PRINCIPLES OF EXPLOSION PROTECTION

Transcription

1 PRINCIPLES OF EXPLOSION PROTECTION

2 Contact Details Definitions... 1 Explosion proof apparatus Hazard zone classifications... 4 Installations Introduction... 1 Maintenance Primary and secondary explosion protection... 2 Temperature classes Types of protection for Zone... 5 Types of protection for Zone

3 1. Introduction We associate the word explosion with an unintended event, the effects of which are beyond human control, and which may cause injury and damage. The danger of an explosion is always present where combustible materials are being handled and where there is a source of ignition. Many industrial plants in which combustible materials are used have electric motors to drive pumps, fans and conveyors, thermostats and pressure switches to control processes, and electrical heating to heat products. Since every apparatus is a potential danger to employees and their environment, the authorities impose measures to prevent the risk of ignition. The fact that explosions seldom occur, despite countless explosion-hazard zones, proves that the safety measures taken are effective and successful. 2. Definitions 2.1 CENELEC standards Standards for explosion-proof electrical apparatus are issued by the European countries (Belgium, Denmark, Germany, Finland, France, Ireland, Italy, Holland, Norway, Austria, Portugal, Sweden, Spain and Great Britain). Members of CENELEC undertake to comply with the conditions laid down in the standards, according to which the European standard is accepted as national standard without alteration. This stipulation des not imply that the results of the European testing institutes must be mutually recognized. 2.2 Electrical Apparatus All items used as a whole or in part for the application of electrical energy. These include the items for the generation, transmission, distribution, storage, measurement, regulation, conversion and consumption of electrical and items for telecommunications. Electrical apparatus for potentially explosive areas is divided into <<Group I>> and <<Group II>> Group I Electrical apparatus for mines susceptible to firedamp, with a maximum surface temperature of 150 C, where coal dust can deposit to build up a layer. If there is no risk of deposits forming, a surface temperature of 450 C is permitted Group II Electrical apparatus used in explosion-hazard zones (excluding Group I). Concerning surface temperature, the stipulations of section 5 <<Temperature classes>> apply Differences from normal design Compared with the electrical apparatus that is not explosion-proof, explosion-proof apparatus must satisfy additional requirements such as: Higher IP degree of protection; Use of tested junction boxes and cable entries (if plastic fittings are selected; Additional costly testing: Testing of flameproof enclosure for explosion proofing and non transmission of internal ignition. Temperature rise testing on junction boxes of the <<increased safety EExe>> type; Additional monitoring devices to prevent excessively high temperatures, sparks and arcs. 2.3 <<Ex>>-components Part of an electrical apparatus for potentially explosive atmospheres that may not be used alone in such atmospheres and which requires an additional certification in conjunction with any electrical apparatus together with which it was used. 2.4 Types of protection 1

4 Modifications applied to electrical apparatus during manufacture in order to prevent the apparatus from igniting an ambient atmosphere that is potentially explosive. 2.5 Explosion-hazard zone Zone whose atmosphere could become explosive (the hazard is potential). 2.6 Explosive atmosphere A mixture with air, under atmospheric conditions, of flammable substances in the form gas, vapour or mist, in such proportions that it can be exploded by excessive temperature, arcs or sparks (the hazard is real). 2.7 Explosive mixture A specific mixture used for testing electrical apparatus for explosion-hazard zones. 2.8 Explosion Independent propagation of the flame in an explosive mixture. 2.9 Maximum surface temperature The highest temperature attained under the most unfavourable conditions by any part or surface of an electrical apparatus. The most unfavorable conditions include the admissible overloads and fault conditions specified in the standard for the type of protection concerned. The maximum surface temperature of an electrical apparatus must always be lower than the ignition temperature of the gas or vapour mixture in which the apparatus is used. If the heating surfaces are insulated, the applicable maximum surface temperature for the assessment is always the relevant heating surface. Normally the insulation is not gas-tight, and this might lead to an explosion in the insulation. Spaces in which steam pipes and the like pass through the explosion-hazard zone are subject to the same requirements. 3. Primary and secondary explosion protection 3.1 Primary explosion protection A combustion reaction requires fuel, oxygen and, to trigger it, a certain amount of additional energy, known as the ignition source. Together, these three components form the hazard triangle. If on of the three conditions is lacking, no combustion or explosion can occur Fuel The rate at which the combustion reaction is independently propagated in the mixture depends on the concentration of the vapour/air or gas/air mixture. A distinction is made between an explosion and a deflagration on the basis of the rate of combustion. The ignition range covers the mixture concentration span within which an explosion can be triggered by means of an ignition source. Primary explosion proofing is a matter of keeping below the lower ignition limit. In this region the mixture is too weak for an explosion to be triggered. The ignition range is between a lower and an upper ignition limit. Below the lower ignition limit the concentration of combustible gases and vapours (in %vol or g/m 3 ) is too low to cause combustion or explosion. Above the upper limit the mixture is too rich in combustible fractions for an explosion to take place Oxygen Oxygen is a gas that must be present in sufficient quantity for an explosion or a fire to take place. 3.2 Protective measures Primary explosion proofing includes the following protective measures: 2

5 3.2.1 Replacing the combustible substance It often possible to replace the combustible substance with a material that is either incombustible or incapable of forming a potentially explosive atmosphere. The main suitable substitutes are: Aqueous solutions Incombustible halogenated hydrocarbons, and Incombustible materials Inserting the apparatus Inserting the apparatus means replacing part of the oxygen in the air in a restricted volume with inert gases. Most fuel/air mixtures are no longer capable of ignition when the oxygen content is below 8%vol (for hydrogen and carbon monoxide below 4%vol). The commercially available gases nitrogen and carbon dioxide are generally used for inserting. The oxygen is displaced in two stages: 1.) Purging of the container or installation before the operation or process begins, for example by evacuation followed by replacing the vacuum with nitrogen; 2.) Maintaining the low oxygen concentration achieved by purging by making up any losses of inert gas during the operation or process. Unless the inerting of the production equipment or tanks is ensured by the process condition, it must be checked and monitored with oxygen measuring devices, for example Use of sealed systems Installations designed as sealed systems, in which combustible materials are present, have the advantage that no gases and vapours an escape. Measures for achieving sealed systems are: Continuous processes Gas compensation pipe Pressure equalization at a safe place in the open Entry through air locks Ventilation measures Ventilation measures can have the effect of greatly reducing the range of potentially explosive atmosphere in the vicinity of installations, apparatus and the like. Ventilation can be provided in various ways: Natural ventilation Artificial ventilation, e.g. room ventilation and Extraction at source Artificial ventilation is required: When handling or processing combustible materials that may form a potentially explosive atmosphere in a non-enclosed system and For the storage of combustible liquids with a flash point below 30 C and the heavier-than-air combustible gases in underground rooms. Artificial ventilation is necessary because it provides a higher throughput and a more focused airflow and natural ventilation. Extraction at source is generally preferable to artificial room ventilation because it is more effective and less costly. This applies especially when dealing with combustible dusts. Ventilation measures that are necessary to protect health often also satisfy the requirements of explosion proofing. 3

6 3.2.5 Concentration monitoring Depending on local conditions, leaks can be detected at an early stage by monitoring the concentration of combustible gases, vapous and mists in the vicinity o possible sources of risk (leakage points), so that the necessary safety measures can be implemented immediately, such as emergency ventilation, fail-safe shutdown of the plant etc. 3.3 Secondary explosion protection Generally, primary explosion protection cannot be completely achieved at all. Steps must therefore be taken to prevent the ignition of potentially explosive atmospheres. Ignition sources may results from, among other things, excessive surface temperatures, electric sparks or exothermic reactions (heat released during chemical reactions). Sparks and excessive temperatures can be avoided by design measures. 4. Hazard zone classifications All areas n which, because of the local and operational conditions, potentially explosive atmospheres may occur in hazardous quantity are deemed explosion-hazard zones. According to the probability, in terms of time and location, of the presence of potentially explosive atmospheres, potentially explosive areas are divided into zones which allow differential evaluation of the explosion hazard. It is the responsibility of the relevant authorities to apply the appropriate rules. The rules cannot be applied to combustible dusts or fibers, nor to the premises used for medical purposes. Since the concentrations decrease with increasing distance from the source of risk the location of the source of risk is of the greatest importance for the zone classification. The term <<source of risk>> is understood to mean the place at which the combustible gases, vapours, mists and liquids arise or emerge, or at which potentially explosive mixtures may form. 4.1 Zone 0 Area in which hazardous, potentially explosive atmospheres occur continuously, for long periods, or for short periods that recur frequently. 4.2 Zone 1 Area in which hazardous, potentially explosive atmospheres are likely to occur occasionally or periodically. This the classical field of application for explosion proof apparatus. 4.3 Zone 2 Area in which hazardous, potentially explosive atmospheres occur only seldom, and then only for short periods (max. 2 hours). 4.4 Factors affecting zone classification No generally applicable definition of hazardous areas can be given, since they are affected by a variety of factors. The following factors affect the zone classification of a potentially explosive area: 1.) The properties of the combustible materials 2.) The quantity of emerging gases, vapours, mists and liquids 3.) The nature of the source of the danger 4.) The nature and effectiveness of the ventilation 5.) The prevention of the propagation of gases mists and vapours. 6.) The promotion of the propagation of gases, mists and vapours by a current of heated air. 7.) Past experience 8.) Maintenance and the internal organization (trained personnel) 5. Types of protection for Zone 0 4

7 In zone 0, only apparatus may be used that is specially approved for this zone. Normally these pieces of apparatus have two mutually independent protection types (such as flameproof enclosures and intrinsically safe power supply). Intrinsically safe circuits must comply with the category <<ia>>. The reference to zone 0 must appear on the nameplate (EEx d [ia] IIC T6 Zone 0). 5.1 Intrinsic safety EN A piece of electrical apparatus is intrinsically safe if all circuits are intrinsically safe. A circuit is intrinsically safe if its amperage and voltage are limited I such a way (function of inductance, capacitance and resistance) that no sparks or thermal effects can occur in it. The energy of such a circuit is lower than the minimal ignition energy required igniting an explosive mixture. For this type of protection, classes A, B and C are prescribed on the basis of the experimentally established minimum ignition current (MIC [see section 6.4.1]) In the case of intrinsic safety <<i>>, a distinction is made between categories <<ia>> and <<ib>>. Category <<ia>> is required to meet more stringent requirements in the event of defects occurring and combinations of such defects. The following safety factors apply: 1.5 In norm al operation and with one defect 1.0 With two defects If assignment to temperature classes is done according to surface temperature, a safety factor of 1.0 is applied. Intrinsically safe connections and conductors must be marked (if color-coded light blue only, RAL 5012). 6. Types of protection for Zone 1 Explosion proof apparatus can be designed according to the various types of protection. Those most frequently used are: 1.) Flameproof enclosure EEx d (Heaters, switches, motors, contactors etc), 2.) <<increased safet y>> EEx e (Terminals, connection housing, motors, some heaters) 3.) Intrinsic safety EEx i (Measurement and control apparatus, monitoring devices) Explosion proof apparatus often has several degrees of protection. The most frequent combination is an enclosure in the form of a flameproof enclosure, and a junction box of <<increased safety>> design. The advantage of this combination is that unauthorized persons cannot tamper wit the flameproof part, since the connections are separately arranged. 6.1 Basic reconditions of EN The conditions and stipulations laid down in CENELEC Standard EN apply without restriction to all permitted types of ignition protection. 6.2 Flameproof enclosure <<d> EN EEx d IIC, EEx d IIB and EEx d IIA Apparatus and components that may ignite a potentially explosive atmosphere are housed in an enclosure which, in the event of the explosion of an imitable mixture inside the enclosure, withstands the pressure and prevents propagation of the explosion to the potentially explosive atmosphere surrounding the enclosure (spark ignition). It is assumed that a potentially explosive mixture cannot be prevented from entering. 5

8 The concept of a flameproof enclosure thus takes into account the possibility of an explosion inside the enclosure, but it is essential that the explosion be restricted to the enclosure space. The following requirements guarantee the necessary safety: a.) The enclosure must withstand 1.5 times the explosion pressure, and b.) There must be no spark ignition Explosion pressure The strength of the enclosure is checked by determining the explosion pressure. In practice, the maximum explosion pressure is never reached, since losses arise owing to thermal conduction and radiation. In addition, the losses are highly dependent on the shape of the enclosure, on any apparatus contained within the enclosure and on the ignition location. In order to obtain corresponding safety factor for the strength of the enclosure, the initial pressure of the gas mixture is raised and the explosion pressure rises proportionally. The explosion pressure must be determined experimentally since those loses cannot be determined by calculation. Besides, explosions vary in so many ways it is impossible to generalize about them. However, measurement technology makes it possible to measure and record the rapidly changing pressures. The free volume of a symmetrical enclosure only has a slight influence on the explosion pressure that may be expected. This effect ceases to exist altogether for enclosures with a volume of more than 5000cm Ignition penetration The gap lengths <<l>> and widths <<w>> occurring at the joints of an enclosure is decisive for its resistance to ignition penetration. The gap length can easily be determined by measurement or by the difference in diameter. It is not affected by the explosion pressure. The gap widths, on the other hand, may be influenced by the explosion pressure. The safety of the design is not only a matter of maintaining and checking the gaps of an enclosure without pressure; in addition, the largest gap occurring in the event of an explosion inside the enclosure must be determined. Deflection of enclosure walls, flanges, etc., may cause the gap to exceed the permitted values. The safe gap widths are known for the usual gases. By using one of these usual gases it is possible to determine whether the gap occurring under explosion pressure is exceeded. On the basis of the experimentally determined safe gap (MESG), enclosures are divided into groups A, B and C. This subdivision determines the allocation of the individual media (gas mixtures) the requirements to be satisfied by the design increase in the order of the letters.most manufactures of flameproof equipment design for the strictest requirements in order to cover all media with one design, since it is uneconomic to have different apparatus in series production. Current international standards distinguish between design in which gaps occur on the one hand without threads and on the other hand with threads. Gap without threads for enclosures with volume up to 2000 cm 3 : Group IIA IIB IIC Gap length l 12.5 mm 12.5 mm 12.5 mm Gap width w 0.3 mm 0.2 mm 0.15mm For thread gaps, there are minimum requirements as to pitch, quality, number of active turns of thread and screw-in depth. Only metric ISO threads are permitted Tests 6

9 In type testing, an explosive mixture is ignited inside the enclosure, and the resulting explosion pressure is measured. The number of tests and the gas mixtures are laid down in EN and are mandatory. For electrical apparatus of group IIA: 3 tests with (4.6 ± 0.3%) propane; For electrical apparatus of group IIB: 3 tests with (8.0 ± 0.5%) ethylene; For electrical apparatus of group IIC: 5 tests with (14 ± 1%) acetylene and 5 tests with (31 ± 1%) hydrogen; The manufacturer must subject apparatus with <<flameproof enclosure>> protection to routine test. The routine tests laid down for the flameproof enclosure include an overpressure test carried out in accordance with the prescribed procedure. The purpose of the routine test is to ensure that, on the one hand, the enclosures, pipes, etc., withstand the pressure, and on the other hand, that there are no holes cracks that provide an uncontrolled connection to the outside. 6.3 <<Increased safety e>> protection EN (EEx e II) With this degree of protection, special measures are taken to ensure an increased degree of safety and to prevent the occurrence of unacceptably high surface temperatures and of sparks r arcs inside or on the external parts of electrical apparatus. The basic difference between <<increased safety>> protection and <<flameproof enclosure>> is that the former totally precludes any ignition sources and thus an explosion. Particular importance is attached to compliance with the maximum surface temperature requirement (see section 8 Temperature classes) and to clearances and creepage paths. Enclosures and junction boxes that contain bare live parts must conform to protection type IP 54. Enclosures and junction boxes that contain only insulated parts may be made to protection type IP Creepage paths The creepage path is the shortest distance between two conducting paths along an insulating surface. The creepage paths between conducting parts of different potential must satisfy the requirements listed in the table below. Grooves in the surface of insulating parts may only be included in measurement of creepage distances if they are at least 2.5mm deep and at least 2.5mm wide. The dimensioning of creepage paths depends on the service voltage, the tracking resistance of the insulation and the insulation s contours. Electrical insulating materials are rated according to their comparative tracking index (CTI) under IEC 112. Because inorganic insulating materials such as glass and ceramics do not leave any creepage current traces, determinations of the CTI can be omitted. These materials are normally assigned to Material Class I. Service voltage, U [Volts] Minimum creepage path [mm] Materials Class I II IIIa U ~ < U ~ < U ~ < U ~ < U ~

10 175 < U ~ < U ~ < U ~ < U ~ Table: Creepage paths for insulating materials Clearances The clearance is the shortest distance in the air between two bare conducting parts. Mains voltage is taken as the basis for determining the clearances between live parts and earth (ground); this also applies for use in systems with solidly earthed neutral point. The table below shows the minimum clearances as a function of rated insulation voltage. Particularly when components with bare live parts are retrofitted, attention must be given to conformance with the minimum clearance requirement. Service voltage, U [Volts] Minimum Clearance [mm] U ~ < U ~ < U ~ < U ~ < U ~ < U ~ < U ~ < U ~ < U ~ Table: Clearances Requirements to be met by terminals As prescribed by Standard EN 50019, all terminals must be secured against working loose. The terminal must be designed so that the conductors cannot become detached from the terminal and sufficient contact pressure is ensured. The conductors must not be damaged in any way by clamping point of the terminal Admissible terminal connections For <<increased safety e>> protection, the following types of internal connections are permitted: Threaded connections secured against working loose Crimping Soldering, provided the conductors are also held together mechanically. Brazing Welding Where aluminum is used, special precautions have to be taken against electrolyte corrosion Junction boxes To make sure the temperature limits in a plant are not exceeded, a maximum dissipation power is set for each and every junction box. The admissible heating limits in these boxes depend on two factors: The number of terminals ad conductors inside the enclosure that create local heating inside it; and The heating of individual terminals and conductors compared with the local temperature around them 8

11 To make things easier for the electrician, the junction bow fabricator provides a table for each box indicating the maximum number of terminals and conductors for various rated currents and conductor cross-sections. Current Cross-section in mm 2 [A] Max. no. of terminals IP protection classification The IP protection system uses two digits. The first digit indicates protection against accidental contact and foreign bodies, and the second protection against water. The degrees of relevant for explosion proof apparatus are: Degrees of protection against accidental contact and foreign bodies (first digit) 4 Protection against penetration of solid foreign bodies larger than 1mm (contact with tools, wires, etc., thicker than 1mm). 5 Total protection against accidental contact, protection against harmful dust deposits; dust penetration is not totally prevented. 6 Total protection against accidental contact. Protection against penetration of dust. Degrees of protection for water (second digit) 4 Protection against water splashed from any direction. 5 Protection against water jets from any direction. 6 Protection against temporary flooding, e.g. due to heavy seas. 7 Protection against water submersion t predetermined pressure for undetermined period. 8 Protection against water submersion at elevated pressure for undetermined period. Example: IP 54 means protection against harmful dust deposits (5) and water splashed from any direction (4) Magnesium content of enclosure alloys Because of possible spark formation due to impact, terminal boxes must not contain more than 6% magnesium. Most enclosures on the market have an aluminum alloy of AlSi12. Light alloy enclosures have been partially replaced by polyester enclosures, which easily meet the higher standards in terms of resistance to impact and high temperatures. 6.4 Intrinsic safety <<i>> EN (EEx ia IIC, EEx ib IIC) 9

12 Electrical apparatus is intrinsically safe if all circuits are intrinsically safe. A circuit is intrinsically safe if, as a result of limitation of current and voltage (a function of inductance, capacitance and resistance), no sparks or thermal effects can occur in it. The energy of such circuits is lower than the minimum ignition required igniting an explosive mixture. Subdivisions A, B and C, which are based on the experimentally determined minimum ignition current (MIC), have been laid down for this degree of protection. As far as intrinsic safety <<i>> is concerned, a distinction is made between the two categories <<ia>> and <<ib>>. Category <<ia>> is subject to more stringent requirements on the occurrence of faults and combinations of faults. Consequently, category <<ia>> is prescribed for use in Zone 0 and <<ib>> for use in Zone 1. A further stipulation is that intrinsically safe connections must be marked as such (if colour is used, only light blue RAL 5012). In practice, this degree of protection permits the use of normal, non explosive proof thermostats, control equipment and monitoring devices, provided that a tested, intrinsically safe arc suppression relay or suitable safety barriers are employed. Associated electrical apparatus with intrinsically safe circuits must be installed outside of the hazardous area. In the case of associated electrical apparatus, the type of protection is stated in brackets: [EEx ib] IIC T Minimum ignition energy For intrinsically safe electrical apparatus, gases and vapours are classified according to the ratio between their minimum ignition current (MIC) and the minimum ignition current of methane. The CMI ratio must be determined using the method and test equipment laid down in the standard. Subdivision A: MIC ratio greater than 0.8 Subdivision B: MIC ratio between 0.45 and 0.8 Subdivision C: MIC ratio less than Zener safety barriers Zener safety barriers separate non-intrinsically safe circuits from the intrinsically safe circuit. Their function is to limit the voltage and the current in an intrinsically safe circuit to such an extent that the energy present is insufficient to ignite explosive mixtures. This limitation must be guaranteed both under normal conditions and in the event of faults. For this reason it is essential to observe the stipulations of the instruction. With Zener barriers, he specified capacitances and inductances in the intrinsically safe circuit must not be exceeded. Fig. 12 shows the schematically the circuit diagram of a safety Zener Barrier. Zener safety barriers are installed either outside a potentially explosive area or in a flameproof enclosure Intrinsic safety protection relays (Switching amplifiers) Intrinsic safety protection relays are used to transmit digital signals or to connect switching devices that are not explosion proof (terminals, pressure switches, micro switches, limit switches, etc.). Intrinsically safe protection relays and switching amplifiers are available with relay outputs and opt coupler outputs. Normally they have a broken-wire interlock and an operation indicator. Intrinsically safe protection relays and switching amplifiers are located outside the potentially explosive area in control equipment or in flameproof enclosures. 6.5 Intrinsically safe electrical system EN

13 An intrinsically safe electrical system is totality of electrical apparatus connected together that is documented with a system description. Those of its circuits that are used entirely or partly in the explosion-hazard zone must be intrinsically safe circuits. It is not necessary for each item of electrical apparatus in an intrinsically safe electrical system to be certified individually, provided that each such item of electrical apparatus is positively identifiable. 6.6 Encapsulation <<m>> EN (EEx m II) Encapsulation is a type of protection in which parts of a piece of a piece of electrical apparatus that are able to ignite an extremely explosive atmosphere by sparking or inadmissible heating are embedded in a casting compound. Thermosets, thermoplastics and elastomers are used for this purpose, with or without fillers (such as colorants). 6.7 Oil immersion <<o>> EN A type of protection in which the electrical apparatus or parts of the electrical apparatus are immersed in oil in such a way that an explosive atmosphere outside the oil cannot be ignited by arcs, sparks or hot gases occurring under the oil. 6.8 Pressurized enclosure <<p>> EN (EEx p II) Sparking apparatus (such as circuit breakers and contactors) and components with hot surfaces are housed in an enclosure in such a way that they can be kept under pressure or purged with air or inert gas. This makes it possible to expunge explosive mixtures that have penetrated the enclosure prior to start-up and to keep such mixtures from entering during operation. Pressurized enclosure, like flameproof enclosure, is a protection type that must be subjected to type testing or acceptance testing. Besides continuous pressurization, the regulations call for monitoring of the air or inert gas supply. In the event of a malfunction, the electrical apparatus housed in the enclosure must be de-energized immediately. Preliminary purging is done with 5 times the enclosure volume. The purging time starts when the rated flow of the purging medium is reached (flow meter incorporated in the pressure switch). Apparatus with simple compressed air connections without proper testing unfortunately occur frequently in practice, but in no way satisfy the relevant stipulations. In view of the new product liability legislation and he low voltage regulations, the use of such techniques to avoid complying with the stipulations may have extremely unpleasant and costly consequences. 6.9 Powder filling <<q>> EN A type of protection I which the parts at risk are surrounded with sand in an enclosure in such a way that ignition of a potentially explosive atmosphere as a result of arcs or excessive temperature is rendered impossible. 7. Types of protection for Zone 2 Apparatus approved for Zone 0 or Zone 1 may also be installed in Zone 2. In addition, the <<non-sparking apparatus n>> type of protection was created for Zone 2. The new CENELEC draft per EN is based partially on IEC Report 79-15, which has already been published. The future harmonized CENELEC standard EN , which will apply all over Europe, will probably become effective in about The new protection type, which applies exclusively to electrical apparatus installed in Zone 2, permits low-cost solutions. Only normal operation is taken into consideration in this zone; there is no need to consider short-term malfunctions. Apparatus with <<n>> protection is classified in five groups: nv for non-sparking electrical apparatus (rotating machines, fuses, light fixtures, measuring instruments and low-energy apparatus) 11

14 nw nr np nl for apparatus producing operational arcs, sparks or hot surfaces (enclosed-break devices, non-incendive components, hermetically sealed equipment and sealed devices) for restricted breathing enclosure for simplified pressurized enclosure for apparatus and circuits with limited energy 7.1 Non-sparking electrical apparatus Ex nv II Zone 2 In the case of this protection type, special precautions are taken to ensure an increased degree of safety and to prevent the occurrence of in admissibly high surface temperatures and sparks or arcs inside or on external components of electrical apparatus in normal operation. Special importance is attached to observance of the maximum surface temperature. Enclosures and junction boxes containing bare, live parts must comply with a degree of protection not less than IP 54. Those containing only insulated parts may be designed to a degree of protection not less than IP Apparatus producing operational internal arcs ad sparks Apparatus with operational sparking or hot surfaces EE nw II Zone 2 Apparatus and components with operational sparking must be encapsulated or sealed in such a way that they will be capable of either withstanding an internal explosion or preventing the external explosive atmosphere from penetrating. In the case of simplified flameproof enclosures, precautions must also be taken to ensure that a permissible internal explosion cannot be transmitted to the explosive atmosphere outside the enclosure (i.e., no spark ignition) Restricted breathing enclosure Ex nr II Zone 2 Apparatus with the internal sparking or arcing or inadmissible internal temperatures during normal operation may be used in Zone 2 if the enclosure complies with a degree of protection of at least IP 54 and an internal gauge pressure of 4 mbar requires more than 80 seconds to decrease to one-half of the initial value (2 mbar). Enclosures and boxes meeting these requirements qualify as <<restricted breathing>> enclosures. Unlike enclosures built to <<EEx e II>> and <<EEx d IIC>>, restricted breathing enclosures are not maintenance-free. The operator must ensure that the restricted breathing enclosures are inspected periodically Simplified pressurized enclosure Ex np II Zone 2 The simplified pressurized enclosure makes it possible to operate an enclosure under pressure. In the event of leakage or pressure loss, an alarm must be given but there is no need to de-energize immediately. Simplified pressurized enclosures normally consist of the enclosure itself, an air or nitrogen inlet nozzle (sintered nozzle to reduce noise) and a pressure monitor with intrinsically safe power supply. As things stood in June 1993, the possibility of tightening the requirements for simplified pressurized enclosures had been discussed at an international meeting. If adopted, these would require fabricators or suppliers to provide a pre-purging system and a flow monitor for purging air or inert gas. That which is prescribed as mandatory for applications in Zone 1 would be left to user s discretion in Zone 2. The user would decide whether the apparatus in the enclosure would be de-energized immediately or alarm would merely be tripped for the application in question. 7.3 Apparatus with limited energy Ex nl II Zone2 A piece of electrical apparatus is intrinsically safe if its amperage and voltage are limited in such a way (function of inductance, capacitance and resistance) that no sparks or thermal effects can occur in it. The energy of such circuits is lower than the minimum ignition energy required to ignite an explosive mixture. 12

15 8. Temperature classes For the commercial use of explosion proof apparatus, the maximum surface temperatures are determined and temperature classes are established. Temperature class T1 has the highest permissible surface temperature, temperature class T6 the lowest. Electrical apparatus conforming to higher temperature class (e.g. T5) may also be used for applications in which a lower temperature class is required (T2 or T3 for example). A common error is to apply temperature classes only to electrical apparatus. Pipes (for steam and hot media) with surface temperatures for a given temperatures class may be just as much a source of risk as the hot surfaces of electrical apparatus. 8.1 Temperature class T1 Mixtures with ignition temperature of t>450 C and a maximum surface temperature of 450 C. T1 includes the substances propane, carbon monoxide, ammonia, acetone, styrene, acetic acid, benzene, methane, toluene, hydrogen and town gas. T1 relates primarily to gasworks and the mining industry. 8.2 Temperature class T2 Mixtures with an ignition temperature of t>300 C and a maximum surface temperature of 300 C. The main substance covered by T2 is isopentane, butyl acetate, ethyl alcohol and acetylene chemistry, which are used in industrially in acetylene chemistry. 8.3 Temperature class T3 Mixtures with an ignition temperature of t>200 C and a maximum surface temperature of 200 C. T3 covers benzene and the corresponding derivatives, which are mainly in the petrochemical industry. 8.4 Temperature class T4 Mixtures with an ignition temperature of t> 135 C and a maximum surface temperature of 135 C. T4 includes mainly ethyl ether and acetaldehyde, which are used in the manufacture of plastics and solvents. 8.5 Temperature class T5 Mixtures with an ignition temperature of t> 100 C and a maximum surface temperature of 100 C. The practical importance of T5 is primarily in the manufacture of textile fibres. 8.6 Temperature class T6 Mixtures with an ignition temperature of t>85 C and a maximum surface temperature of 85 C. This temperature class is of practical importance primarily in areas involving the use of carbon disulphide and ethyl nitrite. 8.7 General conditions Usually electrical apparatus is designed for use in an ambient temperature range of -20 C to +40 C; in this case no additional making is necessary. If the electrical apparatus is designed for use in a different temperature range, this is regarded as a special design. The ambient temperature range must be specified by the manufacturer and stated in the certificate. The marking must then contain either the special ambient temperature range or, if that is not possible, the character <<X>> (see section ) Temperature Class Maximum Surface Safety Clearance for Temperature Permanently hot surfaces T1 450 C 10 Kelvin 13

16 T2 300 C 10 Kelvin T3 200 C 5 Kelvin T4 135 C 5 Kelvin T5 100 C 5 Kelvin T6 85 C 5 Kelvin 9. Explosion proof apparatus 9.1 Capacitors Capacitors that remain connected with the circuits after they have been switched off must have a discharging device regardless of their location (even if they are outside the potentially explosive area). This discharging device must discharge within five seconds to a permanent energy level of: 0.2mJ for electrical apparatus of Explosion group II A 0.06mJ for electrical apparatus of Explosion group II B 0.02mJ for electrical apparatus Explosion group II C 9.2 Transformers Transformers must be protected on the primary side against the effect of short-circuits and on the primary and secondary sides against excessive heating as a consequence of overloading. 9.3 Connectors Connectors must be either mechanically or electrically locked to permit plugging or unplugging in the de-energized state only. Deviations are permissible if connectors are assigned to only one item of apparatus and are secured against unintentional disconnection. In these cases a label reading <<Do not disconnect while energized>> is sufficient. 9.4 Cable entries The particular characteristics of the degree of protection of the apparatus must not be adversely affected by the cable entry. The sealing ring must be correct for the cable used. A distinction is made between cable entries or direct entry into a flameproof enclosure and cable entries for entry into an <<increased safety e>> enclosure. Entries for leading cables directly into flameproof enclosures and cable entries made of plastic must be certified. Metallic cable entries (normally protection lass IP 68) for <<EEx e>> enclosures do not require certification. Only metric threads may be used in explosion proof electrical apparatus. 9.5 Enclosures and distribution boxes Flameproof enclosures (EEx d IIC) and <<increased safety>> (EEx e II) junction boxes must be carefully sealed. No mechanical modifications may be made to flameproof enclosures (the drilling of holes is forbidden). The joints of flameproof enclosures may not be painted; only acid-free anticorrosion agents may be applied. Changes to the internal components (including the additional fitting of EX-e terminals) to EXe enclosures are permissible only if the terms of the certificate allow them. 9.6 Terminals and connections Only one wire may be connected to a terminal, unless the terminal is designed to receive several conductors (example: mantle terminal). The ends of flexible conductors must be fitted with cable lugs or ferrules. 14

17 9.7 Condensation in Ex enclosures Rapid temperature changes may cause the temperature to fall below the dew point, possibly leading to condensation in junction boxes. In the case of junction boxes for electric heaters, this can be prevented by leading in the heating cable directly. Junction boxes that contain only terminals may in special cases be fitted with an approved dewatering plug. 9.8 Seals Since explosion proof apparatus is normally exposed to environmental influences, particular attention must be given the sealing materials. Studies have shown that, for example, nitride rubbers are not suitable for all installations. High temperatures and corrosive ambient air require modern materials such as Vinton seals. These seals can be obtained from the cable gland manufacturers. Their longer life and increased safety justify the extra cost. 10. Installations 10.1 General The general rules and regulations that apply to normal apparatus apply as a matter of principle. Normally the regulations laid down for explosion proof apparatus must be satisfied in addition. It is a matter of <<and>>, not <<either-or>> Use of explosion proof electrical equipment Inscriptions on nameplates The inscription must bear the following inscriptions: 1. The name of the manufacturer or his registered trade mark 2. Ex classification (the symbol EEx, the sign for each type of protection used, the symbol of the group of the electrical apparatus, temperature class and if required also the zone [0 or 2]) 3. The indication of the testing station and the certificate reference 4. The mark <<X>> or <<U>> (if required) 5. Power rating 6. Voltage 7. Amperage where appropriate (important for fuse selection) 8. Instruction number (if required) 9. Extended or restricted ambient temperature range 10. Factory serial number Plus the following additional information for motors: 11. Ratio of starting current to rated current IA/IN 12. Tripping time te for <<increased safety>> motors Or 13. Response time of the temperature monitoring unit ta for thermistor-type protective system (TMS). The nameplate f apparatus approved for Zone 0 or only for Zone 2 must carry reference to the respective zone after EX classification (Ex nr II T5 Zone 2). If the nameplate does not refer to a specific zone, the piece of apparatus may be used in Zones 1 and 2 (EEx de IIC T3) Instructions Additional designation <<X>> If special conditions must be maintained when a piece of apparatus is installed, an <<X>> is added after the certification reference. For the installer, this means that the manufacturer s instruction sheet must be read carefully and its directions relative to safe operation of the apparatus must be followed to the letter. 15

18 Additional design <<U>> The supplement <<U>> tells the user that the apparatus is incomplete and cannot be used on its own. This additional marking is added in the case of components such as enclosures, terminals, microswitches etc, since these only achieve the status of a complete apparatus when installed or assembled Cables At present there are no definitive regulations for installations where there is a potentially explosive atmosphere. But it is advisable to use an appropriate industrial quality that is additionally flame-retardant or even self-extinguishing. Particular attention must be paid to the outer sheaths, so that the cables can withstand the expected mechanical and thermal effects and do not give rise to safety risks in service. Where different plastics are used, it is important to ensure that they are mutually compatible Shielded cables Cables and conductors with a shield or metal braiding must also have an outer sheath of plastic Cables for movable apparatus For movable apparatus with a rated voltage of up to 750V reinforced rubber cables or equivalent PUR cables must be used as connecting cables Fire partitioning of cables Penetrations fire cables and wires into areas that do not contain a potential explosive atmosphere must be sufficiently tightly sealed, for example by sand seals, mortar seals or special compounds. Long runs of cables or wires must be suitably subdivided in explosionhazard zones into individually protected fire sections. 10. Installation of cables and wires At locations that are particularly at risk due to thermal, mechanical or chemical influences, the cables and wires must be protected. This can be done by installing them in conduits, plastic hoses or metal hoses with edge protection (plastic ferrules or guards). Unused openings and cable entries on electrical apparatus must be sealed with approved plugs. In Zone 2, non-certified plugs may be used. Inside control cabinets, and in the interior of switchgear and distribution boards, special measures must be taken where there is a risk of confusion of the wiring of intrinsically safe circuits with that of ones that are not intrinsically safe- for example where there is a neutral wire coloured blue Minimum cross-sections For mechanical reasons, the following minimum cross-sections must be observed: Minimum cross-section for single-core wires 1 mm 2 for stranded conductors 1.5 mm 2 fr single-wire conductors Minimum cross-section for multi-core cables with up to 5 cores (3L + N + PE) 0.75 mm 2 for stranded conductors 1 mm 2 for single-wire conductors Minimum cross-section for multi-core cables with more than 5 cores 0.5 mm 2 for stranded conductors 1 mm 2 for single-wire conductors 16

19 10.7 Fuse protection of apparatus Where apparatus is protected by fuses, the rated current of the loads when stationary and their tripping characteristic must be taken into account in relation to the starting current. In doubtful cases the relevant rated current must be measured. Solenoid valves in Zone 1 must be protected by a separate fuse as marked (appliance fuse for max. 1.5 x rated current) Safety shutdown It must be possible to switch off electrical apparatus immediately (safety shutdown) from a location outside the hazardous area if continued operation of the apparatus in the event of faults would increase the hazard for example by the spread of fires. This safety shutdown facility need not be provided in Zone 2. Apparatus that must continue to operate in the event of malfunctions in order to prevent a spread of the hazard should not be included in the shutdown circuit; instead it should be included an independently switchable circuit Connection of protection and monitoring devices Protection and monitoring devices such as over current trips, safety temperature, safety temperature limiters and pressure switches must, on tripping, disconnect the relevant part of the installation in all phases and may not automatically reconnect it. On switching it on again or unlocking, the operability of the protective device must be checked. Disconnection must not lead to lead to increased risk. Installations must be designed to switch to a safe condition when shut down Equipotential bonding Equipotential bonding is essential within explosion-hazard zones where protective measures are employed with an earth continuity conductor for protection against indirect contact. Conducting structural components such as supports, pipes and containers must be connected together and to the earth continuity conductor. 11. Maintenance 11.1 Disconnection Before any enclosure is opened in an explosion-hazard zone, the relevant part of the installation must be switched to de-energized state. A suitable auxiliary device must be provided to ensure that accidental or unintended energizing of the circuit is not possible. Normally the consent of the operator is needed for maintenance work on installations of this nature Safety precautions On switching off for the purpose of carrying out maintenance work, it is essential to ensure that unintentional switching on is rendered absolutely impossible. The recommended method is to provide safety switches which can be locked by the maintenance personnel with pad locks Ex measuring instruments When carrying out measurements in areas with potentially explosive atmospheres, it is important to ensure that the measuring instruments are explosion proof. For the use of normal measurement instruments, a <<work permit>> must be obtained from the operator. Special measurements, for example those involving high-voltage instruments and insulation testers, may, on connection and disconnection of the measuring voltage, give rise to sparks that possess sufficient energy to ignite explosive mixtures. The same conditions apply to electronic calculators as to measuring instruments, if they are fitted with large enough batteries. In certain cases electronic calculators are permitted for Zone Maintenance and inspection work 17