- In this Q&A video, we're going to answer the question,
does current flow in the neutral conductor of a circuit?
Now this question relates to the video
that Gary and Rick made on the CEENorm VariaBox,
which is a highly flexible and adaptable bit of kit
that you can have made to your own specs
to make installation life easier.
So do go and check one out after watching this video.
Now you may have noticed in that video
that when Gary opened up the enclosure,
and we had a close look at the connections
in the back of the socket outlets,
there was a neutral connection taken
to the back of the three phase socket,
which may have led you to wonder
if it's really necessary,
and if current even flows in that neutral.
After all, some three-phase plug and socket arrangements
don't feature them at all.
So does current flow in the neutral?
Well, the answer is yes and no.
I love how much you all love an ambiguous
it depends type answer,
but hopefully we'll have got some clarity on this
by the end of the video.
Let's start out by taking a look
at a single-phase circuit.
In fact, we're going to look at the supply
to a consumer unit.
So here we've got the line conductor
that feeds that consumer unit,
and if we measure the current flowing
through the line conductor,
you can see we're getting about 9.7 amps,
somewhere around that,
somewhere between 9.7 and 9.8.
And then if we measure the current flowing
in the neutral conductor feeding that consumer unit,
you can see that we're getting about 9.5, 9.6 amps.
So pretty much exactly the same reading there.
There's a slight variance,
but that might be down to a little bit
of natural current leaking away somewhere it shouldn't,
or it might just be a tolerance factor
in the test equipment.
But it leads us nicely to the conclusion
that current does indeed flow inside a neutral conductor.
And if we look at this very simple circuit diagram,
for a single-phase circuit,
it kinda makes sense.
Current flows from the source through the line conductor,
through the load, and then back down the neutral.
It's a common misconception in electrical science
that current gets used up in the load,
and it's simply not true.
In fact, it's a fundamental rule of electricity,
that the current entering a point or node in a circuit,
must be equal to the current leaving that same node,
the so-called Kirchhoff's Current Law.
So here at Line Size Studios in Skipton,
I've asked the boys to set up an experiment
with a three-phase circuit feeding,
three single-phase sockets.
All the neutrals in the sockets are joined to each other,
and go back to the one neutral conductor
in the supply cable.
Each of these sockets is feeding a heater,
and all of these are the same,
and we'll draw the same amount of power.
So we've got one heater connected to L1,
one connected to L2,
and one connected to L3.
Now if we turn on the heater connected to L1,
we can measure the current in the line conductor
feeding that heater.
I'm just gonna bring in hand cam here.
Now you'll notice in the back of here,
that I've undone the cable ties
that were connecting these together,
so that I can access the cables a little more easily.
It normally looks a lot neater than this,
when you get it from CEENorm.
And what I'm going to do here is very carefully,
just clip this clamp meter onto L1 to start with.
So we're gonna turn that on to measure current.
So let's do that.
So that's now gonna measure current
when we clamp it around L1.
So this is L1,
the heater is drawing some current,
and you can see there that it's drawing
about 3.1 amperes.
Now if we move on from there,
and clip that around the neutral conductor,
what do we find?
We're getting pretty much exactly the same amount
of current, just like we were
on that single-phase supply
to that property earlier,
just about 3.1 amps there.
So no surprises there then.
Now let's leave the first heater on,
and let's turn the heater that's connected to L2 on as well.
So now we've got both of these heaters on.
These heaters are the same,
so we should be looking at roughly the same amount
of current in the line conductor for this socket.
So let's have a look at that.
Bring in hand cam again.
Here we are now on L2.
And you can see there
that we are measuring about 3 amps.
There we go. So about 3.0 amps.
So roughly the same current that we had
from the first heater.
Now when we measure the current in the neutral,
what will you expect it to be?
Maybe the current from L1 added
to the current from L2.
After all, both currents are heading
back down the neutral together,
so it makes sense to add them up.
Let's have a look and see what we're getting.
So we'll clip onto the neutral there.
So we're onto the neutral conductor.
And what are we getting?
We're getting about 3.1 amps,
so that's weird.
It's the same amount as just one of the currents
from an individual heater.
So where's the missing current going?
Okay, well, in for a penny, in for a pound.
Let's hook up the third heater.
So we're gonna turn that on now.
Now they're all on, they're all drawing current,
and this third one is the same as the other two.
So we'd expect it to be drawing the same amount of current.
So we'll just confirm it with a measurement.
So we clip that around L3 there,
and again, we're getting about 3 amps,
so we're drawing the same amount of current.
Obviously, there's some slight differences
in the current there,
because the heaters will be manufactured
within certain tolerances.
So let's just clip this on the neutral and see what we get.
You can see there, see it flash there.
0.1 amps is about how much we were getting there.
You'll see that it's gone back to its auto mode.
So essentially, this clamp meter feels like
it's not connected around a conductor at all,
because there's so little current flowing through there.
So the upshot of this is that basically,
there is no current flowing through that neutral.
In fact, in this situation right now,
we could disconnect the neutral completely
and the heaters would just continue running away
quite happily, neutral-less.
So have we just completely destroyed the idea
that the current flowing into a point in a circuit
should be the same as the current flowing out of it again.
Because if we draw this out as a circuit,
what we're saying is that the current flowing
from each of these heaters
into this common neutral connection
somehow becomes zero current in the neutral.
Actually that is spot on.
Those three current values combined does indeed become zero,
because the three currents are out of phase with each other.
There's a few different ways that we can show this,
but I think the simplest way is like this,
in the form of a three-phase wave form.
Here we're showing the three different currents
as sine waves, L1, L2 and L3,
and you can see that they're peaking
at different points in the cycle of the wave form.
Now I've set this so that the peak
of those wave forms represent the value we measured earlier.
Technically, that value would be higher,
as the value we measured was the RMS value,
but more on that another time.
The key point here is that
if we look at any point on these wave forms,
we can see how much current is flowing
in the three line conductors at any given moment.
And if we take a reading of those currents
and combine them as positive and negative values,
every time we get a value of zero.
So if we've got the same amount
of current flowing in each line conductor,
the current in the neutral will be exactly zero.
And in fact, we don't need a neutral
and that is what we refer to as a balanced three-phase load.
That's why three-phase motors don't generally
have a neutral connection,
because each coil in the motor is drawing exactly
the same amount of current.
And so the neutral current is zero.
Clever stuff.
Now that leads us to the question why have a neutral at all?
Well, we've already seen why in this video.
When we only had one heater connected to L1,
or when we had one on L1 and on L2,
there was current flowing in the neutral.
That's because the current
in each line conductor was not the same.
The three-phase supply was not balanced,
and so we had current flowing in the neutral.
So in a big commercial or industrial building,
where there's lots of single-phase circuits connected
to a three-phase supply,
the electrical designer will have tried
as hard as possible to keep the phases balanced.
However, there'll always be some circuits and some mains
with different currents in the line conductors,
and therefore there will always be current in the neutral,
and it will be required in most three-phase installations.
For more information on the CEENorm VariaBox,
please watch this video here,
and thank you very much for watching.