Transcript 06-Grounding 2003 pre

Grounding:
 Voltage by definition is the difference in potential between
two points.
Many schematics make use of taking the earth as defining
zero volt and referred as “earth grounding” .
Any point having a zero volt reference is referred to as
“ground”
 Usually, a single phase power line has 3 lines :
1. hot line.
2. neutral line.
3. ground line (for safety).
Ground faults:
 Ground faults by definition is the return of the current to ground by any path
other than the neutral wire .
for safety , we may use the Ground -Fault -Interrupter
(GFI) ,
as solution for ground fault
 This device compares the current flowing in the hot and the neutral lines.
If the currents (I1,I2) are not equal , then the (GFI) will disconnect the
supply line. As long as (I1=I2), both cancel each others magnetic fields and
no voltage across the winding connected to the relay, on the other hand a
current difference will energize the relay and disconnect the power
source.
 Ground faults may be caused by:
1. Insulation breakdown.
2. Use of 3- wire “cheater” adapter. This adapter converts the
3- wire plug into the old (hot, neutral) out lit.
Testing Instruments:
1. Neon tester:
A simple way to determine if the system is
having a faulty wiring is to use an
“incandescent” bulb.
If the wiring is correct, the bulb will light
(on) if connected as following:
1. Hot and Ground, on.
2. Hot and Neutral, on.
3. Neutral and Ground, off.
Note:
If hot is off and neutral is on , there is a problem the
problem is fault .
This tester is characterized by:
1- Is considered as a voltage controlled device (need 70V
to turn the neon on)
2- Does not detect polarity.
3- It will not give a warning until aground voltage is
greater than 70V.
2. LED tester:
This tester is characterized as:
1- Is a current controlled device
and has a test current capability
of (20 to 50 mA).
2- Emits light only for a forward bias.
3- All 3-LEDs are light when ground voltage is almost 10 V
above the neutral potential.
4- detects approximately an (8-10)V ground to neutral
potential.
Note:
To minimize the risk of fire, the neutral voltage should not
exceed 5% of the line voltage. This is good for LED tester , but
not used for the neon tester.
Instruments Grounds :
1- Power ground:
Is used for a neutral path for the current equivalent.
2- Signal ground:
Is both , the reference point and return path for all signal
currents.
3- Chassis and shield ground:
Shield ground are the connections made at the chassis
instruments.
Note:
As long as the ground wire is connected , the input signal
ground should not be connected to any point that has a
potential difference with respect to the ground.
Ground loops :
The most common causes of noise and interference is the
ground loops. They generally are considered as closed
electrical paths.
 In a typical situation , the transmitter sends a signal to a
receiver. One side of the signal current is grounded. However
, as a safety procedure it is mandated that each piece of
equipment be grounded to earth at its respective installation
location.
This is where the problem becomes.
 Hence , once we ground two pieces or more of equipments , that
have different locations we have set the stage for ground loop
(VG1G2≠0).
Ground loops are created by:
1- Potential difference between two or more points of ground plane to which
external grounds are connected.
2- Inductive coupling.
3- Capacitive coupling.
4- Common - mode - noise.
 Inductive coupling
Is created in a closed loop wire , as a general loop rule , because ground loops are
low impedance paths that cover a relatively large area , inductive coupling from
surrounding magnetic field occur easily.
 Capacitive coupling
This occurs when the ungrounded shielded chassis of an amplifier is
capacitively couple to ground point .
In this network , the amplifier circuit ground is capacitively coupled to
the chassis , which is coupled to ground.
 Common - mode - noise
This is where the signal source is ground through the AC - power line at one – point ,
And the amplifier is grounded at different point .
 The cable shield from the signal source (ground to the chassis signal source ) may
not be connected to the chassis ground of the amplifier.
 Here , the input voltage of the amplifier is the sum of the signal source (Vs) plus
the potential difference between the two ground points (G1,G2) (Vcm)
 This voltage is frequently called common - mode - noise voltage .
 Now , if the cable shield is connected to the chassis ground of the amplifier ,then
a ground loop is formed.
 Interference resulting from ground loops is referred to as common-modeinterference.
 The reduction / elimination of ground loops and common mode interference is
achieved in different ways :
1- Single point grounding (not safe).
2- Use of differential amplifier.
