Electricity and personal safety:
Concept of ac and dc voltage
Direct current (DC)
is the flow of electric charge in only one direction. It is the steady state of
a constant-voltage circuit. Most well-known applications, however, use a
time-varying voltage source. Alternating current (AC) is
the flow of electric charge that periodically reverses direction.
Electricity
requires a complete path (circuit) to continuously flow. This is why the
shock received from static electricity is only a momentary jolt: the flow of
current is necessarily brief when static charges are equalized between two
objects. Shocks of self-limited duration like this are rarely hazardous.
Without two contact
points on the body for current to enter and exit, respectively, there is no
hazard of shock. This is why birds can safely rest on high-voltage power lines
without getting shocked: they make contact with the circuit at only one point.
In order for
current to flow through a conductor, there must be a voltage present to
motivate it. Voltage, as you should recall, is always relative between
two points. There is no such thing as voltage “on” or “at” a single point
in the circuit, and so the bird contacting a single point in the above circuit
has no voltage applied across its body to establish a current through it. Yes,
even though they rest on two feet, both feet are touching the
same wire, making them electrically common. Electrically speaking,
both of the bird’s feet touch the same point, hence there is no voltage between
them to motivate current through the bird’s body.
This might lead one
to believe that it’s impossible to be shocked by electricity by only touching a
single wire. Like the birds, if we’re sure to touch only one wire at a time,
we’ll be safe, right? Unfortunately, this is not correct. Unlike birds, people
are usually standing on the ground when they contact a “live” wire. Many times,
one side of a power system will be intentionally connected to earth ground, and
so the person touching a single wire is actually making contact between two
points in the circuit (the wire and earth ground):
The ground symbol
is a set of three horizontal bars of decreasing width located at the lower-left
of the circuit shown, and also at the foot of the person being shocked. In real
life, the power system ground consists of some kind of metallic conductor
buried deep in the ground for making maximum contact with the earth. That
conductor is electrically connected to an appropriate connection point on the
circuit with thick wire. The victim’s ground connection is through their feet,
which are touching the earth.
Personal Safety
with “grounding”
the presence of an
intentional “grounding” point in an electric circuit is intended to ensure that
one side of it is safe to come in contact with. Note that if
our victim in the above diagram were to touch the bottom side of the resistor,
nothing would happen even though their feet would still be contacting ground:
Because the bottom
side of the circuit is firmly connected to ground through the grounding point
on the lower-left of the circuit, the lower conductor of the circuit is
made electrically common with earth ground. Since there can be
no voltage between electrically common points, there will be no voltage applied
across the person contacting the lower wire, and they will not receive a shock.
For the same reason, the wire connecting the circuit to the grounding
rod/plates is usually left bare (no insulation), so that any metal object it
brushes up against will similarly be electrically common with the earth.
Circuit grounding
ensures that at least one point in the circuit will be safe to touch. But what
about leaving a circuit completely ungrounded? Wouldn’t that make any person
touching just a single wire as safe as the bird sitting on just one? Ideally,
yes. Practically, no. Observe what happens with no ground at all:
rubber-soled
shoes do indeed provide some electrical insulation to help
protect someone from conducting shock current through their feet. However, most
common shoe designs are not intended to be electrically “safe,” their soles
being too thin and not of the right substance. Also, any moisture, dirt, or
conductive salts from body sweat on the surface of or permeated through the
soles of shoes will compromise what little insulating value the shoe had to
begin with. There are shoes specifically made for dangerous electrical work, as
well as thick rubber mats made to stand on while working on live circuits, but
these special pieces of gear must be in the absolutely clean, dry condition in
order to be effective. Suffice it to say, normal footwear is not enough to
guarantee protection against electric shock from a power system.
frequency“Hz” stands for the
unit Hertz.
It is the measure of how rapidly alternating current alternates, otherwise known as frequency. So, the column of figures labeled “60 Hz AC” refers to a current that alternates at a frequency of 60 cycles (1 cycle = period of time where current flows in one direction, then the other direction) per second. The last column, labeled “10 kHz AC,” refers to alternating current that completes ten thousand (10,000) back-and-forth cycles each and every second.
|
Safety devices in
use of electricity :fuse, |
|
earthing, earthing
methods, circuit breakers, MCBs (Miniature Circuit Breakers) for domestic safety |
A fuse is
an electrical safety device built around a conductive strip that is designed to
melt and separate in the event of excessive current. Fuses are always connected
in series with the component(s) to be protected from overcurrent, so that when
the fuse blows (opens) it will open the entire circuit and
stop current through the component(s). A fuse connected in one branch of
a parallel circuit, of course, would not affect current through any of the
other branches.
Normally, the thin
piece of fuse wire is contained within a safety sheath to minimize hazards of arc
blast if the wire burns open with violent force, as can happen in the case of
severe overcurrents. In the case of small automotive fuses, the sheath is
transparent so that the fusible element can be visually inspected. Residential
wiring used to commonly employ screw-in fuses with glass bodies and a thin,
narrow metal foil strip in the middle. A photograph showing both types of fuses
is shown here:
What is earthing?
Why Earthing is Important?
The primary purpose of
earthing is to avoid or minimize the danger of electrocution, fire due to earth
leakage of current through undesired path and to ensure that the potential of a
current carrying conductor does not rise with respect to the earth than its
designed insulation.
When the metallic part of
electrical appliances (parts that can conduct or allow passage of electric
current) comes in contact with a live wire, maybe due to failure of
installations or failure in cable insulation, the metal become charged and static charge
accumulates on it. If a person touches such a charged metal, the result is a
severe shock.
To avoid such instances,
the power supply systems and parts of appliances have to be earthed so as to
transfer the charge directly to the earth. This is why we need Electrical
Earthing or Grounding in electrical installation systems.
The basic
needs of Earthing.
·
To protect human lives as well as provide safety to electrical devices
and appliances from leakage current.
·
To keep voltage as constant in the healthy phase (If fault occurs on any
one phase).
·
To Protect Electric system and buildings form lighting.
·
To serve as a return conductor in electric traction system and
communication.
·
To avoid the risk of fire in electrical installation systems.
Proper way of Earthing.
·
Earth pin of 3-pin lighting plug sockets and 4-pin power plug should be
efficiently and permanently earthed.
·
All metal casing or metallic coverings containing or protecting any
electric supply line or apparatus such as GI pipes and conduits enclosing VIR
or PVC cables, iron clad switches, iron clad distribution fuse boards etc
should be earthed (connected to earth).
·
The frame of every generator, stationary motors and metallic parts of
all transformers used for controlling energy should be earthed by two separate
and yet distinct connections with the earth.
·
In a dc 3-wire system, the middle conductors should be earthed at the
generating station.
·
Stay wires that are for overhead lines should be connected to earth by
connecting at least one strand to the earth wires.
What is a Circuit
Breaker?
Circuit breakers are specially designed switches that
automatically open to stop current in the event of an overcurrent condition.
Small circuit breakers, such as those used in residential, commercial and light
industrial service are thermally operated. They contain a bimetallic strip (a thin strip of two metals bonded
back-to-back) carrying circuit current, which bends when heated. When enough
force is generated by the bimetallic strip (due to overcurrent heating of the
strip), the trip mechanism is actuated and the breaker will open. Larger
circuit breakers are automatically actuated by the strength of
the magnetic field produced by current-carrying conductors within the
breaker, or can be triggered to trip by external devices monitoring the circuit
current (those devices being called protective
relays).
Because circuit
breakers don’t fail when subjected to overcurrent conditions—rather, they
merely open and can be re-closed by moving a lever—they are more likely to be
found connected to a circuit in a more permanent manner than fuses. A
photograph of a small circuit breaker is shown here:
All fuses need to be replaced
with MCB for better safety and control when they have done their job in the
past. Unlike a fuse, an MCB operates as automatic switch that opens in the
event of excessive current flowing through the circuit and once the circuit
returns to normal, it can be reclosed without any manual replacement. MCBs are
used primarily as an alternative to the fuse switch in most of the circuits. A
wide variety of MCBs have been in use nowadays with breaking capacity of 10KA
to 16 KA, in all areas of domestic, commercial and industrial applications as a
reliable means of protection.
MCBs are
of time delay tripping devices, to which the magnitude of overcurrent controls
the operating time. This means, these get operated whenever overload exist long
enough to create a danger to the circuit being protected. Therefore, MCBs
doesn’t respond to transient loads such as switches surges and motor starting currents.
Generally, these are designed to operate at less than 2.5 milliseconds
during short circuit faults and 2 seconds to 2 minutes in case
of overloads (depending on the level of current).
