Transcript CIRCUITS AND SAFETY

CIRCUITS AND SAFETY
SUBSTATION and DISTRIBUTION
GRID
• Find pictures of Orland park substation,
and distribution grid
DISTRIBUTION GRID
ONE LINE DIAGRAM
• Power systems are extremely complicated electrical
networks that are geographically spread over very large
areas. In fact, the power systems are so complex that a
complete conventional diagram showing all the
connections is impractical. Yet, it is desirable, that there
is some concise way of communicating the basic
arrangement of power system components. This is done
by using Single Line Diagrams (also called One Line
Diagrams.
ANALYSIS OF A ONE LINE
DIAGRAM
Equivalent Transmission and Generation Grid
AC
VOLTAGE
138,000V
STEP DOWN
TRANSFORMER
4160 V
Distribution Grid
CIRCUIT
BREAKER
STEP DOWN
TRANSFORMER
240V
Arrows represent Houses
CIRUCIT DIAGRAM vs. ONE LINE
DIAGRAM
• Which lines could fail to produce greatest
outages and least outages
12 V
10-2 ohms
10-2 ohms
10 ohms
10 ohms
10-2 ohms
10 ohms
CURRENT AND RESISTANCE
• If a fault occurs in one of the lines. What
will happen to the overall resistance of the
grid?
• What effect will this have on current?
RESISTIVITY
• Conductive wires which make up circuits also offer
resistance to current. The conducting ability of a
material is often indicated by its resistivity.
The amount of resistance a wire offers depends on the
length, cross sectional area, and material the wire is
made of
•
ρ is the static resistivity
(measured in ohm meters)
R is the electrical resistance
of a material (measured in
ohms)
l is the length (measured in
meters)
A is the cross-sectional area
(measured in square meters)
ρ = RA/l
http://en.wikipedia.org/wiki/Resistivity
RESISITIVITY VALUES
Material
Silver
Copper
Gold
Aluminum
Tungsten
Iron
Platinum
Lead
Nichrome
Carbon
Polystyrene
Polyethylene
Glass
Hard Rubber
Resistivity(ohm•meter)
1.59 x 10-8
1.7 x 10-8
2.4 x 10-8
2.8 x 10-8
5.6 x 10-8
10 x 10-8
11 x 10-8
22 x 10-8
150 x 10-8
3.5 x 10 5
107 - 1011
108 - 109
10 10 – 10 14
10 13
CIRCUIT BREAKERS
• The circuit breaker is an
important safety mechanism,
both on the grid and in your
home. Whenever there is too
much current flowing
through it, these simple
machines cut the power until
somebody can fix the problem.
Without circuit breakers
household electricity would be
impractical because of the
potential for fires and other
mayhem resulting from simple
wiring problems and
equipment failures.
http://static.howstuffworks.com/gif/circuit-breaker-intro.jpg
GROUNDING
• Electricity always follows the path of the least
resistance. You do not want to be that path!!!
• You do not want an exposed point of very high voltage
(relative to what? relative to ground!)in your home. If a
person touched that point, while their feet were touching
the ground a large voltage would be across their body,
driving a large current and shocking them.
• GROUNDING is to electrically attach a circuit to the
lowest "electrical potential" ...mostly referred to as 0 Volt
(zero) potential. If a circuit begins to The ground wire
will provide the path of least resistance should the circuit
become charged, thus safeguarding against electrical
shock.
ELECTRICAL SAFTEY
• Electrocution or electrical shock occurs
when an electric current passes through
the body. The amount of current passing
through the body is determined by Ohm's
Law:
• I = V/R
• I = Current Through the Body
V = Voltage across the body
R = Resistance of the Body
THE ROLE OF VOLTAGE IN
SHOCK
• electricity requires a complete path (circuit) to
continuously flow
Birds can safely rest on highvoltage power lines without
getting shocked: they make
contact with the circuit at only
one point.
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 ROLE OF CURRENT IN
SHOCK
• The level of voltage is not a direct guide to
the level of injury or danger of death,
despite the common misconception that it
is. A small shock from static electricity may
contain thousands of volts but has very
little current behind it due to high internal
resistance.
EFFECT OF CURRENT ON HUMAN BODY
Magnitude of Current
Effect
1 to 8 mA
Perceptible, but not painful
8 to 15 mA
Painful shock, but muscular control is
not lost
15 to 20 mA
Painful shock, Muscular control affected,
cant let go
20 to 50 mA
Severe Muscle contractions, Breathing
difficult
50 to 100 mA
Serious contraction of heart muscle,
Possible death, No known remedy
200 mA and above
Severe burns, Stoppage of heart, Death
certain
THE ROLE OF RESITANCE IN
SHOCK
•
•
Will the 120 volt common household voltage produce a dangerous shock? It
depends!
If your body resistance is 100,000 ohms, then the current which would flow would be:
•
•
But if you have just played a couple of sets of tennis, are sweaty and barefoot, then
your resistance to ground might be as low as 1000 ohms. Then the current would be:
•
•
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/shock.html
Because electric current must pass through the boot and the body and the glove
to complete its circuit back to the battery, the combined total (sum) of these
resistances opposes the flow of electrons to a greater degree than any of the
resistances considered individually.
http://www.allaboutcircuits.coml
AVOIDING ELECTRIC SHOCK
• Indoor safety:
• Check to make sure all electric cords are free of fraying
or cracking. Replace any cords that may be damaged.
• When unplugging appliances, always pull from the plug,
not the cord.
• Keep all electric appliances away from sinks, baths, and
other water sources. If an appliance falls into water,
never reach in to get it.
• to retrieve it. Instead, call your local electric company for
assistance.
•
http://www.puco.ohio.gov/PUCO/Consumer/information.cfm?doc_id=1262
AVOIDING ELECTRIC SHOCK
• Outdoor safety:
• Downed power lines are often caused by thunderstorms or other
severe weather. Always treat a downed power line as if it were live,
and do not touch it or any objects around it. If you see a downed
power line, contact the police department, fire department, or your
electric company immediately.
• Don’t use electric powered lawn tools in wet conditions.
• Always wear rubber-soled shoes or boots when using electric lawn
tools. Never operate an electric tool barefoot.
• Never allow children to climb trees that are touching electric lines.
• Never allow children to fly kites, balloons, or other toys near power
lines.
•
http://www.puco.ohio.gov/PUCO/Consumer/information.cfm?doc_id=1262