Electricity flow and the role of Neutral

[Neptune]

A numpty question....
In domestic electrics, can one be electrocuted by the neutral wire when there is no short circuit fault?

All the online videos suggest that current flows through live and back through the neutral. If the current going out through live is not the same as the current returning via neutral, the RCD will act. This suggests that the N is as lethal as L. Can you please clarify this for me.

Also, with the RCD looking for equal current between L and N, what happens if we have a ring or a radial where one of the sockets is unwired and we have the cables hanging in the air? Would this cause the RCD to trip?

I should add that I treat L and N with equal caution.

Thanks in advance.

 

[littlespark]

Electricity flows in a circuit.

Line and neutral are both live conductors, but neutral is tied to earth somewhere

Touch line before it gets to a load, you get a shock because the path of least resistance to earth is through you.

Touch neutral after a load, you may not get a shock as least resistance is through the wires to earth, rather than through you.

If the neutral is disconnected, and there is no direct connection back, then it will go through you again if touched.

An rcd works by the difference between the line and neutral current flow… if any of it goes through you, from line or neutral… there is an imbalance… and if big enough (30mA) it will trip.


Please don’t try any of the above.

 

[Neptune]

An rcd works by the difference between the line and neutral current flow… if any of it goes through you, from line or neutral… there is an imbalance… and if big enough (30mA) it will trip.

But can the imbalance also be cause by a socket missing and cables hanging in the air?

 

[timhoward]

Also, with the RCD looking for equal current between L and N, what happens if we have a ring or a radial where one of the sockets is unwired and we have the cables hanging in the air? Would this cause the RCD to trip?

No. It might be more helpful to think in terms of "what goes out must come back". The cable in mid air is not drawing any current, so the RCD is not out of balance. It's effectively the same as a plug socket with no appliance plugged in.

I hesitate before complicating things, but what can happen is that Neutral touches the bare CPC, and this provides another route back for some neutral current, and the RCD trips because some of the current is taking another path.

 

[Neptune]

No. It might be more helpful to think in terms of "what goes out must come back". The cable in mid air is not drawing any current, so the RCD is not out of balance. It's effectively the same as a plug socket with no appliance plugged in.

I hesitate before complicating things, but what can happen is that Neutral touches the bare CPC, and this provides another route back for some neutral current, and the RCD trips because some of the current is taking another path.

If I remove a socket on a radial circuit and the 2.5mm at the end is hanging mid-air, how and where is the current going back to the RCD? Is this being returned at the socket prior to the one that is whipped off....so to speak?

 

[littlespark]

Parallel circuits.
If you take a socket off and have a floating bare end… there is a voltage, but no load… even if there is a socket elsewhere on the circuit doing something.

No load, no current.

If you touch the bare end, the voltage passes through a resistance…. You…. And current then flows.

Current always wants to flow with a voltage applied…. And it will find a way… it will jump very small gaps to complete a circuit, where we get arcing.

The trick is to stop it flowing to where you may want to be touching these bare cables. Best place is from the circuit breaker. A nice easy switch that will isolate the power from entering a “final” circuit.
Can also be locked off with the correct device to stop it being accidentally turned back on.

 

[OnlQQker]

This video explains it very well!

 

[timhoward]

@littlespark has done a great answer. As I already typed this you are getting it anyway!

If I remove a socket on a radial circuit and the 2.5mm at the end is hanging mid-air, how and where is the current going back to the RCD? Is this being returned at the socket prior to the one that is whipped off....so to speak?

I'm not sure exactly what you are asking, so here are two questions and answers:
1) How does an RCD work if there is nothing on the end of a circuit and no loads?

Simple, nothing is going out, so nothing should be coming back. But if you touch the bare live, there you are the load returning through earth and the RCD will (should) say hang on a minute and trip.

2) How does removing a socket on a radial circuit affect an RCD?

Think in terms of a complete flow from Live, through a load, and back through the Neutral wire to the rcd and consumer unit.
On a radial circuit exactly what happens depends where the socket is that you remove.
It will either interrupt the live and neutral so the load doesn't work. Or if you removed a socket after the load and none of the conductors touch, then no difference would be observed. Of course as already said this is highly dangerous and we are talking theory here not something that should ever happen (whatever Delroy does on youtube!)

If a load is working and N touches Earth at a socket somewhere, then there are now two possible paths for the return current to take, so not all of it will go through the RCD, and the RCD will probably trip. Most of the current would go down the lowest resistance path which is the normal Neutral wire but it's perfectly likely that at least 20ma will travel on the alternative route and back via the earth (cpc) causing a trip.