Archive for the ‘IEC60079’ category

Cable parameters and all that

June 12, 2008

The intrinsically safe [IS] system standard [IEC60079-25] discusses in detail how to draw up the system documentation and in particular how to calculate the permitted cable parameters. Unfortunately the standard has to take into account all the possible variations and hence the process looks quite complicated. In the majority of applications the simple precaution of using a single source of power with no significant capacitance or inductance across its output terminals [Ci &Li] and field devices with only small input capacitance and inductance [Ci & Li] removes any problems. The permitted cable parameters [Cc,Lc & Lc/Rc] are then the specified output parameters [Co,Lo & Lo/Ro] of the source of power and there is no difficulty. The majority of certified apparatus has low values of Ci and Li and hence in simple systems this simplification is applicable. For this purpose negligible values of Ci and Li are 5nF and 10mH respectively.

The experimental work done by PTB to support the FISCO standard suggests that adding conventional cable to an IS system makes it safer. It would require extensive further testing [possibly reduced by the use of the PTB spark test simulator] to extend the principle established in the FISCO standard to include all other forms of IS system. Unfortunately there is no finance available to do this work. If anyone can raise the funds, I am sure a means of getting the work done could be found. Until this optimum solution is proven and becomes part of the standard it remains necessary to create system documentation which defines the permitted cable parameters.

Frequently installations do not have a problem because the permitted values are higher than can be created by the cables used. The most common limitation is imposed by the permitted capacitance of the higher voltage circuits. The usual limitation is that the permitted capacitance for 28V circuits in IIC for ‘ia’ and ‘ib’ circuits is 83 nF. The IEC code of practice [IEC 60079-14] suggests practical maximum cable parameters of 200pf/m, 1mH/m and 30 mH/W, which suggests a limit on cable length of 400m. A 400m cable can be considered as having an inductance of 400mH which corresponds to a current of approximately 300mA which is not common in IS circuits. Where high currents do occur then the permitted L/R ratio is usually in excess of the 30 mH/W limit and there is no problem If the cable lengths on an installation are less than 400m and the systems comply with the above restrictions then quite a lot of repetitive documentation can be avoided by including in the safety documentation a statement such as ‘The installation does not use cables longer than 400m and the permitted cable parameters are not greater than 200pF/m, 1mH/m and 30 mH/W and consequently no further consideration of cable parameters is recorded.’

Similarly if an ‘ia’ or ‘ib’ system is used in a Zone 2 location then it can usually become an ‘ic’ system and the permitted inductance increases by a factor of 2.25. The permitted capacitance increases by a variable factor which reduces as voltage increases but at 28V the permitted capacitance becomes 272nF and the cable length problem is effectively removed. A general statement can be utilised, such as ‘Where the installation uses ‘ia’ or ‘ib’ systems and the locations only require ‘ic’ systems the need to consider cable parameters is for all practical purposes removed and consequently no further consideration of cable parameters is recorded.’

An even more obvious case for not considering cable parameters is when a IIC system is used where a IIB gas classification is applicable. The permitted inductance is multiplied by 4 and the capacitance by a large variable factor. At 28V the capacitance moves from 83 to 650nF. The argument for not being concerned about cable parameters becomes very powerful

Conclusion

If possible avoid using sources of power or IS apparatus with significant values of Ci and Li. If you are using cable lengths less than 400m there is no real problem. If you are using ‘ia’ or ‘ib’ systems where an ‘ic’ system is acceptable then there is no significant problem. If you are using a IIC system where a IIB system is acceptable then there is no problem.

If in any analysis of an intrinsically safe system the solution shows that there is a significant cable parameter problem then recheck the calculation. Real problems do occasionally occur but there are always very unusual factors.

Finally if you think you have made a mistake sleep easy because the probability is that the cable made it safer anyway

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Thoughts on the design of intrinsically safe junction boxes

April 4, 2008

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

January 31, 2008

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

January 22, 2008

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

December 16, 2007

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.

A non-progress report

June 28, 2007

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 !

A postcard from Rio de Janeiro

October 26, 2006

Apologies for the lack of recent additions to this blog, but an IEC meeting in Rio temporarily stemmed all rational thoughts on real hazardous area instrumentation issues. One of the compensations of attending IEC meetings is that sometimes they are held in interesting places such as Rio. It does provide an opportunity to meet old friends and adversaries, often combined in the same person.

The meetings of the three maintenance teams, which determine the content of the draft standards, dominated the intrinsic safety meetings. Sometimes I wonder how representative maintenance teams are of the real world. A few people [including myself] who do the initial drafting and take an active part in resolving the response to any comments dominate them. These few sometimes have personal axes to grind and/or an advantageous position to defend. As the IEC Ex scheme becomes more readily accepted the IEC standards become even more significant and if left to the few, could become too restrictive and impractical. Active participation by all interested parties –users, manufacturers and certifying bodies, is the ideal way to achieve a practical standard with an adequate level of safety.

The present state of the IEC standards on intrinsic safety is:

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If any of these subjects is of interest then please get the documents from your national committee and provide your comments. Complaining afterwards is too late. And of course if your comments are rejected, you can always say ‘I told you so’ as you go bankrupt or your site is closed on economic grounds

The current ‘in thing’ with the cognoscenti is ‘risk analysis’ and equipment protection levels [EPLs]. There is still considerable enthusiasm for this subject for some inexplicable reason. No doubt it will find some applications when an acceptable method of analysis appears, but meanwhile the ancient technique of area classification used with compatible equipment selection will thankfully prevail