Transcript Basic Electricity II - University of Virginia School of

Basic Electricity II
Continuing our exploration of basic
electricity
Grounding
Grounding is electrically and physically
connecting a portion of a circuit to the
earth’s crust
 Provides a common reference for
voltages (0V)

Grounding in Homes
Silver Screw
Neutral
(White Wire)
Green Screw
Ground
(Green or Bare Wire)
Brass Screw
Hot
(Black or Red Wire)
Typical AC “Wall” Voltages

For Residential:
Phase-to-Ground – 120V
 Phase-to-Phase – 240V


For Commercial:
Phase-to-Ground – 120V
 Phase-to-Phase – 208V


All voltages in Root Mean Square (RMS)
What is RMS?


Root Mean Square (RMS) is the equivalent DC
voltage that would provide the same power to a given
load as an alternating current (AC) signal.
For sinusoidal signals, the peak voltage is given by the
equation:
VPEAK  2 VRMS
Grounding in Homes
Typical Residential 1P Circuit-Breaker Panel
Single-Pole
Circuit Breaker
Main Circuit
Breaker
Panel Schedule
Double-Pole
Circuit Breaker
“Knock-out”
Understanding the “Safety Ground”
Understanding the “Safety Ground”
Understanding the “Safety Ground”
Understanding the “Safety Ground”
Another Example
Sizing Conductors Properly
Current Rating – Wire must be sized
according to how much current it will be
required to carry. The size (known as
gauge) can be picked using an ampacity
table.
 Voltage Rating – The insulation on the
wire determines how many volts it can
safely carry. Check the manufacturer’s
data if in doubt.

Wire Gauge Table
Wire size
AWG
Diameter
(inches)
Ohm's per 100ft
(@ 68º)
Safe Current
(Amperes)
12
0.080
0.16
20
14
0.064
0.25
15
16
0.051
0.40
10
18
0.040
0.64
8
20
0.032
1.02
5
22
0.025
1.61
4
24
0.020
2.57
3
26
0.016
4.08
2
28
0.013
6.49
1.5
30
0.010
10.30
1.3
AWG – American Wire Gauge
Circuit Protection
Fuses – Made up of a thin “filament” wire
inside a glass or ceramic casing. When the
current becomes too high, the filament wire
melts, opening the circuit. Response time can
be very fast (microseconds).
 Circuit Breaker – Uses an electromagnet or
bi-metallic strip to pull a switch open when too
much current flows. Response time is typically
slow (milliseconds).
 Either form of circuit protection must be placed
In Series in series with the load to work.

Circuit Protection – Fuses
5mm x 20mm Fuses
3AG (AGC)
1-¼” x ¼” Fuse
Automotive “Blade” Type Fuses
From www.radioshack.com
Circuit Protection – Circuit Breakers
Pictures from www.lowes.com & www.oselectronics.com
Sizing Fuses & Circuit Breakers

Fuses should be sized
between 125% and 250% of
the rated load
 Circuit Breakers should be
sized at approximately 125%
of the rated load
 On 120VAC systems, the
fuse should ALWAYS protect
the Hot wire