neutral and the other to earth. Plug it into a socket and touch the two wires together. Your RCD will trip; if it doesn't, it's faulty.
Yes you could argue this, but I didn't want to cause any more confusion more than what's perhaps been generated already;)
In fact if there was a heavy earth current into the earth locally than that could be sufficient to cause upset, but its rather unlikely.
In fact I have to alter a circuit thats fed by around 900 metres of overhead low voltage sometime this week I'll see what's induced there.
Snipppppppppppp
No of course if your e and n are "connected" on your premises this loop is very short so no real chance of anything there at all!.
In fact years ago we used to use the earth to carry audio over from mine to a friends house with two stakes set someway apart. Its just a collection of an infinite number of resistor's really:))
Surprising what audio you could get through, but there wasn't that much hum!...
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.