This document discusses electrical safety basics, including thermal and shock hazards. Thermal hazards occur when heat is produced faster than it can dissipate, potentially igniting nearby materials. Circuit breakers and fuses interrupt current flow if it exceeds safe levels to prevent overheating. Shock hazards exist when current can pass through a person, with severity depending on current magnitude. The three-wire system helps prevent shocks by separating voltage input and return paths.

1. THE BASICS OF
ELECTRICAL SAFETY

2. Circuit breakers, fuses, and the
three-wire system are all
designed to minimize damage if
something goes wrong.

3. Electricity has two basic
hazards:
Thermal hazards
Shock hazards

4. Thermal Hazards :
Circuit Breakers and Fuses
Thermal hazards result from
heat being produced faster than
it can be carried away. It is the
rate of heating, or power, going
into thermal energy that is
important.

5. Power in electric resistance is
given by equations
11.2a – 1.2c, repeated here for
convenience:
P = IV = 1²R = V²/R

6. Thermal energy produced in
wires is negligible, but if the
current becomes large enough,
a large amount of thermal
energy can be created in them.
Equation 11.2b illustrates this:

7. Where Pw is the power
converted to heat in the wires
leading to the parallel
connections of resistances and
Rw is the resistance of these
wires.

8. Because power is proportional
to current squared, the heat
heat produced can become
large enough to ignite materials
close the wires.

9. The solution to this problem is to
use a circuit breaker or fuse.
both devices are designed to
interrupt the flow of current if it
exceeds a specific level.
Another problem that creates a
thermal hazard is the short circuit.
A short circuit any very small
resistance receiving full voltage.

10. Ps = V²/Rs

11. SHOCK HAZARDS;
the three-wire system
A shock hazard exists whenever
there there is possibility for
current to pass through a person.
The severity of a shock depends
primarily on how much current
passes through the person.

12. For 60-Hz electricity the maximum
harmless current through a normal
person is 5 mA.
A person can be shocked by such
electricity by touching only one
wire because the feet can
complete the circuit by being in
contact with the ground.

13. The lower the resistance of the
person, the greater is the
hazard.
1p = V/(Rinsul + Rp)

14. The three-wire system was
develop to help prevent shock
hazards.