Maybe not, but in the scenario when the circuit on the other side of the FCU was still live and probably with loads my money is on a parallel path back to the tx
Sanity Check - RCD Trips with Safe Isolation
- Thread starter dizz
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Maybe not, but in the scenario when the circuit on the other side of the FCU was still live and probably with loads my money is on a parallel path back to the tx
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I can see your point but the energized circuits under load would slightly raise the Neutral potential with respect to earth because the Neutral would have some voltage drop across it. When you then short N-E on the load side of the RCD this causes additional current to flow from N-E but you do not need that additional current to trip the RCD.
You can do the following experiment:
On a TT or TN-S system:
1) Connect only a single circuit via one single RCD to the CU.
2) No other circuits connected at all.
3) Short N+E on the load side of the RCD will trip.
4) Now link (yes only for us professionals) the MET with N on the supply side. This is now limiting the N+E loop to inside the property.
5) Short N=E on the load side of the RCD. It will not trip.
While I appreciate you willingness to learn - your first lesson should have been to understand how to properly follow the safe isolation procedure.
By following that you would not have had the sanity issue.
Do you now understand what to do for safe isolation?
What you are describing is a contrived situation that probably would not exist under normal conditions.
The effect of a RCD tripping when Neutral & earth are shorted is usually in a situation where a ciruit or part of circuit has been isolated in a single pole only. This happens when you can't switch off becuase power is required elsewhere e.g. other trades or lights. Neutral current then finds a paralell path back to the TX via earth bypassing the RCD causing an imbalance and the RCD then trips. It is not limited to TT or TN-S systems and the effect happens on TN-C-S all the time. Draw it out, it is really quiet straight forward
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No it isn't. It is an experiment which confirms that you need no other influences within the property on any circuit to cause an RCD to trip when N is shorted to E. It rules out what you appear to say is required to trip the RCD. Though I accept what you say will help trip the RCD. It is very simple. Please try the experiment.
I don't see the point of raising having to switch off just the one circuit because other trades need power etc. I have to do that all the time and understand that. But that is not my point.
The RCD trips because there is a potential difference between neutral and earth on a TT and TN-S system. When you short the N & E you create a situation where a small current flows in the Neutral side of the RCD and not in the Line side so the RCD sees an imbalance and trips. It will do that with other circuits on and drawing current and without other circuits on and not drawing current.
If you then link the Neutral and earth before the RCD then the RCD will not trip when you short Neutral and Earth on the load side. There is nothing contrived about this. It is plain science. I ask yo to try it before you reply. If you fail to get the same result than I will admit that you are right and I am wrong.
Yes, you will see the RCD trip on a TN-C-S system but when the short between N&E is distant from the electrical service entry point (the point where N&E are linked at the cut-out)
I don't need to try a dodgy experiment, the paralell path does not "help" trip the RCD it is the reason it trips
I don't know what you mean, the point I was making in a ideal world you would isolate both live conductors. Single pole isolation is allowed on TN-S and TN-C-S because N-E will be at the same potential or very close, but single pole isolation creates "the effect" if N-E are shorthed for some reason
Quite often on TT system there will be a PD between neutral and earth (hence the requirement for double pole isolation) this does NOT cause a RCD to trip, because as long as what goes in come out then no imbalance occurs. When that current leaks off via some means to earth then the imbalance occurs.
There is no extra current in the neutral, rather it has found another way back to the TX, hence the imbalance
Absolutly not science, as before the imblance is caused because the current has found a paralell path via earth back to the TX .
There are at least 3 external mechanisms by which an RCD will trip ignoring for now the standard L - E short or leakage.
First the way that you state (at least what I think you state)
OTHER ENERGISED LOADS ON THE LOAD SIDE OF AN RCD
If you short the N&E on the load side of an RCD which is carrying current from other energised circuits.
The RCD will trip because the current flow is now greater on the L side than that of the N side. This is because E is carrying some of the current that the N was carrying (E&N are sharing the current while L has it all)
OTHER ENERGISED LOADS NOT ON THE LOAD SIDE OF AN RCD
These other loads can raise the potential of N.
Then despite no other energized load on that specific RCD If you short N&E that specific RCD will trip because N has a higher potential than E. You short them on the load side and a current flows due to PD but this time with current on the N side of the RCD and no current on the L side.
NO ENERGISED LOADS AT ALL THROUGHOUT THE PROPERTY
TN-S &TT
If you short N&E the rcd will trip because there is a small PD between N&E
on such earthing arrangements (imported into the property) Again, it is the N side of the RCD that has all the current with none on the L side. TT offers more PD between N&E but TN-S offers some.
NO ENERGISED LOADS AT ALL THROUGHOUT THE PROPERTY
TN-C-S
If you short N&E the RCD generally does not trip if the short is close to the cut-out. If you move the short further from the cut-out (where N&E are connected) then the RCD sometimes trips.
My point here is to say that in the absence of other local (to the property) reasons for tripping an RCD, when N&E are shorted, will trip.
Why do I say this?
1) I have tried all of these combinations and obtained repeatable results as listed above.
2) I have measured N with respect to E on many TN-S systems and seen as much as 30 volts.
3) I have shorted N&E on the load side of an RCBO with zero load across L&N. It trips.
4) I have shorted N&E on the load side of an RCD with zero load on it. It trips.
All I ask you to do is try one of these, come back and tell me what you observed. If not then I'm sorry that you feel it is too dodgy.
First the way that you state (at least what I think you state)
OTHER ENERGISED LOADS ON THE LOAD SIDE OF AN RCD
If you short the N&E on the load side of an RCD which is carrying current from other energised circuits.
The RCD will trip because the current flow is now greater on the L side than that of the N side. This is because E is carrying some of the current that the N was carrying (E&N are sharing the current while L has it all)
OTHER ENERGISED LOADS NOT ON THE LOAD SIDE OF AN RCD
These other loads can raise the potential of N.
Then despite no other energized load on that specific RCD If you short N&E that specific RCD will trip because N has a higher potential than E. You short them on the load side and a current flows due to PD but this time with current on the N side of the RCD and no current on the L side.
NO ENERGISED LOADS AT ALL THROUGHOUT THE PROPERTY
TN-S &TT
If you short N&E the rcd will trip because there is a small PD between N&E
on such earthing arrangements (imported into the property) Again, it is the N side of the RCD that has all the current with none on the L side. TT offers more PD between N&E but TN-S offers some.
NO ENERGISED LOADS AT ALL THROUGHOUT THE PROPERTY
TN-C-S
If you short N&E the RCD generally does not trip if the short is close to the cut-out. If you move the short further from the cut-out (where N&E are connected) then the RCD sometimes trips.
My point here is to say that in the absence of other local (to the property) reasons for tripping an RCD, when N&E are shorted, will trip.
Why do I say this?
1) I have tried all of these combinations and obtained repeatable results as listed above.
2) I have measured N with respect to E on many TN-S systems and seen as much as 30 volts.
3) I have shorted N&E on the load side of an RCBO with zero load across L&N. It trips.
4) I have shorted N&E on the load side of an RCD with zero load on it. It trips.
All I ask you to do is try one of these, come back and tell me what you observed. If not then I'm sorry that you feel it is too dodgy.
No, you could get quite high currents but the voltage between the earth and neutral wires would be very low. Less than 5 volts worse case.
The current could be as much as half the current from any appliances in use and served by the RCD but probably not more than about a quarter of it depending on lengths and sizes of cables involved
thanks for drawing bernardgreen, it illustrates what I have been saying
And that is what I have described in words in my previous post. Along with other scenarios.
OK - I thought safe isolation simply meant opening the MCB for the circuit involved (after live/dead check, checking equipment etc). Since a neutral-earth fault on the dead circuit could still see current flowing from other circuits at the CU, what does this mean in a practicle sense from a safey point of view?
The risk of serious electrical shock is low PROVIDED you are certain that the circuit you are working on is the one whose MCB has been turned OFF.
However there is always the risk that MCB may be turned on by someone un-aware that you are working on the circuit. ( like a person who wants the TV back on.)
There is a risk there are ( due to bad installation ) two MCB's feeding that circuit.
There is a risk that while working after dark on a circuit that is 95% electrically safe to work on you trip the RCB with an earth to neutral short and lose the lighting circuit. Sudden darkness can result in an accident causing injury to you or someone else in the house.
In addition to the procedure below, think about what can happen if you are holding a neutral that becomes disconnected downstream (at the CU for example) from energized loads.
Disconnect all loads.
Locate the source of supply.
Isolate source of supply.
Lock off and place warning signs.
Test your voltage indicator on a known good source. ( proving unit)
Test the supply is isolated.
Re-test voltage indicator using known good source. ( proving unit again)
Continue to treat all conductors as if they were energized
Make final connections from new circuit to origin last not first and after full dead testing
Less than 5 volts because the voltage is the difference between the potential on the neutral bar and the potential on the neutral block in the cut out where the CPC is jointed the the neutral. This is a very short and very low impedence length of circuit, typically less than 0.1 ohms, so with 50 amps flowing through it the potential developed between those points is typically less than 5 volts ( 50 A x 0.1 ohms ).
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