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Introduction to Earthing of Sensitive Electronic Equipment Electrical Standard Products/ (Computers/IT Loads/PLCs/Other Similar Sensitive Electronic Equipments)

Electronic Earthing

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Page 1: Electronic Earthing

Introduction to Earthing of Sensitive Electronic Equipment

Electrical Standard Products/VR

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

Page 2: Electronic Earthing

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.

Page 3: Electronic Earthing

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).

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

Sensitive Electronic EquipmentBasic Issues

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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

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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

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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.

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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

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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.

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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

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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?

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Isolated Earthing

Earth Fault

Computer Modules

Isolated Earthing

20A CB

IF

415V/240V

5 Ohm 5 Ohm

IF = 240/10 = 24A

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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.

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Single Point Earthing

R

Y

B

N

G

Isolation

Transformer

Computer

Modules

Equipment

Earthing Conductor

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FROM MAIN415V SOURCE

240V COMPUTER SYSTEMPOWER PANEL

RECOMMENDED POWER DISTRIBUTION – COMPUTER SYSTEM

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Computer Isolation Transformer

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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...

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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

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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

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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.

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Single Point Earthing

R

Y

B

N

G

Grid Network in Raised Floor Structure

Computer

Modules

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SRG - Resonance reduction

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Distributed Processing System

Power Wiring & Data Cable Wiring

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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.

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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.

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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.

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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.

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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.

Page 30: Electronic Earthing

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.