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