On would hope not very many '10s of volts', particularly given that there will be an equal VD in the L - since one needs some L-N difference ('the supply voltage") left to run one's installation
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RCD trip with Live isolated
- Thread starter richardindorset
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Doing the maths is interesting. We are looking at the resistance line to earth of less than 1.44 Ω so a 3 kW kettle will draw around 12 amp, so
V = Ω x A so the maximum voltage neutral to earth is likely less than 17 volts. So we can consider 10 volts as around the most we will see.
So 30 mA at 10 volts, well let's say 27 mA as it has tripped at 30, = 370 Ω. So try it with some damp bread in a toaster, and we find it is clearly variable, but around that area, the same applies to damp plaster in sockets, spiders etc.
When I got my clamp-on able to check at 1 mA increments, I started to test, and what was somewhat of a surprise, never seemed to get the same reading on two consecutive days. Clearly something getting damper or drier. I know the one supply in the whole house not RCD protected, to the central heating, has the highest reading. But this is not a winter job.
However, the main point for this thread has to be line and neutral are both considered as live. So to isolate, the neutral must also be switched off, some RCBO's do switch the neutral, and one can get duel pole MCB's essential with a UK site 110 volt supply, as we don't have a neutral just line 1 and line 2, but with domestic the MCB is normally single pole, so does NOT isolate.
V = Ω x A so the maximum voltage neutral to earth is likely less than 17 volts. So we can consider 10 volts as around the most we will see.
So 30 mA at 10 volts, well let's say 27 mA as it has tripped at 30, = 370 Ω. So try it with some damp bread in a toaster, and we find it is clearly variable, but around that area, the same applies to damp plaster in sockets, spiders etc.
When I got my clamp-on able to check at 1 mA increments, I started to test, and what was somewhat of a surprise, never seemed to get the same reading on two consecutive days. Clearly something getting damper or drier. I know the one supply in the whole house not RCD protected, to the central heating, has the highest reading. But this is not a winter job.
However, the main point for this thread has to be line and neutral are both considered as live. So to isolate, the neutral must also be switched off, some RCBO's do switch the neutral, and one can get duel pole MCB's essential with a UK site 110 volt supply, as we don't have a neutral just line 1 and line 2, but with domestic the MCB is normally single pole, so does NOT isolate.
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There doesn't need to be any difference between them for it to trip when you touch them together, it's related to how much load is being drawn on other circuits(on the same RCD) and the relative impedances of earth and neutral
It's not a fault condition - other than you have caused a fault by touching two wires together.
It is just how things work.
No, the fault condition where there is an earth to neutral fault in one applicance which only trips the RCD when a different applance(s) are switched on, bringing the total current up high enough that 30mA goes via the fault...
I asked our lecturer about this (my original question) and they hadn't got a scooby how or why it would happen...
I now completely understand why the RCD operates now, thanks to everyone's input!
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The greater the load, the higher the volt differential between earth and neutral, the higher the volt differential the more current will flow.
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It's nothing to do with that. Earth and neutral can both be considered 0 volts. The current will travel back to 0 volts any way it can and if you connect a neutral to the earth post RCD some of that current will travel back up the neutral to earth and back to the transformer. If the resistance of the neutral-earth fault path was 30x higher than the design path through the neutral (for example), you'd still get around 30mA of current flowing down the earth at 1A load.
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Have you actually read the volts between earth and neutral? Or even read the current in the earth cable? I remember having to do the calculations in university, can't remember the two calculations now, it was a long time ago, but returning to original question, answer is simple, the lives were not isolated, only one live was isolated, the other live was still connected. As soon as one realises both line and neutral are lives, the question goes away.
The current obviously always travels, by some route(s), back to the 'neutral side' of the transformer, and if there is a neutral-'earth' (really neutral-CPC0 fault within the installation, that current will be shared between neutral and earth, according to their respective impedances.
However, that's not what eric was talking about. In the absence of any 'N-E' fault, there will be a vol;tage drop in the supply neutral due to any loads within the installation, and that will raise the potential of the neutral within the installation to appreciably above the potential of the neutral side of the transformer. However since, in the asbence of faults, no current flows through the 'earth' path, that remains at the voltage of the neutral side of the transformer.
Hence, the greater the load within the installation, the greater will be the potential difference between neutral and earth within the installation - and it is that potential difference which will drive (and determine) the fault current if a 'N-E fault' appears within the installation.
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Yeah, just seems like a strange way of thinking about it. Seems a lot more practical just to consider the neutral and cpc added to the neutral path.
It would take 0.135 Volts to draw 30mA in a 4.5 ohm resistance, which is probably about the max you could expect for a cpc return path. So, barely measurable will be enough to trip.
... pressed button prematurely!
There appeares to be a little confusion going on here, hopefully I'll not add to the confusion.
The sketch is a fairly representative of many installations except I show an excessive loop impedance of 2Ω
Thr current in the load prior to the RCD is shown in orange, this may be the other loads in your own property on the other RCD in the 17th CU or in the neighbouring properties.
Lets say the load is drawing 20A and the source voltage is 240V, the volt drop across the 1Ω cables will be 20A x 1Ω=20V ∴ point 'N' will be at 0+20V=20V and 'L' will be at 240-20V=220V RELATIVE TO 'E'. Additionally the point 'n' will also be at 20V relative to E.
Shorting 'n' to 'E' will result in the current taking 2 routes back to neutral except the current will be shared by the neutrl and earth impedances, in this instance 10A each or dropping 10A x 1Ω (same as 20A x 0.5Ω as resisters are in parrallel) meaning there is 10A flowing through the RCD.
Obviously these figures are manufactured for the explanation and potentially excessive.
The sketch is a fairly representative of many installations except I show an excessive loop impedance of 2Ω
Thr current in the load prior to the RCD is shown in orange, this may be the other loads in your own property on the other RCD in the 17th CU or in the neighbouring properties.
Lets say the load is drawing 20A and the source voltage is 240V, the volt drop across the 1Ω cables will be 20A x 1Ω=20V ∴ point 'N' will be at 0+20V=20V and 'L' will be at 240-20V=220V RELATIVE TO 'E'. Additionally the point 'n' will also be at 20V relative to E.
Shorting 'n' to 'E' will result in the current taking 2 routes back to neutral except the current will be shared by the neutrl and earth impedances, in this instance 10A each or dropping 10A x 1Ω (same as 20A x 0.5Ω as resisters are in parrallel) meaning there is 10A flowing through the RCD.
Obviously these figures are manufactured for the explanation and potentially excessive.
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Thanks for the drawing @SUNRAY , no confusion here. Your drawing has the load on the wrong side of the rcd and your maths doesn't match up with it - the current is still returning to the RCD neutral bus but travelling back up the neutral and out through the earth. Really it's the difference of your ze/2 to your r1 r2 of the isolated circuit (neutral and cpc in this case).
No, the load is the correct side and nothing to do with the circuit being discussed (and possibly not in the same property). the op stated line is isolated, ( presumably the following MCB is switched off or line disconnected)
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Back in 1970s 10-20V was very common between N & E where I lived. We had a spare car battery in the garage and a battery charger with a failed mains transformer...On would hope not very many '10s of volts', particularly given that there will be an equal VD in the L - since one needs some L-N difference ('the supply voltage") left to run one's installation
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