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Lightning

  • Lightning is the transient passage of electrons between a cloud to

· Surface of earth, 50% of all lightning are cloud to surface

· Another cloud

· Air

  • There are also ground to cloud strikes but these are very rare
  • Lightning are formed when the atmosphere has high variation in pressure and temperature. Large particles will move down and small particlaes will move up. This causes and imbalance of charge as large particles are more negative. This creates a large potential difference which if large enough will cause all the negative charges to move down to the surface of the earth

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  • Lightning causes damage to equipment in 3 different ways

· Direct lightning strike

· Carried lightning strike -lightning is carried by cable to an equipment

· Induced lightning strike – lightning induces electromagnetic field which creates surge currents. Magnetic fields can also induce

  • Interesting Facts on Lightning

· 99 % of strikes are of intensity exceeding 3000A, 50 % of strikes exceed intensity of 28000A and 1% of strikes exceed around 200,000A

· The duration of stroke are typically 100 microseconds while in exceptional cases it may exceed 1 second. The voltage present on a charged cell will be in the magnitude of 100MV

· The high current flashes start first from tall structures.

  • The damage lightning does are as follows

· For human , massive current flow causes disturbances in the human electrochemical system, nerve damage. The heat generated from lightning burning can turn sweat instantaneously to steam. These steam is known to blow of people’s boots, shoes and clothing.

· For Electronics, it can cause vaporization of PCBs, transistors and fuses due to the heat it generates when arcing through high resistance insulation.

· For other equipments, it can create sparks and if there is combustible gasses nearby, it will cause fire

  • It is experienced several times that lightning can travel in long distance electrical cables

Lightning Protection System

  • Lightning strikes equipment by the following
    • Direct strike – There is no way to minimize damage dobe by a direct strike. One can only divert the strike to somewhere else
    • Direct Energization – Lightning strikes on a cable/piping/tracks which there are equipment connected at it’s ends. Even when the lightning reaches the ground, it can still travel through underground cables and pipes.
    • Induced / Near Field Coupling(EMP) –Electric fields are developed adjacent of the strike path due to the tremendous negative potential. The electric field may rise up to 8000 V/m within a 1 mile distance. Voltage may not be enough to cause major arcing but enough to vaporize PCB, Transistor and fuses.
    • Ground Potential Raise – When lightning strikes on the ground, it causes a ground potential gradient. This will cause some small surges. Example : A 100MV lightning strike at 30,000A can cause a ground potential of Rise of 75000V
  • Lightning damages electronics/instruments by the following
    • Memory discharge – Data loss, software corruption
    • Degradation – Degradation of electronic components due to transient currents
    • Damage by heat – High current flowing can create heat
  • Although lightning is devastative, 95% of electronics fail due to degradation of equipment are not from lightning, but from transients currents, which occur momentarily from
    • Collapsing magnetic field in inductors
    • Charging and discharging of capacitors
  • The most important thing that needs to be done to avoid lightning strike is to ensure that all critical equipments potential rise together and no potential difference is created. Totally eliminating lightning is not possible. When there is a potential difference, this will cause arcing which is a source of ignition and extreme heat. It is important that all metallic equipments are connected together with the earthing grid
  • A lightning protection system is a system which protects a structure from lightning strikes either by safely conducting the strike to ground or preventing the structure from being struck.
  • The main components of a lightning protection systems are
    • Air termination / electrode / lightning rod
    • Down conducter – to conduct lightning for the air termination
    • Earthing / grounding system – to dissipate the charge to earth
    • Bonds and clamps – to connect all the systems together
    • Surge arrestors
  • The down conductor must be put around a building, distanced
    • 10m apart for tall buildings (>20m)
    • 20m apart for short buildings (<20m)
  • Grounding System
    • All plants have a single earthing grid buried inside the soil of the earth. In between these earthing grids, an electrode will be plunged to the ground. The earth electrode is a galvanized steel pipe. The rod’s are >9m
  • Surge Arrestors
    • A SPD (Surge protection device) or lightning arrestor is a device which limits the current going through the line by channeling the excessive current to ground. Lightning arrestors must be installed at every line
    • The most effective protection is combination of SPD (surge protection device) at both ends, together with a low impedance grounding system
      • SPD shall be before the barrier
      • Instrument – SPD – Cable – SPD – Barrier - DCS
      • SPD is not used in off shore since there is no lightning in off shore platform
  • Instrument Earth and Electrical Earth is connected by a High frequency choke
    • Lightning is a very high frequency voltage
    • The choke is actually a HF inductor which give low impedance at high frequency
    • This allows IE and PE to be equalized during lightning
    • The choke shall be protected by a 100v surge protector put in parallel with the choke.
      • The surge arrestor is normally a GDT type
      • At high voltage, it will give low impedance
      • During lightning, the surge arrestor will have low impedance and hence allows current to flow through it
      • This reduces the current flowing through the choke hence protecting it from damage
  • Types of SPD
    • GDT (Gas discharge Tube)
      • Has a gas which becomes highly conductive at high voltage
      • Diverts the charge to ground
    • Zener Diode (Most Commonly used for instrument)
      • Zener diodes allow current to flow in the opposite direction at larger than the zener voltage
      • When high voltage occurs during lightning, the zener diode (arranged upside down) will flow through to ground
    • Metal Oxide Varistor (MOV)
      • At high voltage, it has low resistance but at low voltage has high resistance
      • When high voltage occurs during lightning, the MOV will have low resistance and route all the current through it to ground

SPD Type

Advantages

Disadvantages

GDT

Can be used for AC

Zener

Provides hardest clamping

Cannot be used for AC

MOV

Can be Used for AC

Grounding / Earthing

  • Earthing is the conduction of fault electricity current to earth.
  • Earth is used because it is the most reliable low impedance path of the fault current
  • The purpose of equipment earthing is
    • In electrical circuits it is used as a current sink since it has the ability to absorb a high amount of current without changing it’s potential. Though this is rare
    • For electrical supply, it is used as a common neutral reference. The grounding is done at a distribution sub station. As a normal user, we will see this as neutral terminal in our 3 pin socket
    • For lightning protection i.e. to ensure safe route of lightning and transient current to ground so that it will not effect other equipments
    • For EM reduction i.e. to sink electrical noise in signal transmission cables (such as telephone, microphone and etc)
  • Every plant will have a plant earth ring. This will be one of the first things people will do during construction of a plant. The earth ring are made of conductors sized at 70mm2.
  • Instruments are connected to the plant earth ring via a earthing cable. The size of the earthing cable for instruments are recommended at 25mm2.
  • The earthing resistance of any earth electrode or ground rod is made up of:
    • Resistance of the (metal) electrode – This is usually neglected
    • Contact resistance between the electrode and the soil – This is usually neglected
    • Soil resistivity
      • The temperature effect on soil resistivity is negative but is negligible for temperatures above freezing point.
  • In the plant we also have an instrument earth
    • Instrument earth is used to quickly discharge noise from screen wire cables to the ground. The screen cable are usually grounded at the control room and should be left hanging on the field side
    • These noise are normally from electromagnetic coupling between wires. As wires are typically bundled close to each other, current flowing through them will create electromagnetic field. These electro magnetic field can cause current in other wires
    • It is a common practice to have instrument earth totally isolated from the electrical system earthing grid. This is because if there is stray currents in the earthing grid, this may induce interference current in the instrument signal
    • The earthing system for instrument earth shall be as clean as possible. Therefore, it is important to put the instrument earth as far as possible from other earthing points. >5m should be okay
    • The earth resistance for instrument earth is very low typically <2 ohms. This is typically achieved by having an array of parallel electrodes planted. The electrodes will be connected by cable. Typically for instrument earth, there will be up to 10 earth electrodes
    • Should instrument and electrical earth be connected?
      • Latest technology has promoted EMC certified devices where they are more susceptive to signal noises. With the use of EMC certified devices, we do not need screen cables anymore
      • Normally in a plant, instrument and electrical earth are connected by an inductor and surge clamper.
        • The inductor is used to filter out high frequency noise from the earth grounding system

Electro Magnetic Interference

  • Electro magnetic Interference (EMI) is unwanted disturbance that affects electrical circuit due to electromagnetic conduction or electro magnetic radiation
  • Electromagnetic Compatibility (EMC)
    • Is a study branch in electrical engineering to try an ensure equipments are protected from EMI (electro magnetic interference)
    • The word EMC is commonly referred on a 2004 directive by the European parliament that:-
      • States the requirements of certified equipments do not emits electromagnetic signals that disturb other signals such as radio and television
      • States the requirements and governs the immunity of certified equipments to radio emissions
    • Manufacturers will usually seek EMC testing to ensure that they’re products are EMC certified
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OPEN-PLANT PTY LTD

Perth, Australia

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info@open-plant.com
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