Grounding

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.

Inductive coupling 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-mode-interference. 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.