Thoughts on the design of intrinsically safe junction boxes

Junction boxes are ‘simple apparatus’ when used in intrinsically safe circuits, and given a little thought they can make life simpler for the installation and maintenance engineer.

The first rule is where at all possible only include intrinsically safe circuits in the junction box. It is possible to design a box for use with both intrinsically safe and other types of circuit but there are considerable complications and questions such as segregation, maintenance procedures and certification have to be addressed. [a subject for a future blog?]. A positive effect of the introduction of the ‘ic’ concept is that circuits, which were previously considered to be ‘nL’, can now be included in the same cable and junction boxes as other intrinsically safe circuits.

The choice of type of box is relatively simple. The way to avoid any discussion as to its adequacy from an ‘explosion- proof’ viewpoint is to use an enclosure which is already component certified ‘Exe’. The fundamental question is however is it suitable for its intended environment? In almost all circumstances the choice of a stainless steel box solves the problem and is not too painful from an economic viewpoint. Plastic enclosures are acceptable but in this case the ‘Exe’ component route is almost unavoidable to satisfy the anti-static and other requirements.

The ingress protection [IP] rating of the enclosure is theoretically determined by the application but the majority of available enclosures claim IP 65 and this has become the norm. In some circumstances additional protection is necessary. For example where a junction box is subject to intense solar radiation some form of shade is desirable or the temperature inside the enclosure can become too high for the wire insulation. All enclosures breath when subjected to variations in ambient temperature and where the air is humid condensation within the enclosure occurs. Consideration to fitting a suitable breather/drain plug should be given and if there is any doubt one should be fitted. The other situation where a breather is desirable is if there is any possibility of process gas feeding back along the interconnecting spur cable. Ensuring that the inside of the enclosure is at atmospheric pressure ensures that the leaking gas does not continue down the multicore. The enclosure needs to be clearly identified so provision for a plant identity label is essential and a warning label such as ‘Intrinsically safe circuits – Take care’ desirable. There is a tendency to paint all things intrinsically safe blue but this is invariably badly done and should be avoided, particularly if the box is plastic or stainless steel.

The choice of size of enclosure is determined by a number of factors. Obviously it has to be big enough to accommodate the number of terminals required. A separate terminal so that each screen can be properly terminated is very desirable. Less obviously and more difficult to determine is that there has to be space to bend the cables to line up with the terminals without putting stress on the connections. It is also desirable to be able to read the cable marking without having to lever apart the other cables. The gland plate on the box has to be big enough to accommodate the large number of glands required and if fitted a drain plug. Some designs require special spanners to tighten the glands. This is preferably avoided since these spanners are never available when they are needed. For example a sixteen pair multicore with individually screened pairs requires 48 terminal blocks, 1 large and 16 other glands and one drain plug and the enclosure needs to be twice as big as was first anticipated. The remaining golden rule is if some of the cores in the multicore are spare then fit all the terminals so that all the cores can be properly terminated. Also drill all the holes required for the maximum number of glands and close off the unused holes with an appropriate stopper plug. Without these precautions using the spare cores at a later date becomes extremely difficult.

It is common practice to use terminals of the type used in low powered ‘Exe’ circuits [sometimes blue] because these satisfy the requirements of segregation between circuits and clearance to ground. Their construction ensures a degree of operational reliability, which is desirable, and there is usually an adequate provision for marking. Terminals carrying intrinsically safe levels of current are allocated a temperature classification of T6 [850C from 400C ambient] and consequently the temperature classification of intrinsically safe junction boxes is never a problem.

The choice of glands is determined by the cables used and the requirement to maintain the integrity of the enclosure. In practice the use of a large number of glands frequently lowers the enclosure integrity from IP 65 to IP54. The use of anything other than round cables should be avoided

The siting of junction boxes is a long neglected art. They should be mounted in an easily accessible position at a reasonable height and in a not too exposed location since junction boxes are frequently the most convenient point at which to do maintenance checks. Avoiding situations where it is necessary to erect scaffolding or hang down from elevated walkways to gain access is desirable. Usually cables are supported on cable trays and the cables are supported where they leave the cable tray so as to limit the stress on the gland. The junction box should be positioned so that the cables can leave the tray in a smooth curve and sharp bends avoided.

Conclusion

Use a large [twice as big as you first thought] stainless steel IP65 enclosure with a sufficiency of ‘Exe’ terminals and glands, position it carefully and all will be well.

Field wiring- Segregation of IS cables and other cables

Recently there has been concern expressed about the possibility of inducing unacceptable levels of energy in intrinsically safe cables from power cables and it seems opportune to review current practice in light of these concerns.

The UK view [and possibly the international view] on this subject was determined many years ago and was largely based on research done by ERA. There is a paper; “Project 3051 Induction in intrinsically safe circuits” dated October 1974, which is still probably the best reference on the subject. The paper does not indicate the authors but from memory was probably a combination of H Riddlestone and A Bartels so has a high level of credibility. Basically the paper argues that the principal problem is magnetic coupling from low frequency [50Hz] high current power supplies. It is argued that higher frequencies use lower currents and require higher values to produce ignition and hence are not the primary risk.

In practice the need to avoid interference problems decides the layout of instrumentation cables in general and IS cables in particular. The majority of instrument systems are adversely affected by interference at levels well below the energy levels necessary to cause ignition.

It is normally considered good practice to run instrument cables in separate cable trays from power cables. From an IS viewpoint this has the advantage of making inspection and maintenance a much better defined task. If IS and non-IS instrument cables are in the same tray they are usually segregated by securing them to opposite sides of the tray or using dividing plates. [ IS cables are frequently identified by using a bright blue outer sheath. The code of practice states ‘light’ blue but the colour used would not be recognised as light blue by anyone from Cambridge University].

The majority of instrument cables have an outer screen and use twisted pairs, which has the dual effect of reducing emissions and preventing interference from entering the system. The code of practice [IEC 60079-14] suggests that if either the IS or non-IS cable is screened or armoured then no segregation is required. In practice the IS cable is normally screened and the power cable is normally armoured and hence there are two mechanical barriers between two circuits and hence interconnection is very improbable. If a power cable is carrying both supply and return balanced currents then the magnetic field generated is not large and this is further reduced if the cable is armoured. There is no requirement to use twisted pairs in the instrument cable but this is beneficial and practicable hence is recommended. The toughened outer sheath frequently used for Exe power circuits is an adequate mechanical barrier but is not included in the acceptable single layer protections, presumably because it has no electromagnetic shielding effect

From an IS viewpoint there is no requirement for segregation from power cables of screened IS cables but for interference reduction reasons avoiding long parallel runs and some segregation is desirable. The advice on segregation distance varies but even small distances [5cm] are beneficial and a general guide of greater than 50cm has a number of advocates.

To summarise, if a belt and braces approach is feasible, use a separate cable tray for instrument cables, and use cables with an outer screen and twisted pairs. The power cables should be armoured and fixed to a separate cable tray at least 50cm away. It is unlikely that this ideal situation can be maintained throughout the cable runs but they can be safely brought together for short distances. Suitably protected power and IS cables can be safely run in close proximity if this is unavoidable but this is not desirable for interference avoidance and clarity of identification reasons.

An Afterthought. The code of practice warns against exposure to intense magnetic fields and cites proximity to overhead lines as an example. The existence of strong magnetic fields in a hazardous area [hopefully only a Zone 2] would presumably cause significant currents to flow in any conducting object for example handrails. Presumably the risk analysis of the plant would ensure that any IS cables were protected from such fields possibly by enclosed metallic trays. Making sure the trays are non-incendive is an interesting problem. It is interesting to speculate what faults in an overhead line have to be taken into account when doing the risk analysis.

Intrinsically safe multicores

It is an opportune time to review the use of multicores containing more than one IS circuit because of the introduction of the ‘ic’ concept. [For the uninitiated ‘ic’ is intrinsic safety without faults intended for Zone2 use and will replace the ‘nL' concept of the type ‘n’ standard in time] A further factor is that the subject has now been introduced into the draft documents of the system standard [IEC 60079-25] whereas previously guidance was confined to the code of practice [IEC 60079-14]. The reason for this is that the information is required for Group I [mining] applications and IEC 60079-14 is only applicable to surface industry. The alignment of the two documents is being carefully monitored, in the vain hope of avoiding conflicting requirements.

As an aside the CDV [voting document] of the next edition of the system standard should emerge in the near future. If you are involved in IS installations you should obtain a copy from your local standards body and make your opinions known since this is the last opportunity to influence the content of the document.

 

There are a number of basic requirements for IS cables. At first this seems strange since the open circuit and short circuit of field wiring is taken into account in the design of an IS system. The requirements are intended to reduce the probability of multiple faults such as random earth faults to an acceptable level. It is necessary to establish the inductive and capacitive parameters of a cable and this is difficult unless the cable is of a known conventional construction. There is a need for all cables to be suitable for their intended environment. Resistance to chemicals and extremes of temperature is a frequent requirement. In practice the requirements have not been challenged because they align with the requirements for a reasonably operationally secure installation. The origins of many of the requirements in the IS standards are a pragmatic mixture of safety and operational needs.

 

The basic requirements of an IS cable is that the insulation shall withstand a 500V insulation test and that the minimum strand diameter should be 0,1 mm. The latter requirement is concerned with all the current flowing through a single strand causing hot wire ignition. There is little or no justification for this requirement but it has been there since 1960 and is impossible to dislodge

 

Multicore cables have additional requirements of a minimum insulation thickness of 0,2 mm and a modified insulation test. The sensible approach is to use the two types of multicore, which are considered to prevent the interconnection of the different, IS circuits within the multicore. The most easily defended multicore is a Type A cable in which each IS circuit is contained within an individual screen. There are then two screens between the circuits and the probability of an interconnecting fault penetrating both screens is considered to be acceptably low. Where the cable can be well supported and protected against mechanical damage a cable without interposing screens, known as a Type B cable can be used. The conventional multicore between the IS interface and the distribution junction box protected by a cable tray is an example of this type of installation. There is no requirement to protect against the rampant fork lift truck so beloved by standards writers, a modicum of common sense on the level of protection should be applied.

It is permitted to use a multicore, known as a Type C cable, without screens or mechanical protection but it is then required to consider faults between the IS circuits [up to four open circuits and two short circuits]. This analysis is difficult if all the circuits are resistive limited, and if one or more of the circuits is non-linear then the analysis is even more complex. All the circuits within this type of cable take the gas classification and the level protection of the least well protected circuit. For example if one of the circuits is ‘ic’ then all the circuits become ‘ic’. This type of cable is permitted but is best avoided. There are occasions when several IS circuits have to be taken via the same route and continuously flexed, such as the monitoring circuits on a robot arm. In these circumstances using separate cables within an hydraulic tube rather than a multicore has much to commend it.

It is necessary to establish the cable parameters of the multicore. Where Type A or B cables are used and the cores used for a particular circuit can be readily identified then the parameters of that combination can be identified and used to determine the safety of the system. It is therefore desirable to choose a cable with readily identifiable combinations of cores [fortunately this is usually possible] If this is not possible then the parameters of the worst possible combinations of cores must be used. It is usually cable capacitance, which becomes a problem. If a Type C cable is used then the worst possible combination must be used, which is another good reason for not using this type of cable.

Conclusion

Whenever possible avoid the use of a Type C cable. It is permitted but the safety analysis is quite complex and the probability of fully complying with all the requirements low. A well-supported Type B cable is usually the most economic solution. There is a small question of cross-talk between signals if high frequency signals are used. [This is a practical problem, not a safety problem] The use of separately screened circuits as required by Type A cables is technically commendable but may not be practicable for availability and economic reasons.

The use of mechanical tools in hazardous areas

Purpose of note
From time to time concern is expressed about the possible risk of frictional sparking caused by the use of steel tools in hazardous areas. This note attempts to put this risk in perspective and make a positive proposal on acceptable practice.

Background
It is recognised that frictional sparking between certain materials can ignite a flammable gas with only a very small amount of mechanical energy. The classic example is the early cigarette lighter, which with a flick of the thumb ignited petroleum vapour by rotating a serrated steel wheel against a flint. The materials which cause a lot of concern are light alloys because heating small particles of these materials causes a thermal reaction which produces a higher than expected temperature. The use of such alloys in electrical equipment for use in hazardous areas is discouraged in the General Requirements [IEC 60079-0] which imposes limits on the use of aluminium, magnesium, titanium and zirconium in Zone 0 and 1. Interestingly there are no restrictions on the use of these alloys for Zone 2 applications except for fans and ventilating screens, presumably it is accepted that the risk of frictional sparking causing ignition is low enough for it to be ignored.

Most of the available technical evidence supports the view that ignition by small hot particles is related to spark ignition energy and not to ignition by hot surfaces. That is the IIC gases [hydrogen, acetylene and carbon disulfide] are easier to ignite than IIB and IIA gases. The minimum level of impact required to produce ignition is difficult to establish but a glancing blow is thought to be most incendive. A recent analysis of the available literature plus some experimental evidence carried out by a German Test House suggests that a steel upon steel impact has to be greater than 3Nm for ignition to occur. At 10Nm there is a 10% risk of igniting a IIC gas and a very low probability of igniting a IIB gas. [3Nm is created by a 1Kgm weight falling through 30cm]. These are significant levels of impact and are not likely to occur unintentionally in the maintenance and inspection of instrument systems.

In the conditions where maintenance is normally performed hydrogen is unlikely to achieve the ideal concentration for ignition [20% by volume in air]. Hydrogen has very low density and low viscosity and hence disperses rapidly on release in a non-enclosed location. It has to achieve a concentration in excess of 10% before its ignition energy is less than that of ethylene, hence in the majority of Zone 1 locations where servicing of instrumentation is carried out, it becomes effectively a IIB gas.

The use of tools
The use of those tools normally used in servicing instruments is not likely to produce an impact which will could create a frictional spark capable of igniting any gas/air mixture. The weight of such tools as screwdrivers and keys to socket headed screws [Allen keys] are such that they would have to achieve a significant velocity before producing an impact of 3Nm. The use of a correctly chosen socket spanner is a safer practice than the use of non-steel alternatives which are frequently unsatisfactory because they are not sufficiently robust. It is advisable to avoid the use of rusty tools, but this is normal practise. This manually operated type of tool is normally considered capable of causing a single spark. Activities, which cause multiple sparks such as grinding and sawing, do require special consideration, since they pose a greater risk.

There are obvious areas of risk, normally outside the activities of the majority of instrument technicians such as the use of a hammer and cold chisel to remove rust from an enclosure or the dismantling of aluminium scaffolding , both of which require some thought.

Guidance within IEC standards on the use of tools is not available. Several petroleum industry codes do give guidance and they all suggest that there is no significant risk. A CENELEC standard EN 1127-1 has an informative Annex A which takes an extremely cautious and impracticable view on the subject. It proposes that the use of steel tools should not be permitted in a Zone 1 without a gas clearance certificate where the gas producing the risk is a IIC gas. Possibly this prohibition can be justified if the tools involved are heavy and of the kind which might be necessary for work on major pieces of mechanical equipment. A blanket approach, which includes the tools normally, used by an instrument engineer is not justifiable.

EN 1127-1 adds a number of IIB gases to the prohibition. These are hydrogen sulfide, ethylene oxide and carbon monoxide. Why these particular gases are singled out is not apparent and it would be interesting to find the basis of this selection.

Summary
The use of steel tools of the type normally used to maintain instruments does not cause an unacceptable risk of ignition due to frictional sparking in Zones1 and 2 whatever the gas/air mixture creating the possible hazard.

There is a need for anyone working in hazardous areas to be aware of the risk associated with frictional sparking, but the careful use of steel tools is acceptable

Footnote
In anticipation of a possible comment, I realise that gases are not classified and that it is equipment, which is classified for use with certain gases. However if this note was written using phrases such as a ‘gas which requires the use of IIC equipment’ it becomes even less comprehensible. This justifies the use of ‘IIC gases’ even though it is not pedantically correct.

The future role of the humble paper clip in Hazardous Areas

Apart from their traditional use of holding the multiple sheets of paper that one is required to have with them when performing inspections on equipment in hazardous areas, what are the other uses for the humble paper clip these days ?

The first one that came to mind was a piece of test equipment for IS switch circuits. Chris Towle has always suggested the use of a straightened out paper clip to fault find cabling or other faults on switches connected to IS interfaces. Acting as a simple shunt and being classed as Simple Apparatus, it can be used at convenient points along the loop starting at the Hazardous Area terminals of the interface to see at what point the problem occurs.
A second use came to an end this week with the release of the OSHA Directive on Dusts (link). Previously NFPA654 (Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids ) has used 1/16″ or the thickness of our favourite tool, as being the sufficent depth of dust required on a piece of equipment for it to be considered to be posing a significant risk of flame spread or secondary explosions. The recent OSHA document has now reduced this to be 1/32″ based on recent tests. This is of course a rule of thumb and the actual thickness will depend on other factors but it has reduced the applications for our paper clip. We therefore need you to come up with other uses, suggestions please via the comments section.

Steve

PS The good news is that the OSHA Directive does suggest the use of Intrinsic Safety as the protection technique to be used for instrumentation in areas where there are dust hazards so not everything is that bad. Perhaps this will finally increase the use of the technique in North America

Earthing –bonding and all that

The perennial problem of earthing and bonding is raised from time to time. The basic principles that are applicable to all bonding problems is that it is potential difference that causes problems and explosions and that all currents should be provided with a well defined low impedance return path. Within Europe there is a strong movement to train cows to stand on one leg during thunderstorms so that they avoid the potential difference which could ignite the methane in their stomachs. These learned ‘certified ‘ beasts will be branded with the appropriate ATEX marking.

A recent situation where these principles were applied was to consider the bonding of an IS installation on an off-shore rig which was subject to lightning induced surges, the primary concern being the bonding of the screen. The installation, which emerged, is illustrated in this diagram

 

NOTE. The numbers shown on the diagram and used throughout the text are indicative of the order of values commonly encountered. They are intended to assist in understanding the subject and are not intended to be precise or applicable in all circumstances.

Almost all offshore installations are electrically well-bonded structures, which normally carry the relatively small [500 mA] leakage currents associated with heavy current equipment. There are tales of significant voltages appearing between deck plates but these are outside the author’s experience. Significant circulating currents have been experienced in the immediate vicinity of the electrical generators on some less well constructed installations but these are very localised and do not affect the major part of the installation.

 

In normal operation the circuit when connected as illustrated follows the usual rules for intrinsically safe circuits with the screen being bonded at one point only. In this case, multiple bonding of the screen would not cause an incendive current to flow in the screen because the voltage between the bonding points A and B would not be high [50 mV]. It remains good practice to bond the screen at one point in order to avoid low frequency interference being induced in the measuring circuit. In this type of circuit the measuring circuit, the screen and the structure are maintained at or about the same potential by SPD 2 and its bond to point B. The measuring circuit within the field device is usually isolated from the enclosure, [except for RF interference capacitors which are normally negligible from an IS viewpoint], however the SPD is usually bonded to the enclosure and its bond is common with that of the enclosure.

 

When a lightning strike occurs then a significant current flows in the rig structure, and this current has a fast rise time. [100 kA/ 10ms]. The return path for this current is either the discharge of a locally accumulated induced charge between cloud and sea or part of a circulating current through the earth and troposphere or a combination of both. The usual simplification is that it disappears into the sea or seabed and the rig makes a good connection for it to disappear through. The current sub-divides in the structure and the lower current [10kA] generates a voltage difference in the structure, which is almost entirely due to the inductance of the structure [0,1mH/m] and the rise time of the current [10ms]. Using the suggested figures and assuming a distance of a 100m between A and B gives a voltage difference of 10kV between the two points. The SPDs prevent this voltage difference damaging the measuring equipment but allow a further subdivided current to flow through the interconnecting cable [100A]. It follows that during the short period of this transient current [100ms] the cable is not intrinsically safe, but this is considered an acceptable risk in Zones 1 and 2. An advantage of a well-bonded structure is that the SPD bonds can be kept short [1m] and straight thus minimising their inductance [1mH] which results in a small transient voltage drop [10v] which can largely be ignored. The bonds need to be mechanically robust and secure. The forces generated by these transient currents are not negligible. In practice any transient voltage difference between the field cable and the computer is absorbed by the isolation in the interface.

In normal operation the screen is bonded to the structure and provides an effective electrostatic screen.When the screen is connected to the cable via SPD2 as illustrated, it tracks the potential of that end of the cable during the transient. Consequently there is a potential difference between the screen and the cable at the field end. This is not quite the full voltage difference between points A and B [10 Kv] because the transient voltage is attenuated by the distributed capacitive and inductive of the screen and cable but it is still significant.[5kV] The cable insulation will usually easily withstand this transient but it is important that flashover does not occur at the field termination. It is desirable therefore that the screen should be carefully cut back and insulated and preferably secured in an unused terminal so as to minimise this probability.

The system illustrated describes an effective bonding technique, which is adequately safe in normal and abnormal conditions. A practical installation will almost certainly differ in detail because of the difficulty of ensuring adequate bonding connections. However providing that the basic principle of minimising potential difference is considered then an adequately safe installation should be created.

A non-progress report

My apologies for the long interval between entries on this blog, but my contemplating time has been taken up preparing papers for a seminar in Australia, which I rashly promised to provide. If you are involved in Intrinsic Safety now is a good time to get involved in rewriting the standards, which you [or is it possibly just me ?] are always complaining about.

The IEC 31G committee has drafts for comment [CDs] on both the apparatus standard IEC 60079-11 and the system standard IEC 60079-25 in progress. These should be available from your national committee [usually contactable via a trade association] and it may be in your interest to make your views known. Comments made at this stage are the most effective because it is easier to make changes and introduce new concepts.. Later the document acquires its own ‘momentum’ and changes are less acceptable and tend to be deferred until the next edition. This introduces a five to ten year delay, which is not ideal but almost inevitable. A system of maintenance teams was introduced to speed up the process but the interval between editions is still about five years.

Any comments will be discussed in Kuala Lumpur in November this year. There is also a voting document [CDV] on the Fieldbus standard IEC 60079-27 but while this is passed the point of fundamental changes, it is worthwhile reading if you are involved

There is some thought being given to the subject of the risk presented by combined gas and dust hazards. There are a number of questions were the required technical knowledge is not available and consequently anyone who can make a contribution in this area would be welcomed with open arms (Contact your local committee)

The whole process of creating IEC standards is far from ideal, but is the only system in existence and the known alternatives are no better or even worse. Inevitably the majority of the delegates to IEC committees have one or more of the following characteristics. They are passed their sell by dates, have particular interests to defend, have large egos and epitomise their national stereotypes. The selection process is not ideal. What is needed is some new blood with new ideas to counteract the existing inertia and combine with the more experienced participants to produce better standards. The negative side of participation is that it takes time and attending meetings is expensive. The positive side is that some of the participants are worth knowing and are good company, plus the meetings tend to be in interesting places !

ATEX instructions- a paper chase

There exists an ATEX working group/ standing committee, which promulgates interpretations of the ATEX apparatus directive. Quite what its powers are and how binding its decisions are is not clear but if we assume they are significant, then its latest decision is a nuisance. At a recent meeting [November 06] under “Any Other Business”, without a lot of discussion and certainly without any prior consultation it was decided that the only permissible delivery method for instructions was on paper. The Directive does not specify any format but does require instructions to ‘accompany equipment’ so the interpretation is ‘intuitive’ or divine.

The problem of getting instructions to the person who needs them is well known to all manufacturers. In practice different people need the instructions at various stages in the life of the equipment and it is virtually impossible to ensure that the instructions are in the right place at the right time. Many manufacturers welcomed the possibility of using the Internet to ease this problem. By making instructions available on the Internet it becomes possible for anyone with access to the Net to obtain an up to date set of instructions in a relatively short time from a well defined source. The instructions can be used and then thrown away since they can be reproduced with certainty at any time. The avoidance of using out of date information is a requirement of most quality control systems.

The other advantage of having the information on the net is that instructions are frequently changed in response to new applications or to clarify their intention. One of the arguments for not including detailed instructions as certified documents is that it inhibits change and consequently changes determined by experience of use [which may be safety related] do not get included.

The decision is all the more puzzling in that the IEC Ex scheme posts all its certificates on the web so the certification state of any equipment can be easily checked. This has proved successful, is widely used and sets a worthwhile example.

The committee appears to have decided that out of date untraceable paper is the preferred solution and rejected a potentially safer technique. The- likely outcome is that manufacturers will still produce the paper so as to satisfy this requirement, confident that it will get lost anyway and that the end user will then use their website to find the necessary information.

The end result will be safe and only a few trees will be wasted. It could be worse the committee could have decided that the instructions had to be carved on tablets of stone in line with the rest of their pre-historic thinking.

The Three Day Expert

It is generally acknowledged that there is a lack of expertise in defining and installing electrical equipment in hazardous areas. In the last few years the facilities for the training of technicians has improved, in Europe this has been largely due to the EEMUA initiated courses but supplemented by some other organisations. The problem of training engineers to design equipment is not too severe since most equipment is subject to third party certification, and hence there is a possibility that any significantly dangerous errors in the design will be detected. This does presuppose the certification authority is competent but that is a perennial question.

The current need is to adequately train engineers to select and specify equipment for specific applications. The problem is that no one person can know enough to ensure a satisfactorily safe installation and yet current proposals suggest that a few days training can produce a “certified” engineer who can do it all. It is obvious that this is absurd, for example the skills necessary to select and specify an intrinsically safe level gauge for installation in a Zone 20 location are a world apart from those necessary to select and specify a large electric motor for a Zone 2 location. In practice many instrument engineers who work on plants have detailed knowledge of relevant aspects of their particular installation and only a cursory knowledge of other techniques and installations. They are usually in the best position to assess and ensure the safety of their installation, and the difficult part is to sustain a responsible attitude to safety in the face of other pressures. Possibly the more difficult situation is that of the engineer working for a design contractor who may have to work on plants with quite different problems in rapid succession. In these circumstances wide experience and a network of knowledgeable acquaintances is the only hope for creating an adequately safe design.

Area classification is a part of most applications and is an art which is best practised by a team of people with complimentary skills and experience, it  is not predominantly an electrical engineering problem. There are some basic concepts but every application is different and the necessary skills cannot be taught in a few days. The situation becomes even more difficult if the “instant expert” tries to apply the risk analysis approach since this requires the Wisdom of Solomon and the direct help of whichever the expert worships. 

There is a need for training of engineers in the basics of ‘explosion protection’ techniques, and for frequent updating as the techniques develop. The case for testing to establish that a level of understanding has been achieved is less well established. However it must be recognised that training courses are only one starting point and the most that a training course can achieve is to make an engineer aware of the risk and the available solutions. Possibly the greatest benefit to be derived from training courses is the realisation of how much there is to learn and the recognition of personal limitations. In practice most engineers continuously build up experience by working on specific projects and interacting with colleagues. Training courses make only a small contribution.

The available training courses are usually organised by certifying authorities or training organisations. The better ones use engineers [usually semi -retired and not quite passed their sell-by date] who have worked on installations and have a realistic approach to the known problems. Some courses deteriorate to intensive training on how to read completely unintelligible labels and nauseating detail on the interpretation of apparatus design standards. This practice is frequently led by the obligatory multiple choice questions that form the end of course test.

Possibly a website containing basic training material could be set up by one of the engineering institutions and that could draw from the experience of its members. It might even be possible to update the information on the site and encourage discussion on known problems. The cost would be considerable and it would need a driving personality behind it, and so it is unlikely to happen. 

The fundamental need is for engineers to behave as professionals, acquire the knowledge they need, accept responsibility for what they do, and to recognise when they need to consult with colleagues. The possibility of anyone being ‘certified’ as competent in all aspects of “safety in explosive atmospheres” is too remote to be contemplated.

Ex d – a non solution to a hot problem

Background

Very occasionally someone notices that the IEC apparatus standards usually only consider ambient temperature conditions of  -20 °C to + 40 °C. This range can be extended when this is considered desirable and noted on the certificate and by marking. The usual limits are –40 °C and + 75 °C. Some concern has recently been raised as to whether measuring temperature outside this range using thermocouples [TCs] and resistance thermometers [RTDs] is adequately covered by the certification and/or is adequately safe.

Note: An interesting aside is that both the ATEX Directives apply only to gas/air mixtures ‘under atmospheric conditions’ so the Directives are not applicable and do not consider this problem

Some thermocouple installations are located at the interface between Zone 0 and Zone 1. In these circumstances the inside of the thermowell is usually a Zone 1 and the outside a Zone 0. Hence from a thermal ignition risk viewpoint it must be suitable for Zone 0

The intrinsically safe solution

It is known that ignition energy of a gas/air mixture decreases with temperature, although the available experimental data is limited. At the ignition temperature it is zero, and the way it changes between atmospheric temperature and that point is not well defined [a translation of that statement is that the author of this note does not know the answer since the limited information is partially contradictory]. Within the normal testing range for apparatus [up to +75 °C] it is usual to assume that the change is small and adequately accommodated by the large safety factors inherent in the intrinsic safety technique.

There are obvious thermal ignition problems where the process temperature approaches the ignition temperature of the gas and consequently the temperature rise of the measuring element under fault conditions must be restricted to as low a level as practicable.

Usually the TC or RTD is regarded as simple apparatus and it is the output parameters of the transmitter or IS interface, which determines the acceptability of the measurement. Fortunately most of the recently designed temperature converters, temperature alarms &c have very low safety output parameters, which make them useable with TCs and RTDs at higher temperatures. For example the MTL5074 temperature converter in TC mode has output parameters of  Uo: 6.6V, Io: 76mA and Po:17mW. The voltage is well below the ignition threshold of any gas [10 -12V], and the current is too low to create an inductance problem. The limited power means that the possible rise in temperature of the TC under fault conditions is small.

It seems reasonable therefore that provided that the output parameters of the connected instrumentation are small then the measurement of temperatures approaching the ignition temperature of the gas/air mixture is acceptably safe. The output parameters of the connected device are in accordance with the certificate; hence it can be argued that the installation is partially ‘certified’. It is adequately safe to the normally accepted level and no problem exists.

The flameproof  [Ex d] non-solution

The performance of flameproof enclosures at temperatures other than those for which they are tested is not well documented. The emphasis on the need for testing in the Ex d standards suggests that the number of interacting factors make the prediction of performance difficult. Most of the recently expressed concern relates to low temperatures and hence possibly high temperature effects are not so critical if metal enclosures are used. In the particular case of TCs and RTDs then the relatively small heat capacity [compared with a large metal box] and the possible intimate thermal contact with the process fluid calls into question the adequacy of the flameproof practice of allocating a temperature classification without considering faults.

If this argument is accepted then the use of flameproof thermocouple wells and heads is only marginally acceptable at atmospheric temperatures and the possibility of an unspecified temperature rise under fault conditions makes them unsuitable for use at elevated temperatures. The questionable performance of flameproof enclosures at temperatures lower than that for which they have been tested makes their use in these circumstances not acceptable.

Some thermocouple installations are located at the interface between Zone 0 and Zone 1. In these circumstances the inside of the thermowell is usually a Zone 1 and the outside a Zone 0. Hence from a thermal ignition risk viewpoint the installation must be suitable for Zone 0, which means that a flameproof installation is not acceptable.

The inevitable conclusion is that the majority of flameproof TC or RTD installations do not achieve the level of safety implicit in the IEC standards. Fortunately they do not have to comply with the ATEX Directives but they are probably not adequately safe.  However there are no explosion incidents attributable to the use of this type of installation known to the author so perhaps there is no call for undue panic.

Conclusion

The only acceptable technique for the measurement of temperature in using TCs or RTDs in Zone 0 and Zone 1 hazardous areas is to use intrinsically safe equipment where the interconnected interface has output parameters which have a considerable safety factor over those permitted for gas/air mixtures at atmospheric temperatures. If this is done then the safety level achieved is not well defined, but it is covered by the well-known practice of ‘making it as safe as is practically possible’. However it is unlikely to be covered by the certification.