(electricity hissing)
(electricity buzzing)
- In this Q&A video, we're going to answer the question,
what's the difference between an AC and a DC isolator?
It's a sign of the times that we're living
in that this question needs to be asked at all
because up until a few years ago,
installing and controlling DC circuits
would've belonged to electricians
working on industrial controls
and perhaps vehicle technicians.
However, with the rise of photovoltaic panels
on private properties,
DC is increasingly making its presence felt
and getting the right isolator
for the right application is really important.
So we're going to answer this question partly by experiment
and partly by taking apart these isolators from Scame.
Now, it may be tempting to think of all electricity
as pretty much the same.
However, there is a fundamental difference
between AC and DC.
Put simply, if you connect a resistive load to a DC supply,
a set amount of current will flow
in one direction around the circuit
and it won't change much until it's turned off.
If you connect a resistive load to an AC supply,
the current is constantly changing value
from zero to a maximum value in one direction
and then it drops back to zero again,
reverses the direction that it goes around the circuit in,
rises back to a maximum value,
drops back to zero,
and keeps repeating this process.
That's why we call it alternating current,
because the direction it moves in alternates.
In the UK,
this entire cycle happens once every 0.02 of a second,
or repeats 50 times every second,
what we refer to as the frequency of the supply.
So what difference does that make?
Well, it makes a huge difference
when it comes to switching
the different types of electricity on and off.
I've set up an experiment here
to show you how much it differs.
Now please, please don't try this at home,
as this is just spectacularly dangerous to replicate.
Here I've got a 1.25 kilowatt load
connected to a 200 volt AC supply,
and it's wired through this incredibly technical
wooden switch that is not made by Scame.
It's made by me.
Now, when I connect it and then disconnect it,
you can see it generates a little bit of a spark
as the contacts come apart.
This spark is generated
because as the contacts start to separate,
the voltage is high enough to ionize the air
between the contacts,
which means this little bit of air
becomes a bit more conductive than usual
and electricity can pass through it to the other contact.
It very quickly, however, reaches the point
where the electricity can't jump the gap anymore
and so the spark is extinguished.
Now watch what happens when I swap over to a DC supply
of the same voltage connected to the same load.
Now when I pull the contacts apart,
there's a big old arc generated between the two
which is sustained until the contacts are much further apart
than they were when we were switching the AC.
So why is this arc so much more sustained
and harder to break in a DC circuit?
Well, it comes back to that fundamental difference
between the two types of electricity.
Because the AC is changing value all the time
and even not flowing for short periods,
it means that the flow of current is easier to interrupt.
Because the direct current is always flowing
in the same direction
and doesn't fluctuate very much at all,
there's never a moment when the electricity isn't flowing,
and so it's harder to get it to stop.
So how does a DC isolator differ from an AC isolator?
And is it a big deal to get them wrong?
Well, here I've got two isolators from Scame,
one of them is a regular triple pole isolator
for emergency switching and locking off
and the other one here is a DC isolator.
Now, apart from the coloring,
these two isolators look pretty much the same.
The yellow and red here doesn't indicate
that this is an AC isolator.
And if we open them up,
they still look very similar inside.
The DC isolator has four contacts
as well as these links in here.
You'll see they both connect one contact to another,
so connecting an incoming conductor here
and an outgoing one here
means that the conductor gets disconnected in two places,
as the contacts are in series with each other.
This effectively makes the air gap
between the contacts larger.
We saw earlier how far the contacts in my wooden switch
needed to be apart to disconnect the flow,
and this effectively larger air gap in the isolator contacts
means that the direct current can be broken
using smaller switches.
For more information on PV installations,
take a look at our sister channel eFIXX ENERGY,
and the AC isolator has three poles,
in this case with a neutral terminal down here.
But even the housing of the internal switches
looks exactly the same,
same torque information, same casing.
It's all written in the same font.
In fact, while researching this video,
I literally took these switches completely apart,
and I couldn't see any physical difference
between them at all.
Only when you start to read the information off the front
do you realize
that they have different applications for use.
As we saw earlier,
to break the flow of DC current is a bit harder to achieve,
and so even though these switches look incredibly similar,
the DC switch is generally rated
to break lower current values,
even allowing for the larger air gap
created by the interlink contacts.
Looking at the markings on the front of the switch,
there's a number of different ratings,
and we're not gonna get bogged down
in all the details of what these codes mean,
but the two ratings on the isolators
that are the most comparable are DC-22A and AC-22A.
Both of these mean switching of mixed resistive
and inductive loads,
including moderate overloads.
And you can see that the AC switch
is rated at 20 amps at 690 volts,
and the DC switch is only rated at 10 amps
and at a slightly lower voltage of 600 volts.
So that really illustrates that two very similar switches
will disconnect very different values of current
depending on whether they're rated for AC or DC.
So it's absolutely critical
that we don't use a switch design to operate AC
for a DC supply.
The manufacturer's information simply must be followed
in this case.
If you'd like to see
the range of Scame isolators in action,
then check out this video right here,
and thank you very much for watching.