Hey there guys, Paul here from TheEngineeringMindset.com. In this video we're
going to be looking at, a typical single phase electricity supply into a domestic property. We're going to look at
the distribution cables and transformer, the phase neutral and earth, the main fuse, the electricity meter, the isolation switch, the consumer unit as well as the RCD's and MCB's. Now this is a common design
used across the UK, Europe, India, Australia, New Zealand, et cetera. There are some slight variations and these components may
look a little different between the countries, but essentially it's very similar. However, North America
is a little different because they use two
voltages in the homes. So we're going to look at that in detail in a separate video. You can still follow along in this video, to understand the basics though. I'm going to be using
the European colour codes for this video, that may be different from
your local regulations. Remember, electricity is
dangerous and can be fatal. You should be qualified and competent to carry
out any electrical work. So electricity is generated
far away at the power station.
The electricity leaves the power station and the voltage is increased
in a step up transformer, where it will then be distributed over long distance transmission lines. We generate and distribute
AC, alternating current because it is more
efficient and convenient than DC, direct current. Once it reaches the town, the voltage will be reduced in a substation step-down transformer. If you want to learn
how transformers work, we've covered this in a previous video, do check that out, links in the video description below. From the substation the electricity will either be distributed locally, through overhead or underground cables. Depending on the local design and the voltages being used. A home might be connected
directly to a small transformer located close to the property, or alternatively, a group of houses will
share a larger transformer.
The electricity is
distributed in three phases, but in this case, we're looking at a
single-phase installation, which means the property is connected to just one of these three phases as well as the neutral. We've also covered how
three-phase electricity works in a previous video. Do check that out, links in the video description below. Now, each house in the street might be alternately
connected to a different phase or different streets might be
connected to different phases. This is just to try and balance the demand on the transformer. A smaller service cable comes
off the distribution cable and will feed the property. This service cable will again be overhead or underground depending
on the local installation. The service cable contains the phase and neutral wires. There's also a metal protective
sheaf around the cable in most cases, especially
if it's buried underground. The surface cable comes in, the electricity is going
to flow through the phase, pass through the main fuse, then through the electricity meter, and then into the consumer unit.
The service head or cut out, holds the main fuse or service fuse. This fuse provides
protection to the property and ensures only a set
amount of current can flow into the property. For example, in the UK it's typical to have a fuse rated
between 60 and 100 amps. The electricity distribution company may also remove this fuse
to isolate the property. They may do this for example, to replace the electricity meter. The phase and the neutral then
enter the electricity meter which quantifies how much
energy is being consumed. You may find that this meter
is mechanical or electrical, or even a digital smart meter. There are a lot of variations
of design for these. The phase and the neutral will then leave the electricity meter and enter the consumer unit or fuse board. The consumer unit or fuse board varies in size depending on the size of the property, and how many circuits
there are being controlled. Inside the consumer unit, we first have the main switch or main double pole isolation switch.
This controls the supply of electricity to the rest of the consumer units, and therefore, all the
circuits feeding the property. This switch is not automatic and must be manually
flipped to cut the power. This switch will disconnect both the phase and the neutral line together. The cables typically enter the main switch via the top terminals. At the bottom we find the neutral wire, which will connect to the
neutral block connection. We might find one or more phase wires coming out of the bottom of
the main switch to feed RCDs. If there aren't RCD's being used, then a bus bar will feed
the circuit breakers, but we'll look at that again shortly. The phase line enters the RCD or residual current device.
Again usually entering by the top. This RCD switch is constantly monitoring the electrical current. It's checking if the
current in the phase line is equal to the current
in the neutral line. If these two currents are not equal, then there was an electrical fault and the device will quickly and automatically cut
the power to everything past the switch. Typically, an RCD will break the circuit if it measures a
difference of 30 milliamps, as anything above this
is dangerous for humans. If you touch a live wire and electricity flows
through you to the ground, then the current is
bypassing the neutral wire, so the phase and the neutral
currents will not be equal. The RCD will notice this
and then cut the circuit to reduce the risk of
electric shock or death.
It's now increasingly common to have two or more RCDs in a consumer unit. In such case, the RCD
will only cut the power to the circuits connected
directly after it. So the other RCD will still be powered and only some parts of the
property will lose power. The RCD will trip when it
believes the current is unsafe even for a fraction of a second. It does need to be manually
reset to restore the power, but this does not solve the problem and you should locate and
remove any faulty appliance or fixture.
From the bottom of the RCD, We have a busbar. This is just some conductive metal which the electricity flows along, and connects into each of the MCB's which just makes the installation easier rather than having a lot of cables. The MCB or miniature circuit breaker controls individual smaller circuits. For example, connected to one RCD maybe we'll have one MCB
for the downstairs lighting, one for the upstairs lighting and one for the kitchen plug sockets. On the other RCD, maybe we have one for
the stairwell lighting, one for the upstairs lighting, and one for the downstairs plug sockets. These switches will quickly and automatically trip to cut the power, but they also need to be manually
reset to restore the power The MCB protects the circuit in two ways, overload and short circuit. The MCB is rated to handle
a certain amount of current passing through it.
For example, 32 amps for the plug sockets. If this value is exceeded on that circuit, for example, by gradually
plugging in too many things, then the MCB will trip and cut the power to protect itself. The other protection it offers
is short circuit protection. In the event of a short
circuit for example, the live touches the neutral then the circuit is bypassed and there may be a large and instantaneous increase in current.
This will create a magnetic
field inside the MCB which will cut the
power to protect itself. The phase then Leaves
via the top of the MCB and will flow through the circuit for example through some lamps. It then returns via the neutral cable and into the neutral block. All the circuits do this with the phase coming out
of the circuit breaker and heading off around the property, and the neutral lines coming back and meeting at the neutral block. The neutral block is
then connected to the RCD which checks if the current flowing in is equal to the current flowing out. The neutral then flows from the RCD to the main neutral block, and from there back to the main switch, which is connected to
the electricity meter and the service head.
So, electricity can then flow from the main distribution phase line, up through the service head and the main fuse. It then flows through
the electricity meter and into the consumer unit main switch. From the main switch it
flows through the RCD, along the bus bar and into the MCB. It then flows up around
the MCB separated circuits. The electricity can then come
back via the neutral lines and into the neutral blocks. It then flows through the RCD and back into the main block. From there back into the main switch, and then the electricity meter. Will then pass through the
service head and the fuse, and back into the neutral line of the main distribution cables. Now you may have noticed that
there are some other cables with green and yellow stripes. These are called the earth cables. These earth cable will usually
runs along with the phase and neutral wires into the fixtures such as light
switches and plug sockets.
Some appliances will
also use an earth wire for added protection. Typically, if the device
uses a metal casing. The earth wires will
connect from these fixtures into the neutral block
with a consumer unit. All the earth cables for each circuit then connect into the
earthing block in the unit. Another earth cable will then connect from this earthing block in the consumer unit, over to the main protective
earthing terminal, which is typically located somewhere near the electricity meter. Other earthing wires will connect from this main earthing terminal over and onto metal pipes
such as the plumbing and the gas pipe work. That way if a person touches a live wire in a metal pipe in the property, the electricity will flow
through the earth wire and should be detected by the
RCD which will cut the power.
There are a few ways the main
protective earthing terminal is connected to ground. The first option is shown here with the main earth
terminal being connected to the neutral wire of the service cable, within the service head. This means that the phase to
earth fault is effectively now a face to neutral fault instead. Another option is to use
the metal protection sheaf around the service cable
as the earth conductor. So the main earth terminal is
connected to the metal sheaf and this carries the phase to earth fault back to the transformer. The other option which is
when the electricity supply doesn't provide an earth path. So instead the main earth
terminal is connected to an electrode road which
is installed into the ground and provides a direct earth path. Okay, that's it for this video, but if you want to continue your learning then check out one of
the videos on screen now and I'll catch you there
for the next lesson.
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