electronic earthing

Introduction to Earthing of Sensitive Electronic Equipment

Electrical Standard Products/VR

(Computers/IT Loads/PLCs/Other Similar Sensitive Electronic Equipments)

Sensitive Electronic EquipmentSystems to be Earthed...

1) Equipment earthing - metallic enclosures, or frame of electronic equipment.

2) Signal Common Earthing - the zero reference system for data lines & the signal portion in general.

3) DC Power Supply Reference Earthing - the electronic equipment may have different DC voltage systems 12V/24V.

Sensitive Electronic EquipmentBasic Issues

1 Susceptible to random voltages far below the levels that are perceptible to humans & that have no effect on electrical power equipment (even the static voltage charges generated by lightning strokes within several thousand feet can cause damage).

2 Problem of noise coupling (capacitive & inductive) between signal circuits & power circuits.

Sensitive Electronic EquipmentBasic Issues

3 Impedance considerations related to the power frequency safety aspects of a earthing system may not necessarily provide the desirable low impedance at the high frequency signals.

Sensitive Electronic EquipmentBasic Issues

4 Several elements at different locations, but linked by a data cable that carries it own zero reference - a conductor linking the earthing connections at different locations. Under certain conditions substantial differences can exist between distant elements of the system - leading to component failures.

Sensitive Electronic EquipmentBasic Issues

Earthing Methods

1) Multiple Earthing Connections

# In a commercial/non-industrial building it is not unusual to find neutral connected to earth/panel enclosures at more than one location.

# Results in noise causing errors or worse.

# Whether accidental or intentional, this practice is not recommended.

Multiple Earthing

Building Steel (Earthed)

Signal & Ground Wire

0.1 Ohm Z

5V Pick-Up if (say)

50A stray current

flow

Ground Loops

The N current gets

divided & a part of it

will be flowing thru

ground/ground wire,

even w/o earth fault

Earthing Methods

2) Separate/Isolated Earthing Connection

# To earth computers to isolated earthing electrodes separate from the power system earthing electrode system.

# While continuous low-level noise are eliminated, other catastrophic incidents are encountered.

# Since unsafe, therefore not recommended.

Isolated Earthing

Building Steel/Power

System EarthingSignal & Ground Wire

(Insulated)

Isolated Computer Earthing

Building to Computer

Capacitance

1 Ohm

10 KV

10 KA Stroke to

Building

Isolated Earthing

• Large voltages impressed on computer components under thunderstorm conditions (direct strike or charge induced because of clouds overhead).

• With isolated earthing, a fault in a computer requires fault current to pass thru the resistance of both the isolated & power system ground in series. In this case the current would be insufficient to operate the protective device.

Why Unsafe?

Isolated Earthing

Earth Fault

Computer Modules

Isolated Earthing

20A CB

IF

415V/240V

5 Ohm 5 Ohm

IF = 240/10 = 24A

Earthing Methods

3) Single Point Earthing Connection

# To prevent stray/circulating currents from affecting the computer signals & operations, it is necessary to keep the computer ground system separate from the equipment ground components & connect together at only one point.

# Recommended method, as it eliminates problems of earlier two methods.

Single Point Earthing

R

Y

B

N

G

Isolation

Transformer

Computer

Modules

Equipment

Earthing Conductor

FROM MAIN415V SOURCE

240V COMPUTER SYSTEMPOWER PANEL

RECOMMENDED POWER DISTRIBUTION – COMPUTER SYSTEM

Computer Isolation Transformer

Single Point Earthing

Due to length of the grounding conductor from the computer units to the grounding point, there is possibility of computer conductors being subjected to high-frequency resonance with RF signals.

Only Limitation...

Single Point EarthingIn addition requires...

• Since equipment earthing employing long earthing conductors exhibit higher impedances at higher frequencies

• Therefore, requirement of equipotential earth plane to ensure minimal voltage variances exist among the connected signal circuit & interconnected equipment

Equipotential Plane

Includes:

• Conductive grid embedded in or attached to a concrete floor

• Metallic screen or sheet metal under floor tile

• Ceiling grid above sensitive equipment

Earthing Methods4) Single Point Earthing Connection plus

Connection with Grid in the Floor

# For each group of computer equipment, provide a grid network in the raised floor, with tie wires from each computer unit to the grid & one from the grid to the room grounding point.

# Recommended method, as it also eliminates problems of interference from radio frequencies.

Single Point Earthing

R

Y

B

N

G

Grid Network in Raised Floor Structure

Computer

Modules

SRG - Resonance reduction

Distributed Processing System

Power Wiring & Data Cable Wiring

Use of Isolation Transformers

Need...• For establishing the power ground

reference close to the point of use. This greatly reduces common mode noise thru ground loops etc.

• Ability to transform or change the input to output voltage level &/or to compensate for high or low site voltage.

CASE HISTORIES

1)PC experiencing frequent lock ups & unexplained failures

2)Erratic computer operation within a large office building experiencing several lightning storms

3)Intermittent data memory errors & data transmission errors between remote terminals & a central computer system.

Problem 1

• No problem was found in the utility supply; earthing was also found to be okay.

• The power outlet serving the computer was examined for polarity, revealing the phase & the neutral conductors were reversed at the receptacle.

• Once the conductors were correctly connected, the PC ran w/o problems.

Problem 2

• The earthing conductors of all the computers in the building were connected to isolated earthing electrode system (consisted of 6 rods driven into the earth away from building).

• The computer manufacturer was persuaded to permit earthing of the computer system to the building earthing system, which had concrete encased earthing electrodes. Immediately the erratic operations ceased.

Problem 3• In the installation of a multistory office building, several

floors of modular workstations were powered from a common, 3 phase step down transformer with a shared neutral. Mainframe terminals in these workstations experienced intermittent data memory errors & data transmission errors, & occasionally hardware failures.

• It was found that considerable (& variable) neutral current existed. RMS values of N to earth didn’t exceed 3.7V, but the peak voltage ranged up to 10V.

Problem 3• Each floor of the building was isolated into 2

sections via shielded isolation transformers. Individual N conductors were installed for all workstation branch circuits.

• Re-establishment of the N-E bond at the new isolation transformers, combined with the reduced neutral current in the dedicated neutrals, reduced N-E voltage to less than 2V. The problem was solved.