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The IEE Buildings Electrical Technology Professional Network

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Earthing In Industrial and Pharmaceutical Plants

Kieran Fallon, DPS Engineering & Construction Ltd

0 The IEE Printed and published by the IEE. Michael Faraday House, Six HiIls Way,

Stevenage. Herts SGI 2AY, UK

EARTHING IN INDUSTRIAL AND PHARMACEUTICAL PLANTS

Kieran Fallon, Chartered Engineer FIE1 Electrical & Instrumentation Manager, DPS Engineering & Construction

(www.dps. ie)

Abstract - Earthing (or grounding) is an essential part of any electrical installation. Within industrial and pharmaceutical plants with potentially explosive atmospheres earthing plays an essential role in maintaining the electrical and instrumentation systems in safe condition. The earthing system, although a single physical system, it carries out many different functions including automatic detection and clearance of electrical faults, prevention of dangerous potential differences which could cause injury, prevention and dissipation of static charges, screening and reduction of interference in instrumentation systems and provision of a reference point for intrinsically Safe Zener barriers systems.

INTRODUCTION The basic reasons for earthing and bonding in an electrical installation are

To provide a reliable low impedance return path for fault currents so that the fault can be detected and the source of power removed as quickly as possible. To prevent potential differences which would create a possible electrocution hazard. To minimise damage due to lightning strikes and associated surges. To prevent the build up of static charges to a level where their discharge would be a source of ignition or potential safety risk. To minimise electrical interference to instrument and communication signals. To provide a reference point to allow Intrinsically Safe systems to limit energy available in Potentially Explosive Atmospheres.

On industrial or pharmaceutical plants with Hazardous Areas the requirements for earthing and bonding are higher than in normal industrial or commercial environments. This is in order to minimise potential sources of ignition from static charges and lightning and to ensure rapid clearance of faults.

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The earthing system provides a low impedance return path so that any fault that results in metal parts not intended to be live, becoming live will result in a high current forcing the circuit protection to operate. This “earth loop impedance” and the time taken for protection to operate is defined by the within the regulations. A low earth loop resistance is necessary for a system to operate safely.

Faults within an electrical system may result in an area or object rising in voltage in comparison with other adjacent areas or objects. Such a fault would create a danger for any person who is near or in contact with items at different voltage. To prevent a voltage gradient within a plant all metal items are connected (bonded) together and to earth. This equipotential and supplemental bonding is a requirement of a safe electrical installation.

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The earthing system also prevents the build up of static charges within plant areas. Any build up of charge may result in sparks which may ignite the potentially explosive atmospheres which exist within the plant. Anti-static earthing, an essential safety requirement, extends to a larger range or items and equipment than equipotential bonding.

Lightning strikes and associated electrical‘ surges in a building have the botential to cause significant damage and be a source of ignition where potentially explosive atmospheres occur. To reduce the effect of a strike a lattice of conductors is installed around the building. This Faraday Cage conducts the surge currents safely to .earth. Following the lightning strike “flash over” may occur to any other earthed metalwork creating-a parallel conductive path to earth. To prevent this a definite bond is installed between the lightning protection system, equipotential bonding system and electrical safety earth. This ensures that all earthed equipment is at the same potential in the case of a lightning strike or surge.

One technology for elimination of ignition sources in Hazardous Areas (Potentially Explosive Atmospheres), Intrinsically Safe Systems, reties on the concept of limiting energy available within the hazardous areas. One of the methods to limit energy requires a reference voltage to be available to which voltage and current is referred. This reference voltage is always earth potential, zero volts. Without an effective and reliable earth system some intrinsically safe systems may become unsafe.

REGULATIONS, STANDARDS AND RULES The electrical installation regulations referred to in this paper are the Electro-Technical Council of Ireland (ETCI) Rules for Electrical Installations 3d Edition ET101-2002. These rules, derived from a CENELEC document, are similar but not the same as the IEE Regulations.

Other standards referred to are generally euro norm standards IS-EN-XXXX which are identical to the UK versions BS-EN-XXXX or British Standards which are used and accepted in Ireland.

HAZARDOUS AREAS In the many industrial processes and pharmaceutical plants where flammable materials are handled, any leak or spillage may give rise to an explosive atmosphere. To protect both personnel and plant, precautions must be taken to ensure that this atmosphere cannot be ignited. The areas at risk are known as ’hazardous areas’ or ‘potentially explosive atmospheres’. The materiats that are commonly involved include solvents, natural and man-made process gases, alcohols, metal dusts, carbon dust, flour, starch, grain and fibres.

Protection of workers within Hazardous Areas or Potentially Explosive Atmospheres is now governed by statute throughout Europe as a result of EU Directives. The directive requires sources of ignition including static discharges, lightning and equipment under fault conditions to be gives specific consideration.

’ EARTH ELECTRODES The regulations require that all TN systems be earthed. The physical connection to the general mass of earth is achieved by use of an earth electrode. The concept is that the mass of earth is at zero potential and everything connected to it is thus also at zero potential.

The regulations require connections to earth electrodes to have the following properties;

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1 Adequate electrical conductivity 1 Mechanically robust = Protected against corrosion I

1 Be accessible for inspection Clamp does not damage earthing conductor

Labelled “SAFETY ELECTRICAL CONNECTION - DO NOT REMOVE

Although there are a variety of earth electrodes available generally driven rods or buried tape are used. The number of earth electrodes and dept to which they have to be driven is dependant of the quality and type of soil in the area. Although theground surveys required by the civil engineers may give an indication of the soil types details of the soil resitivity 1s not generally available at the electrical design stage of a project. From a practical point of view a decision is required prior to tender as to how many earth electrodes should be supplied, usually an earth nest is specified with three to four driven 2m / 3m rods.

The earth electrodes will be installed usually as in straight line or in a square / triangular formation, the latter been preferred for space reasons. Where a number of earth electrodes are provided they should be spaced apart a minimum of the driven dept. Thus to install say a four electrode nest a linear space of eight to ten meters. Thus spatial requirements for earth electrodes can be significant and should be indicated on drawings (together with earth cable routes).

The number of electrodes required for power system earthing is usually low, however when combined with lightning protection there may be in excess of a hundred electrodes on a site. Maintenance, even inspection of the earthing system may thus be a considerable maintenance task, Earth electrodes are an essential part of the electrical installation it is not unusual for sites to give little or no maintenance to these items unless there is a specific problem with the site earthing. Testing of the main earth electrodes may require disconnection of the electrodes from the system and comparison with reference electrodes. This may require isolation of the supply and disruption to the site. For some sites a second earth nest may be required.

Sizing of the earth conductor is a similar procedure to the sizing of a CPC conductor, however considering it5 importance it should always be mechanically robust.

Typically industrial sites grow and change with time as production expands or changes. This often results in the electrodes been lost or covered during building expansion, provision of hard-standing or concreting over of yard areas.

In addition to the safety notice consideration should be given to providing a reference number for each earth electrode such that traceable records may be kept of the testing and inspection of each item.

Key Points to Note 1

1 Co-ordination with architectural features

Clearly label electrodes

Establish whether ground is ‘made up’ or rock or whether there are other potential problems Ensure sufficient space is available for earth nest

Ensure a suitable route is available from main switchgear to earth nest

Consider maintenance, inspection and testing

EARTH BARS The regulations require that a main earth bar be provided in every installation for the

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interconnection of the following conductors:

General Earthing Protective Conductors

0 Main Equipotential Bonding Point Functionat Earthing Bonding Conductors for Lightning

For large sites there should also be a sub-mai? earthing bar within each building. In addition it is normal practice to provide earthing bars throughout a plant to which vessels and equipment may be bonded. These earth bars are generally connected in loop back to the main building earth bar using a robust earth cable, say 50 mm sq. copper / PVC. These earth bars as with the main earth bar are generally mounted on walls or structural elements using insulated supports. The earth bars must be both mechanically and electrically capable of carrying the high currents and stresses possible in case of large fault currents. Care is required to ensure that the effective earth cable csa is not reduced at the bar even where the largest lug bolt is fixed.

Often the earth bars and cables are not identified other than the green/yellow colour coding. It is recommended that each bar and associated cables be identified. This will allow traceable records to be kept of inspection and tests on the bars.

Key Points to Note

Label Mechanically and electrically robust

Access for maintenance and additional cables

INDUSTRIAL ELECTRICAL POWER SYSTEM Electrical power is generally delivered to large electrical consumers at medium voltage and is stepped down to 400V / 230V via a transformer either owned by the utility or the customer. Key elements of a power distribution system is the earthing consisting of;

Medium Voltage Earthing Transformer star point earth (Neutral Earth link) Earth distribution throughout the Site Generator Earthing Earthing of Final Circuits

Medium Voltaae The Medium Voltage (1OkV or 2OkV) system in Ireland is operated as an earth free system. Although earthing of the MV system is not required a reliable earthing installation is required for the cables and MV switchgear enclosures. All electric cables become charged in the same way as a capacitor. On Medium Voltage systems the effect of charging is more significant and charge retention lasts longer. Voltages are such on Medium Voltage systems that the stored charge on cables may be dangerous. For this reason before work is permitted on Medium Voltage cables, the cables are always earthed and facilities must be provided to allow such earthing.

In addition the 20,000 Volt system currently been rolled out may in future be earthed. Where low voltage generators (400V) are used to feed a site MV system (1OkV / 2OkV) via step up transformers, facilities must be provided such that the site system (if 20kV system is earthed) retains an earth on disconnection from grid supply. This may entail the provision of W or DY step up transformers.

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Transformer Star Point Earth (Neutral - Earth.Link) The regulations requires that "where an installation is supplied from a source such as a generator or transformer forming part of the installation, the exposed conductive parts of the installation shall be connected to the neutral conductor at one of the following points

at each sub-distribution point"

earthed neutral point of the source, or main distribution point of the installation, or

._. The regulations (542.5) also require that the neutralising conductor be insulated and of the same cross sectional area as the cable to the earth electrodes. The neutral earth link must be of sufficient cross sectional area to be capable of carrying the maximum fault current without damage or significant volt drop.

This link is usually specified as part of the main switchboard, however unless specifically instructed may be omitted. Sites may, due to increases in electrical loads, move from a utility owned transformer (supply at low voltage) to customer owned transformers (supply at medium voltage). In these cases, if the existing main distribution board is retained as the main board, provision of a Neutral-Earth link may be required.

Key Points

Disconnection link is provided

With transformers or generators ensure N-E link is provided (TN) Ensure size is adequate and robust

Generator Earthing Standby generators are provided on a number of sites sized to handle the entire site load or essential loads depending on the site. Generatly the standby generators are low voltage sets (400V) feeding the distribution boards directly or occasionally stepped up to medium voltage and distributed across the site. (Generators feeding the grid on an occasional or full time basis are subject to specific requirements and outside and not considered here)

As with site based step down transformers the star point of any generator on a TN system must be earthed. Care must be taken to ensure that in the event of disconnection of the grid supply the system and generator remain earthed. Similarly where a standby generator only supplies a specific sub-distribution board on site arrangements are required to ensure that at this point N-E link is provided (while the generator is running)

Earth loop impedance should always be considered to ensure that protection will continue to operate under all supply conditions. Where parallel sets are provided circu tation currents need to be considered as requirements of BS7430 taken into account.

Mobile generators also require facilities for earthing and these should be considered where fixed facilities are provided or generators are brought to site for a specific project.

Key Points Ensure that when disconnected from grid standby generator I CHP remains earthed Ensure star point is earthed

Earthinn of Final Circuits The ETCl regulations (553.3) require that "circuits supplying socket outlets shall be protected by

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one or more residual current devices having a rated operating current not exceeding 30 mA. The ETCl regulations (553.4) also include a requirement that the earthing contact in socket outlets be connected to earth. This should be checked during periodic testing and inspection of the sockets.

The maximum value of the earth loop impedance is defined in relation to the time taken to trip the protective circuits rather than in absolute terms. With each electrical supply within a plant there will be a earth or CPC cable sized to ensure that protection will operate within a specified time ( 0.4 sec

‘or 5 sec) disconnecting the mains supply. The CPC cable is designed to carry the full fault current, however fault currents will not be restricted to this single CPC and may take parallel paths via other CPCs, or other conductors to earth.

In addition within hazardous areas conductor temperatures under fault conditions must be considered. The ETCl National Rules for Electrical Installations in Hazardous Areas specifically required that the “conductor temperature under fault conditions shall be lower that both the ignition temperature of the explosive atmosphere and the manufacturers short time temperature rating”

Key Points

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RCDs required on all circuits feeding sockets (ETCI) Check cable temperature under fault conditions (Hazardous Areas)

EQUIPOTENTIAL BONDING The object of equipotential bonding is to ensure that all adjacent metal work is at the same potential (voltage) under normal or fault conditions. In case of fault which involves the outer metal enclosure of an electrical apparatus becoming live there is a risk that, by contact with this live component and another earthed piece of metal or conductor, a person could become the primary or a secondary earth path for the fault current. To prevent any risk of a person becoming a current path (and thus receiving an electrical shock) all exposed conductors and extraneous conductive parts within an area are bonded together and to earth. This ensures that in case of fault,. when the outer metal enclosure becomes live and rises in potential to mains voltage, everything in the area atso rises to mains voltage, thus there is no potential difference between any metal parts,which a person may be in contact. Without potential difference there is no current and no shock. The minimum size for these bonding conductors is specified in the ETCl rules.

Equipotential bonding can be considered .as protection against indirect contact, that is, protection against shock in the case of exposed conductive part or extraneous conductive part becoming live.

The ETCl regulations (544) require that main bonding conductors be insulated copper with cross sectional area at least 50% of the largest protective conductor. However the main bonding conductor need not exceed 25 mm sq. The ETCI regulations also require labelling of all main bonding conductors with a label “Safety Electrical Connection - Do Not Remove”.

At various points throughout a plant there will be supplementary bonding where all local metalwork is bonded together and possibly bonded again to earth. The Supplementary bonds are always in addition to main bonding. Bonding in addition to creating an equipotential zone will also, in case of fault which results in direct contact between live conductor and bonded metalwork, carry some fault current. Both the main equipotential and supplementary bonding installation is required to be robust and capable of surviving in an industrial environment.

Bonding conductors and supplemental bonding conductors tend to be of different sizes the latter may be significantly smaller than main bonding conductors. In the case of bonding of say lined pipe

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running within a plant area, this would be generally considered to require supplemental bonding and anti-static bonding. It would be usual to provide a minimum of a 4 mm sq copper conductor would be adequate to meet ETCl requirements. However the pipe system also requires main bonding conductor which would be as discussed above.

Within the ETCl National Rules for Electrical Installations in Potentially Explosive Atmospheres there.is a specific requirement that in Zone 0 and Zone 1 areas the resistance, R, in ohms between any two simultaneously accessible conductive parts not exceed;

Z = phase earth loop impedance of relevant circuits U = phase earth voltage

Bonding throughout a plant area usually takes the form of a number of items of equipment, pipework and ductwork connected directly to a local earth bar. This type of installation has the advantage over a ‘looped’ system in that removal or modification to plant items will not compromise the overall bonding. Care must be taken to ensure that pipework, vessels etc are electrically continuous throughout or that suitable bonding straps are provided.

Key Points to Note

0

Provide local earth bars

1

Specific Hazardous Area requirements

Equipotential bonding required for safety Supplemental bonding will also be required throughout the plant

For each piping system ensure that at tease one equipotential bond is provided. Ensure electrically continuous piping systems or bonding straps

LIG HTNl N G PROTECT1 0 M Provision of lightning protection for a building is normally based on a risk assessment of the building and its contents. The risk assessments and requirement for lightning protection on specific buildings iooks at the probability of a lightning strike in the area, relative size of the building, structure and contents and is clearly described in the standards BS 6651 1991 Code of Practice for Protection of Structures from Lightning. The standards recommend lightning protection for all buildings with flammable or explosive contents. It states that ‘structures with inherent explosion risks usually need the highest possible class of lightning protection.

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Lightning protection of buildings operates on the ‘Faraday Cage’ principal where the lightning strike is conducted to earth around the structure without entering it. The protection works on the theory that for perfect conducting enclosure, electromagnetic radiation incident on the enclosure or current flowing in the enclosure does not generate a fjeld within the space inside the enclosure. With lightning protection what we try to achieve is something approximating this conducting enclosure by placing a lattice of conductors over a building. For the protection to be effective the lattice must be regularly spaced (10m or 20m spacing), uniform as far as possible and earthed. It is essential that the down conductors are terminated in a low resistance earth system and the lightning transient does not discharge to adjacent metal work or find alternative lower resistance paths to earth.

The object of the lightning protection system is to conduct the large surge of electrical energy to earth without damage to the building or contents. The nature of the current and its very steep rate

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of rise and decay are such that a low impedance earth electrode system is required. The nature of the waveform and frequency is such that what appears as a low impedance path at 50 Hz may appear as medium or high impedance to a lightning surge. For example sharp bends in copper tape may give rise to high impedance from a lightning perspective.

If the lightning protection system is hit by lightning then the current flowing through the system and impedance offered by the conductors will determine the potentiat difference on the conductors with respect to true earth potential. This potential difference may be in the order of megavolts (1,000,000V) with respect to earth potential. Within the buildings and adjacent to the lightning protection conductors there will be metalwork, pipes, cable armour etc that are connected to earth via the equipotential bonding system. If the lightning surge current in the lightning conductors sees a lower impedance path to earth via adjacent cables, metalwork, instrument cable screens etc. it will attempt to flow down this path. To achieve the connection to this path it may arc across air gaps, damp concrete, instrument circuit boards etc.

To minimise the risk of side flashing the impedance of down conductors must be minimised. The impedance is minimised by taking the most direct route from air termination to earth and avoiding, where practical, sharp bends in the conductors In addition equipotential bonding is linked to the lightning protection so that as the potential on the lightning system rises the potential of all conductors within the building will also rise. This is intended to reduce the possibility of the lighting surge seeking alternative earth paths. Without a potential difference between the down conductors and the parallel path current should not flow between them. Pipework and potential parallel earth paths should be bonded to the lightning protection at both top and bottom of the structure. Offend due to pipe supports from the structure of the building this is achieved without additional specific bonding.

The concept of a Faraday Cage as a perfect box without openings or protrusions is not practical when considering industrial and particularly pharmaceutical buildings which tend to have a number of vents, pipework cables and process equipment entering and leaving the building. Any lightning strike to say, the pipe bridges will result in a pulse of electrical energy travelling to earth via the bridge supports. This surge will also travel to each building connected to the pipe bridge to find a low impedance path to earth. This path wilt include equipotential bonding, lightning conductors, instrument cable screens etc, within the building. It may also cause side flashes and resultant damage to electronic and instrument systems. To prevent surges entering the building via the pipework, pipe bridge supports or cable tray a low impedance path to earth should be provided at the point of entry into the building. All pipework entering a building on external overhead pipe- bridges should be bonded (together and to earth) at the point of entry and at 20m intervals. Steelwork forming part of lift structure or guides should also be bonded to the lightning protection / earthing system.

In the same way as a surge originating on a pipe bridge can travel to buildings connected to the structure, surges originating in any building on the site may be transferred around the site. In addition to pipework and metalwork buildings are linked via the armour of power and signal cables. The armour is normally bonded to earth at the steel frame of control and power distribution panels which will allow surges to dissipate to earth without entering the panel.

The earth electrodes are considered as a network and specific individual earth resistance is not specified as with power distribution earth electrodes. The entire network is required to have an overall resistance to earth of less than 10 ohms. Individual earth electrodes should have a resistance (ohms) not exceeding ten times (10 x) the number of electrodes provided on the building. However when testing if a single earth eiectrode is found to be significantly higher resistance that ail

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others consideration should be given to replacement of that specific rod. The British Standards also recommend buried ring electrodes.

The lightning protection system cannot be considered as a stand alone earthing installation since it utilises a number of components (parallel earth paths) also used as main bonding and supplementary bonding points (exposed conductors and extraneous conductive parts). The regulations require that the lightning protection and power system earth be bonded together at the main earth bar. This bonding cable should be clearly labelled as the lightning protection bond and facilities provided for disconnection to allow testing.,

Surne Protection Devices In addition to protection of the building structure, sensitive electronics within a building may also require protection. This again is based on a risk assessment and business contingency requirements. Surge diverters are fitted to the incoming power, telecoms and control signal lines. The surge protection devices will also require a reliable and robust earth connection.

Key Points to Note Lightning operates at a higher frequency than mains electricity and thus ‘sees’ different impedances for the same strip of copper. Buildings do not form perfect ‘faraday cages’ and protrusions need to be considered Parallel earth paths need to be bonded Hazardous Areas need higher levels of protection Bonding to electrical power system earth is required

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AN TI -STATIC EARTH I NG Static discharges within Hazardous Areas have the potential to ignite the potentially explosive atmosphere and need to be controlled and eliminated. Static electricity is generated when like or unlike materials move against each other causing electrons the be stripped from one material and attached to the other. This results is one of the materials having a positive electrical charge and the other a negative. For significant charge to be developed one of the bodies must be a conductor and insulated from earth. This creates a hazard if the body becomes sufficiently charged to create a spark capable of discharging to an adjacent conducting body.

Potential Energy on Typical Plant Items (1 OkV) Road Tanker‘ 250 mJ Person I O mJ 100 mm Flange 0.5 mJ

Minimum Ignition Energy (Vapours) Propanol 0.65 mJ Methane 0.28 mJ Methanol 0.14 mJ

The most common source of danger from static electricity is the retention of charge on a conductor because virtually all the stored energy can be released in a single spark to earth or another conductor. The accepted method of avoiding the hazard is to connect‘all conductors to each other and to earth with sufficiently low resistance to permit the relaxation of the charge.

In addition, as with any electric shock, a person discharging a static charge may as a result suffer fall or other injury..

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The British Standard 855985 Control of Undesirable Static Electricity Part 1 I991 & Part 2 1991 define the requirements for anti-static earthing installations. The standards generally require a resistance of 10 Ohm from conductors (tanks, pipework etc.) to earth. However resistances up to I O 6 Ohm will relax static charges.

The anti-static earthing system is part of the Overall earthing system. The system which provides electrical safety earths and equipotential bonding will also provide anti-static earthing provided the resistance to earth is sufficiently low.

Fixed Plant Items In general the equipotential bonding and supplemental bonding within a plant area is sufficient for static charge relaxation of fixed equipment. Attention is required for items which are not normally considered within the scope of equipotential bonding systems such as vessel agitators (normally sufficiently low resistance to allow relaxation but should be tested), vessel on weigh cells which may be insulated from earth and removable components (chutes etc).

Glass Pipework Some process applications require the use of all glass pipework and vessels. These pose specific problems for anti-static earthing in particular earthing of flanges which are generally metal. All flanges must be bonded to earth.

Mobile Items of Plant Mobile items such as movable equipment, drums, mobile vessels and raw materials pose significant risk for build up and generation of static charges within a plant. These items often have no connection to the electrical network and must be specifically earthed at their point of use and storage. This is generally achieved using earthing reels or earth bars within the plant areas. The British Standard BS5989 and Cenelec technical report CLC/TR 50404 both give detailed guidance on avoidance and control of static on mobile items. Typically wheels on mobile plant must be anti- static.

Tanker Bays Loading and unloading of aviation, automotive and other petroleum fuels requires particular precautions to ensure static discharges do not occur or, result in ignition. These installations are governed by specific standards and legislation and are not addressed here. However tanker loading and offloading is a common requirement on industrial and pharmaceutical sites and anti static earthing facilities should be provided where ever flammable liquids are handled.

To eliminate this risk when earthing road tankers, a system has been developed which confirms a secure connection by the earth clamp to the tanker body before zero earth potential is applied to the clamp. The system operated by passing an Intrinsically Safe current through the prospective earth circuit prior to application of the earth potential. These systems can also detect, by checking the capacitance of the earthed object, whether the clamp has been applied to a tanker or just local metalwork and can be used for interlocking of plant and equipment. These systems are commonly referred to as “earth rite systems” (earth rite is a trade name of one of the system suppliers).

Control of the Generation of Static Electricity It should be noted that earthing is not the only method for control of static electricity in internal plant areas. Although humid air is not a conductor it may be used to control static. With high relative humidity a moisture film forms on the surface of solids with a resultant reduction in surface resistivity to a level where relaxation occurs. Standards recommend that RH s maintained above 65% and not be the only method of static control. There are also numerous methods applied on

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sites to reduce or eliminate the generation of static. These include anti static footwear and clothing, reduced pumping rates, reduced splashing on charging vessels, use of metal containers for storage and transfer of product and so.

INTRINSICALLY SAFE SYSTEMS Industrial or pharmaceutical plants that use or store solvents, fine powders or flammable materials are required to zone the plants into areas that contain potentially explosive atmospheres, Hazardous Areas. Within these Hazardous Areas'all ignition sources are eliminated or controlled. One of the methods of controlling ignition sources is by use of intrinsically safe installations. The concept of intrinsically safe systems is that the energy available in the hazardous areas is limited such that it is not capable of igniting the explosive atmosphere. The energy is limited by use of safety barriers of which there are two types either with or without galvanic isolation. The safety barriers without galvanic isolation require a high integrity earth to operate.

The shunt diode barrier limits the energy available by use of fuse and resister to limit current and a diode (zener diode) to limit voltage. The principle of operation of the zener barrier is based upon the reference voltage of the zener barrier not been exceeded in normal operation. If this voltage is exceeded by for example a fault within the safe area then the diode will avalanche into conduction and break the fuse which isolates the circuit. In order for the circuit to operate it must be connected to a secure and effective zero reference point. This zero reference point is always the earthing system with the plant.

In a shunt diode circuit fault, currents are returned to the neutral star point within the safe area. However during that transient period the voltage difference between the star point and the barrier earth bar is transferred to the hazardous area. In these circumstances it is necessary to maintain the barrier earth connection as low as possible to prevent a source of ignition developing. The usual limit on this potential difference is 1OV. The intrinsically safe earth bar must be secure, reliable and paths to star point must be of low resistance.

The National Rules for Electrical Installations in Potentially Explosive Atmospheres ET1 05:2001 specifically requires that "the conductor used shall be insulated to prevent invasion of the earth by fault currents, which might flow in metal parts with which the conductor could come into contact (for example control panel frames)".

A secure earth is normally provided by connecting the barriers directly to the main distribution board earth bar (transformer star point) via two separate earth cables with diverse routing. In addition to a secure earth this arrangement allows testing of the earth installation (one cable at a time)'while the plant is live.

Key Points to Note;

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IS Safety barriers without galvanic isolation require a high reliability earth to operate Earth connection must be less than 1 ohm Insulated and isolated to prevent invasion by earth fault currents

INSTRUMENTATION EARTHS AND SHIELDING Instrument cables in an industriaVprocess plants operate within an environment with electrical noise emitted from power cables, motors, lightning and other electrical equipment.

The two types of induced noise affect signal cables are electrically coupled capacitively interface

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and magnetically coupled interface.

To minimise the capacitively coupled interference a screen is required around the signal cables with a connection to earth. It is normal practice to earth this screen at one point only to prevent circulating currents. To achieve a single earthing point instrument cables screens are normally earthed on earth bars mounted on isolators. One bar, normally at the source of the cables i.e. PLClDCS panel is connected to the earth system.

The reduction of magnetically couple interference can be achieved by creating an opposing magnetic field. This can be achieved by providing on screen earthed at both ends to allow current flow. The magnetic field will induce a current that will reduce magnetic flow within the shield area. Correctly bonded cable tray and earthed cores within multicore cables will also contribute to the shielding achieved by induced currents.

In addition to the requirements to reduce interference and noise the National Rules for Electrical Installations in Potentially Explosive Atmospheres ET1 05:2001 restricts earthing of screens on Intrinsically Safe cables to one point only, normally within the non-hazardous areas. It also recommends that armour be bonded to earth at both ends.

REFERENCES

ETCl ET101 2002 National Rules for Electrical Installations - Third Edition

ETCl €TI 052002 National Rules for Electrical Installations in Potentially Explosive Atmospheres

BS 7430: 1998 Code of Practice for Earthing

BS 5958 Code of Practice for Control in Undesirable Static Electricity

BS 6651 1991 Code of Practice for protection of Structures from Lightning

IEE Guidance Notes on the Wiring Regulations.

Institute of Petroleum - Guidelines for the Control of Static Hazards arising from Static Electricity Part 21 2nd Edition 2002

Cenelec CLCnR 50404 Electrostatics - Code of Practice for the avoidance of hazards due to static electricity. June 2003

USEFUL WEB SITES Electro-Technical Council of Ireland - www.etci.ie National Standards Authority of Ireland - www.nsai.ie Institution of Electrical Engineers - www.iee.org

Kieran Failon may be contacted at kfallon@dm.ie or kifallon@iee.orq

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EARTHING IN AND PHARM

PLANTS

Kieran fallon, Chartered Engineer FIE1 Electrical 8 Instrumentation Manager,

DPS Engineering & Construction

WHY EARTH ELECTRICAL SYSTEMS

Low Impedance Return Path for Fault

* Prevent Potential Differences

Lightning Protection. * Relax Static Charges. .e Minimise electrical interference to

instrument signals.

Cu rren ts.

(electrocution hazards).

Reference for Intrinsically Safe Syste I

REGULATIONS, STAND RULES

Electro Technical Council of Ireland e Euro-Norm Standards (IS-EN-0000 or BS-

EN-0000) British Standards Irish Standards (NSAI)

EARTH ELECTRODES

Adequate Electrical Conductivity Mechanically Robust

0 Protected Against Corrosion Clamp does not damage Earthing

Accessible for Inspection Label

Conductor

%

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EARTH ELECTRODES v

Quantity Location

e Co-ordination with Architect & Services Cable Routes

e Testing ti Maintenance

EARTH BARS

0 General Earthing * Protective Conductors e Main Equipotential Bonding PO,J

High Integrity Earths (IS Systems) Bonding Conductors for Lightning

Main site earth bar Individual building earth bars Plant area earth bars

Robust Clearly Labelled

ELECTRICAL POWER SYST

Medium Voltage Earthing Neutral Earth link Earth distribution throughout the Site Generator Earthing Final Circuit Earthing

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ELECTRICAL POWE SYSTEM Medium Voltaae

Discharge of Cables Future Earthing of 20kV

ELECTRICAL POWER SYSTEM Neutral - Earth Link

Required for site generators and

. Size Usually in'Main Distribution Board

transformers

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k PO SYSUEM

0 Earthing if Grid I Utility supply disconnected

0 Loop Impedance BS7430

ELECTRICAL POWE Y Final Circuit Earthinq

0 . RCD protection on all sockets (ETCI) 0 Loop Impedance

Fault Currents, Conductor & Temperature (Hazardous Areas)

3/20

EQUIPOTENTIAL ,c 5ONDING

Safety requirement Protection against Indirect Contact Insulated copper Safety label Size - 50% largest CPC Need not exceed 25 mm Sq Robust

EQUIPOTENTIAL BONDING

Supplementary bonding throughout plant

Small cable size requirement than main

9 Specific Hazardous Area requirements 0 Electrical Continuity of Piping Systems

areas

bonding cables

3/21

0 BS6651 Code of Practice for the Protection of Structures against Lightning

* Risk Assessment - Always required on buildings with Hazardous

Areas - Always required where extending buildings

with existing protection - Cost of loss of production

LIGHTNING

Faraday Cage - Conducting box - No openings - Current flows over the outside - No effect on interior

3/22

LIGHTNING B OUECTION

Structures in Pharmaceutical Plants - Steel framed or RC Concrete - Often insulated steel cladding - Usually no requirement to ‘hide’ down

conductors

LIGHTNING PROT

* Structures in Pharmaceutical Plants - Usually contain Hazardous Areas - Protrusions through building including vents,

ductwork, pipe systems, cable tray

3123

Down Conductors -Steel Frame - Cladding - Copper Tape - Bond all parallel paths - No sharp bends - IOm Grid

LIGHTNING PRQTECTI

Air Termination - Cladding or lattice of copper conductors - Cross bonding at roof level of all equipment - 1 Om spacing for hazardous areas

3/24

LIGHTNING PR TECTlON

* Earth Termination Network - Overall network I O Ohm - Individual electrode resistance = lox number

- As close as possible to building - Ring electrodes should be considered

of elect rod es

ANTI-STATIC EARTHING

e Contact electrification - Materials brought together and separated - Flow of fluids through pipes - Mixing, agitation - Movement on conveyer belts -Walking, Moving equipment or drums - Crystallization - Dust collection & Pneumatic Conveying

3125

0 Potential Energy on Typical Plant Items (I OkV) - Road Tanker 250 mJ - Person 10 mJ - 100 mm Flange 0.5 mJ

- Propanol 0.65 mJ - Methane 0.28 mJ - Methanol 0.14 mJ

Minimum Ignition Energy (Vapours)

Source- IChemE

ANTI-STATIC EARP

Relaxation of Charge

Ohm Minimum)

Equipotential Bonding

static outside scope of this discussion

-Achieve I O Ohm bonding to all items ( I O s

- Same earthing system as used for

- Control measures to reduce generation of

3/26

ANTI-STATIC’EARTHING

Mobile Plant Items Removable parts on Equipment Road Tankers Personnel Drums Raw Materials & IBCs -

ANTI-STATIC EARTHING

Earth%Reels and Clamps Monitored and Interlocked bonding for

Sampling procedures and equipment Anti-static footwear and test stations

Tankers

3/27

Safety Barrier without Galvanic Isolation - Zener Diode & Fuse Circuit - Requires Zero Volt reference - Usually two earth cables from main earth bar -Less than I Ohm - Isolated such that earth fault currents cannot

invade circuit. * - Clearly labelled & identified

INSTRUMENTATION E AND SHIELBI

Shielding from capacitive and inductive noise - Bonding of shields at source only - Bonding of armour, tray and parallel earth

paths at both source and destination - IS Requirements reinforce this concept

3 128

EARTHING IN INDUSTRIAL AND PHARMACEUTlCAL

PLANTS

Kieran Fallon, Chartered Engineer FIE1 Electrical & Instrumentation Manager,

DPS Engineering & Construction I