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Basic difference between Zone-2 and Zone-1 equipment
Exd/Exe equipment for Zone-2 and Zone-1

Friends, the query is:

1. What is the basic difference between an Exd/Exe equipment for Zone-2 and Zone-1 for the same gas group?
Is there any constructional difference, i.e. gap between two flanges, thickness of the housing etc. or is it only the extent of testing done?

2. Suppose I have an Exd motor suitable for Zone-2 in a Zone-2 area. Now there is gas leak. Do I need to shut down the motor? My understanding is, if I don't shut down, gas may enter into the casing and there may be explosion inside the casing. Though it is not a safety issue, but the equipment will be damaged. So if I want to save the equipment, it is better to shut down. But normally there will be thousands of transmitters on the field and on confirmed gas in the plant we don't shut down the transmitters.

What are the recommendation from international standards? Has this issue been discussed anywhere?

Appreciate if someone can help.

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Here is an explanation of zones that I hope you will find helpful.

Zone-1 contains a hazardous gas/air mixture under normal operating conditions. Zone-2 is hazardous only in abnormal operating conditions, and if so, it will only exist for a short time. I like to use a gas station as an example. At the pump, it is normal for vapors to be present so this is a Zone-1 hazardous area. At the pay station there may be vapors present only during abnormal conditions like after a gas spill. Normally this area is non-hazardous, therefore Zone-2.

There are two main methods of protection. Explosion-proof enclosures to contain the explosion and energy limiting methods (like intrinsically safe and non-incendive circuits) that ensure that if a spark is generated, there is not enough energy in the spark to ignite the atmosphere.

For explosion-proof equipment, I don't believe there is much difference between Zone-1 and Zone-2. Conduit seals are required at all Zone boundaries. The reason for sealing between zone-1 and Zone-2 is not only to stop an explosion from propagating between locations but also to prevent the migration of gas or vapour through the conduit. Regulatory bodies approve equipment for saftey rules only. If it will avoid an external explosion it can be approved. As far as the certification is concerned, it doen't matter if the equipment works or breaks due to a contained explosion in the enclosure. Therefore, you do not have to shut down Zone-2 equipment when the area becomes hazardous, although you may want to to protect the equipment.

For energy-limiting equipment the difference between the zones is much greater. This equipment does not have the heavy explosion-proof enclosures so its circuits are exposed to the atmosphere. Zone-1 equipment must be non-incendive. This means it must not contain enough energy to ignite the atmosphere. This is usually done by running all wiring through intrinsically safe barriers.

Zone-2 equipment does not have to be non-incendive. It can contain lots of power as long it it doesn't normally arc or spark or have any surfaces that get hot enough to ignite the gas.

Using the gas station example, a low-wattage 110Vac light could be installed at the pay station since it doesn't normally create sparks. A light switch, however, could not be approved since it sparks under normal conditions. A hemetically sealed switch could be used. The light bulb example is not ideal since its surface temperature may be too high to allow its use.

Gerry Corraini
Cameron Measurement Systems


Thanks for the explanation. But if there is no difference between Zone-2 Exd and Zone-1 Exd equipment, why Zone-2 equipment can't be used in Zone-1?

Difference between Exe in Zone-1 and Zone-2 is also not clear. Suppose a zone-2 switch is installed at the gas pay station. Now due to any reason there is gas leak at the pay station. So the area is now Zone-1. In this case Zone-2 switch will not create any risk? If not, why?

As I understand... this is my disclaimer... zones are typically not mentioned for flame-proof equipment. This equipment is certified as flame-proof and can be used in Zone-0, 1 or 2.

For Exe installations, a Zone-2 switch is sealed from the atmosphere and will not expose a spark to the hazardous gas. Therefore it would take 2 faults to create a dangerous situation. First, a gas spill or some other abnormal event must make the normally safe atmosphere hazardous. And second, the high energy (incendive) circuits that are normally non-sparking must have some abnormal fault to create a spark or a hot enough surface to ignite the gas. The chance of these two abnormal situations happening at the same time is very remote.

2 faults must exist to create a dangerous situation. This applies to Zone-1 as well. Zone-1 circuitry must be designed with redundant components so multiple comonents must fail before the circuit becomes incendive (since the area is always hazardous). This is why you see multiple protection diodes in I.S. barriers.

I hope this helps,


I have to correct you, Gerry. Ex d equipment CANNOT be used in Zone 0 - it is good for Zone 1 and Zone 2 only. (At least in the IEC world).



Dear Bruce,

I've been reading your responses to questions regarding haz area applications and thought perhaps you could help me out.

I am using field instruments connected through barriers located at safe area. The haz area falls in Zone 2, gr.IIA/IIB. Please let me know whether I can use a Ex ia instrument in place of Exd ia instrument which will be working on 24vdc through barrier.

Also please let me know why a Exd instrument can NOT be used in zone 0. Or a Exd with ia can work in zone 0. I have lot many other questions in mins and if you don`t mind can I have your email id?

Request to reply me on
Thanks in advance.
Sandeep Datar

By Bruce Durdle on 14 January, 2014 - 2:07 pm
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Hi Sandeep,

An Ex d ia classification sounds as if it belongs to equipment with a high-power drive or supply circuit, and a low-power signal circuit. If you give me more details including an equipment model number or similar I can give a more specific answer.

Assuming it is something like the above, then you will not generally be able to supply enough power through a barrier to the high-power circuit. Even if you could, it is not permitted to connect any equipment that is not certified as IS on the field side of the barrier. This is because equipment designed to meet Ex i requirements has very limited capability to store energy in internal inductance and capacitance - a low-voltage solenoid, for example, may have sufficient inductance to develop an incendive spark if the circuit is broken and would not meet these requirements.

You should be able to get detailed installation specifications from the manufacturer's instruction manual, which should be available to you as it is a necessary part of the Flammable Atmospheres dossier for the installation.

The difference between the zones where different methods of protection can be used is generally based on the integrity of the method - what is the likelihood of the protection being made ineffective because of something like inadequate maintenance or an operating error? This is made clearer in the recent changes to the IEC standards referring to Equipment Protection Levels. In Zone 0 or the dust equivalent of Zone 20, the equipment is exposed to the flammable atmosphere most of the time and must remain safe with at least 2 independent faults. In Zone 1, there is a lower probability that the atmosphere will be flammable and it is adequate for the equipment to remain safe with 1 fault, while for Zone 2 it is acceptable for equipment to be safe in normal operation only and not necessarily if it is faulty (it is expected that the fault will be found and corrected before the equipment is exposed to a flammable atmosphere).

Hope this helps,

By Bruce Durdle on 15 January, 2014 - 2:42 pm
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Hi Sandeep,

I've had a look at the installation details given in the operating

and can see why you have a problem! - they are not very clear. I have also checked with the equipment certification from

My interpretation is that this is a simple 2-wire connection not requiring a separate external power supply.

The Ex d ia unit is divided into two sections - components located outside the process vessel, and those exposed to the atmosphere inside the vessel.

The Ex d element is used to provide explosion protection for the terminal chamber, and is rated as safe for the external atmosphere where the sensor is installed. External wiring must meet Ex d rules rather than Ex i. This part of the system has Equipment Protection Level EPL Gb - good for Zone 1.

The Ex ia rating applies to the probe and means that the probe can be inserted into a tank or silo where the atmosphere is Zone 0 (EPL Ga)

With the Ex ia unit, there is no separation between the two components. This unit will need a barrier on the external wiring.

Note that this is my interpretation only - you need to get a definite answer from someone who is familiar with the detailed design of the equipment. If you send this query to the manufacturer, I am sure that you will get a suitable response - most reputable manufacturers want to make sure that their equipment is installed and operated correctly, as any incidents attributed to their equipment will affect their reputations.


Hi Bruce,

I liked your explanation. I glad that you and I have the same understanding regarding the Exia rating for the sensor/elements which actually go inside the vessel. Now, I have 2 queries.

1. I have looked for description of this explanation in various standards but no standard explicitly differentiates between the enclosure and the probe part. They all talk about supply and limiting the energy etc. Do you know any reliable technical document which speaks about this?

2. The E&H FTL51 has this certification, ATEX II 1/2G Ex d IIC T6/IECEx Zone0/1. I would like to now if this is Exd, how can it be used in Zone0?

By Bruce Durdle on 4 December, 2014 - 2:30 pm


The question oh having different certifications for different parts of the same equipment is not specifically set out (as far as I can tell) in IEC60079-0, where I would expect it. However, this standard, does refer to "enclosures" as the basic entity being

"all the walls, doors, covers, cable glands, rods, spindles, shafts etc. which contribute to the type of protection and/or the degree of protection IP of the electrical equipment."

So if you have an item of equipment made up of more than 1 "enclosure" then each "enclosure" can be treated as a separate entity. In the case of a level probe, there is an enclosure for the terminal compartment, and another for the probe (this may be a separate entity completely, with a cable connection, or part of the same physical object but with a physical barrier between it and the terminal compartment).

As far as specifics of any manufacturer's assigned certification goes, you need to ask the manufacturer this. It is common for a manufacturer to assign a wide range of certifications to equipment to satisfy different site standards. The primary document has to be the equipment compliance certificate as this may contain references to specific applications or other details - there is only a limited space on the equipment itself for these details.
Hope this helps,


By Hamid Sarukhani on 28 August, 2007 - 2:00 pm


It seems to me that the prices of the equipment which can be used in Zone 1 and that of those which can be used in Zone 2 are not that different.

Do you have any rough estimation on this? e.g. prices for Zone 1 are almost %x more expensive than for Zone 2.



Regarding your question: "Do you have any rough estimation on... prices for Zone 1 are almost %x more expensive than for Zone 2."

This is not something that can be easily estimated. It depends on the type of equipment. For example, simple apparatus like RTDs can be approved for zone 1 without modification since they do not store or create power. Other more complex equipment like flow computers or handheld communicators can be made I.S. but the design and approval can be costly. The prices are completely up to the manufacturers.

Gerry Corraini
Measurement Systems


Thanks for the reply and sorry for the late follow up of mine!

My question is for any substantial price difference for equipment like Process transmitters (PT, TT, FT, LT, Etc.) and electric valve actuators for MOVs suitable for Zone 2 and not for Zone 1 with those suitable for Zone 1 (which of course suit zone 2 as well).



I am at Goro Nickel Project new Caledonia. Some one has installed Ex nR luminaires in zone 2 areas. They have now realised that there is a statement at the bottom of the CE that after install the conditions of install call for enclosure to be pressurised for 3kPa after a period of 27 seconds the internal pressure shall not be less than 2.7kPa - a lot of lights for this plant for them to go back to - what is your opinion?


By Steve Yates on 18 July, 2007 - 11:36 pm

What equipment you can use in which zone depends on the type of protection used, some techniques are allowed in all zones, some are restricted. Basically it is a case of matching the level of risk of a gas/air mixture existing to the level of safety that the technique used offers so as to arrive at the best solution in terms of cost and ease of use.

In Zone 0 there is a permanent or long term risk so you need the safest possible solution (typically Intrinsically Safe equipment certified Ex ia ). In Zone 2 where the risk is infrequent (and to refer back to the OP) will not last for a significant time), all of the equipment allowable in the other 2 zones is acceptable PLUS Ex n which is only allowed in Zone 2.

Flameproof equipment (Ex d) and Increased Safety equipment are only allowed to be used in Zone 1 or 2 but the design design standard does not differentiate between the 2 zones.

If you care to email me off line I can send you some of our application notes.

syates @ mtl-inst. com

By W.L. Mostia on 18 July, 2007 - 11:57 pm

The primary difference between Zone 1 and Zone 2 areas is that in a Zone 1 area, a flammable/explosive mixture is expected to occur under normal conditions whereas in a Zone 2 area, a flammable/explosive mixture is expected to occur under only under abnormal conditions. In regards to equipment, the difference is generally related to the above availability of a flammable/explosive mixture.

In a Zone 2 area equipment, you cannot have an ignition source under normal operating conditions (i.e. if you have a spark sufficient to cause ignition, then you enclose it in an explosion-proof/flameproof box, make it not available (hermetically sealed, purge, or other method) or you limit the ignition energy in the spark (intrinsic safe, non-incendive). This leads to you needing two abnormal events to have a combination of an ignition source and a flammable/explosive mixture in a Zone 2 area.

In a Zone 1 area equipment, you cannot have a source of ignition under normal operating conditions or abnormal conditions (equipment failure). This means that the whole device must be protected against serving as an ignition source whether it is operating correctly or it has failed in some manner. This requirement precludes some of the Zone 2 methods (non-incendive, sealed, etc.) but allows explosion-proof, purging, and intrinsic safety. Here it is assumed that a flammable/explosive mixture is available and therefore you cannot have an ignition source available under normal or abnormal operating conditions.

In general, equipment certified for Zone 1 is allowed in Zone 2 for the same area (materials) as a protection means without specific Zone 2 certification.

William (Bill) L. Mostia, Jr.
WLM Engineering Co.
This information is provide on a Caveat Emptor basis.

By Jeff Jannsen on 28 August, 2013 - 4:56 pm

Do you know if a cross-reference exists to cross from ATEX Zone to NEC Class and Division?

By René Kerbel on 26 September, 2013 - 3:38 am

> Do you know if a cross-reference exists to cross from ATEX Zone to NEC Class and Division?

See the following link on page 42 & 43:

By Steve Yates on 17 July, 2007 - 1:01 am

I am not aware of any Ex"d" or Ex"e" equipment certified specially for Zone 2 applications, do you have examples you can cite?

As for what happens when there is a gas leak, that will be down to the regulations that apply to the plant you are working on and could be country or company specific. An important part of the definition of the Zone 2 classification is that any explosive gas/air mixture will only exist for a short time so any leak is expected to be dealt with promptly. If it isn't, then you do not have a Zone 2 Area.

MTL Instruments

There is no difference between Ex d or Ex e equipment used in the different Zones. The requirements for equipment to be certified as Ex d or Ex e are set out in various Standards - IEC 60079-1 for Ex d Flameproof equipment, and IEC 60079-7 for Ex e Increased Safety equipment (and documents based on these). These standards make no reference to the zone where the equipment will be installed, and any equipment certified as Ex d or Ex e can be safely used in Zone 1.

The method of protection that is permitted in Zone 2 is Ex n non-sparking which is built to requirements set out in IEC 60079-15. This in fact can use a number of different methods of protection internally to deal with possible sparking components. The difference is not in the detail such as clearances etc but in the integrity - Ex d and Ex e are both safe under specified fault conditions while Ex n equipment is safe only if fault-free.

As you state, an Ex d motor is safe in that it can contain the results of an internal explosion. It is designed and intended to be used in an atmosphere where there are frequent releases of gas. On the other hand, an Ex n motor should not be operated if there is a prolonged presence of gas in the area - especially if it uses the subsidiary "restricted breathing" approach (Ex nR) which relies on reasonably good seals to give a slow exchange of gas between the motor internals and the external atmosphere. The external atmosphere for Zone 2 must have any explosive atmosphere present for only a short time. The question of damage to Ex d enclosures resulting from an internal explosion is a good one. Equipment tests include a number of checks carried out after an internal explosion but these are designed to look for damage that could make the protection void. Damage to equipment is not considered - but anything such as internal shorts would cause excessive heating and invalidate the temperature rating of the equipment.

If you are concerned about these effects, use Ex e (for Zone 1 or Zone 2) or WEx n (for Zone 2). These methods of protection do not allow ignition (hence no need for a gas group rating). But be aware that there have been some recorded instances of Ex n motors exploding on start-up - they were driving pumps using seals that relied on rotation to maintain the seal, and there was an obscure mechanism in the motor end-windings causing sparking on starting.



By Ingrid M. Gilstrap on 19 September, 2007 - 12:52 pm


I've been reading some of your responses to questions regarding haz area applications and thought perhaps you could help me out. My question concerns using a Zone 1 approved device in a Zone 2 area. NEC explicitly states that this is allowed as long as Zone 1 wiring techniques are used in the installation. I assume that means if the device is Eex d (method of Zone 1 protection is flameproof)a conduit installation must use explosion proof fittings and conduit seals, eventhough it is only in Zone 2. But what do European codes allow/require? I cannot find where instructions are explicitely given for the case of using a Zone 1 apparatus in a Zone 2 area.

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A lot of the detail depends on where in the world you are, and what the local jurisdiction has to say about permitted practices. You may even find differences in interpretation between inspectors in the same area - I've had an inspector say that he wouldn't accept something already approved by one of his colleagues in the same company!

My background is in Australia/New Zealand and we are in the process of recognising the IEC 60079/61241 series of standards in place of Australia-only ones.

Conduit can act as a duct to transfer hazardous materials from the zoned area into the heart of your switchboard or marshalling racks, and seals are required at any zone boundary (or at a point in the conduit system where the effect is equivalent). Conduit seals are also required between enclosures to avoid pressure piling where the piston effect of a flame front increases the pressure in an enclosure, resulting in much higher pressures than design when the mixture in the second box ignites.

The current Australia/New Zealand standard also requires barrier glands on cabling systems where the process pressure can be applied to the end of a cable (such as in a pressure switch where the process fluid is separated from the electrical compartment by a single diaphragm). This may appear to be overkill but I have had reliable reports about petrochemical liquids coming out of a field junction box after a switch diaphragm failure. My general approach is that if you are using Ex d equipment you apply Ex d rules regardless of zone. That way, if the zoning changes the installation is acceptable, and there is also no possible confusion for those who have to install or maintain the equipment. It is possible for example to treat an Ex d switch as a "simple device" on an IS circuit, in which case the full catastrophe of explosion proof installation is not warranted, but in this case it needs to be made VERY clear on labels etc that the circuit is not in fact protected by Ex d but is in fact Ex i.

Hope this helps... Bruce

Dear all,

I am reading with great interest the replies given by so many people. Thanks for your time and interest. Some of the thoughts are really interesting. But I think my 1st basic question is still not answered. I am not asking the definition of Zone-1 or Zone-2 hazardous area or the difference between Exd or Exe equipment. More or less we generally understand the subject and most of us have practical experience applying the techniques of Exd, Exe, Exn, IS (ia), IS (ib) etc. in various plants at various parts of the world. Basically I am trying to find out:

1. What is the exact difference between an Exd equipment suitable for zone-2 and an Exd equipment suitable for Zone-1? What are the basic constructional difference? Difference in testing rigorousness? Material?
2. Similarly difference between Exe equipment in Zone-1 and Exe equipment in Zone-2?
3. Surely there are manufacturer who just built the equipment for Zone-2 and use it for Zone-1 or Zone-2. But there are many manufacturers who have equipment suitable for Zone-2 only.
4. The question is equally applicable for Div.1 & Div.2 equipment as per NEC.

I am still searching!

Your basic question is not answerable, as there is no such entity as Ex d equipment designed for Zone 2. Ex d equipment is designed to comply with IEC 60079.1 or one of its derivatives. Equipment so designed is suitable for use in Zone 1. It is more than adequate for use in Zone 2, and can be safely used there. The only protection method suitable for use in Zone 2 only is Ex n. The requirements for this are given in IEC 60079.15. This has a number of subcategories all designed with a lower integrity than Ex d or Ex i. Ex d equipment must remain safe under normal or fault conditions: Ex n equipment is guaranteed safe only under normal conditions and does not have to be safe when carrying electrical faults. "Ex e equipment designed for Zone 2" is a good working definition of the basic Ex n requirements - again, the required integrity is less.


>1. What is the exact difference between
>an Exd equipment suitable for zone-2 and
>an Exd equipment suitable for Zone-1? <

I have never seen Ex d equipment certified for zone 2 only.

>2. Similarly difference between Exe
>equipment in Zone-1 and Exe equipment
>in Zone-2? <

I have never seen Ex e equipment certified for zone 2 only.

>3. Surely there are manufacturer who
>just built the equipment for Zone-2 and
>use it for Zone-1 or Zone-2. <

Manufacturers don't "use" equipment, users do and how they do so is out of the control of the manufacturer. If a user is using zone 2 equipment in zone 1 they are running risks they shouldn't be.


Sorry for my late response. 1st of all thanks a lot for your explanation. I have also gone through the IEC standards and haven't found any relaxation for use in Zone-2. Exd or Exe, once it is manufactured as per the standards should be suitable for Zone-1 & 2. But the user must look carefully that they are certified for the gas group and temperature class of his area classification.

Having said that, I will cite an example of an equipment from a famous manufacturer. Refer to Rosemount's Electromagnetic Flowmeters of 8700 series. Product data sheet (No. 00813-0100-4727, Rev NB, catalogue 2006-2007), page-24 indicates that Rosemount 8742 transmitter is FM approved for "Explosion-proof Class I, Div. 1, Gr. C & D" but "Class I, Div. 2, Gr. A,B, C & D". Div. 2 is equivalent to Zone-2 as per IEC. So for gas Gr. A & B (Gr. IIC as per IEC) this product is certified only for Zone-2. What is the technical explanation for this? In my experience I have seen in the past other manufacturers also giving similar types of products. Even they are very very few, still they exist. I don't know why!

Lastly, I want to mention a typing mistake in my last query. I wanted to say "Surely there are manufacturers who just build the equipment for Zone-1 and use it for Zone-1 or Zone-2". By typing mistake it appeared as, "Surely there are manufacturer who just build the equipment for Zone-2 and...", which changed the meaning completely. Anyway you had corrected my mistake rightly.


A number of years ago, we had discussions with Rosemount re differences between transmitters certified to CSA standards as opposed to those certified to Australian standards. The reply was essentially "The transmitters are physically identical - you tell us what certification you want, and we'll attach the appropriate label." This was apparently the situation with the full range of possible certifications. More recently - I have seen a transmitter with a single label showing certification for Ex e (so Zone 1 and Zone 2) and Ex n (Zone 2 only). Don't forget - equipment may also be certified as compliant with conditions - the latter have an "X" appended to the certificate number. Best thing is to RTFP (Read the full paperwork) and make sure that the application is in total compliance.



By Steve Yates on 4 October, 2007 - 8:34 pm

If it is possible to do without it costing too much, it is in everyone's interest that a manufacturer makes only one version of a product that meets the requirements of the various standards (Ex d, Ex e, Ex i etc)for the various certifcation bodies because cost of manufacturing and inventory are reduced. However this does mean that the certification label is going to carry a lot of information, some of which may end up being condensed and it becomes even more important to obtain and read the documentation/certificate as Bruce has said (far more politely than I would have).


By Steve Yates on 4 October, 2007 - 9:02 pm

Mr. Hait,

I don't think I understand your question.


By Dave Anderson on 19 September, 2010 - 4:15 am

Exd and Exe are designed to be used in zone 1 areas. Because of this they can also be used in Zone 2 areas.

If you have a small plant it is safer to use the superior designs in Zone 2 hazardous areas. if you have a large plant it may be cost effective to use equipment rated for Zone 2 only in Zone 2 areas.

With the smaller plants if you use equipment with different ratings you can guarantee that sooner or later an inferior design will be fitted into a zone 1 area and have to suffer the consequences.

On small plants where the cost saving is not so large it is safer to use equipment rated for Zone 1 in both zones 1&2.

By Ingrid M. Gilstrap on 19 October, 2007 - 4:52 pm


Thanks for your response to my question about Zone 1 flameproof approved product use in Zone 2 areas. It's been helpful. I like your site and exchange of information.


By Ali Fotovat on 15 April, 2008 - 12:41 am

Can we attach a label to a Zone 1 certified device? If so, what type of label should we use?

In Ex d the symbol "d" is derived from the German word "Druckfeste kapselung". Some people use the term explosion proof synonymous with the flame proof encloser.

The flame path is the shortest path along a flame proof joint from the inside to the outside of an encloser. Normally flanged covers, spigotted cover and the threaded cover are used in flame proof enclosers. The flame path shall be

Min 12.5 mm for IIA & IIB enclosers.
Min 25 mm for IIC enclosers.

The flanged joints are not permitted in IIC areas. Threaded joints shall have more than 5 engaged threads and an axial length of 9mm minimum. This is applicable for cable glands as well. The gap in the flameproof joint is the distance between two mating surfaces of the encloser. The maximum gap permissible for flame proof joint is

0.20 mm to 0.40 mm for IIA enclosers.
0.15 mm to 0.20 mm for IIB enclosers.
0.10 mm to 0.15 mm for IIC enclosers.

The different methods are adopted to safe means of connecting the electrical equipment with a flameproof encloser to the external circuit. the Indian standard makes it mandatory to have an Ex d terminal chamber, either integral to the housing or as two separate enclosers joined together by means of sealed or moulded bushings.

In European standard, the use of Ex e terminal chamber interconnected to Ex d encloser is permitted. The connecting wires from the main enclosers are passed through sealed or moulded bushings to the terminal chamber. In American standard the metal screwed conduit is directly connected to E x d encloser.

All protection techniques are not suitable for all zones. Following is the recommendation for different zones.

ZONE 0 Ex ia
ZONE 1 Ex ib, Ex d, Ex e, Ex p
ZONE 2 Ex n, Ex o, Ex q

By Paul Spresser on 1 July, 2009 - 8:04 pm

The Flameproof (Ex d) technique allows the use of any commercial electrical or electronic equipment inside of the Ex d enclosure. If the atmosphere becomes explosive and seeps into the enclosure, a spark will only ingnite the mixture inside the Ex d enclosure because the combination of the long joint length (flame path) and the small gap (<<MESG - the maximum experimental safe gap) cool the exhausting gases to below the ignition temperature for the group of gas.

The Ex e technique simply ensures that a spark or thermal heating which will ignite an explosive mixture is most unlikely to occur in the first place. Improved terminal types are used to minimise the likehood of terminations loosening and marking fortuitous contact with other conductors, terminal types, wire gauge and current combinations are limited to certified combinations to maintain the temperature rating, the enclosure has a minimum IP (ingress protection rating).

Ex d enclosures do not need one to be safe - simply for normal electrical reasons, and the enclosure parts are all boded to the system earth, including the gland plate, the door and any other extraneous metal part. Ex e enclosures are usually sheet metal or plastic, with the exception of motor terminal boxes which some may mistake as being flameproof when they are not. They are simply of robust construction.

The standards Australia document - HB13-2007 -0 available from SAI Global as a PDF document - is an excellent resource for coming up to speed on IEC (or CENELEC - that is Europe including the UK, or Australian/New Zealand requirements) which are reflected in most of the world except the Americas.

As to the Zone 2 example with the gas leak, the Ex d motor is certified for Zones 1 and 2. It depends on the gas leak as well. Is this something that was allowed for in the original hazardous area classification - such as the venting of a PSV - or what this a catastrophic failure?

The classification of hazardous areas does not take into account catastrophic failures but does include a study of safety devices (the action of which is classed as "normal operation").

Also note - the definition of Zone 1 includes the presence of an explosive atmosphere - ocasionally. From personal experience in testing such equipment (explosion and flame transmission testing of flameproof equipment - including motors)a properly designed Ex d motor which has been certified as complying to an IEC standard (or any national clone thereof)would most likely not cause an issue with a one off catastrophic release of flammable gases.

The real issue is that such a release will cause the orignal hazardous area classification to go out the window, as the hazardous area will (however momentarilly) expand to include equipment and installations never intended to be in an explosive atmosphere - and will most likely find a source of ignition there.

Most Ex protection types have belts and braces built into them in terms of the methodology behind the technique and the requirements of their respective product standards. An inadvertant release of a flammable gas about a well maintained, certified Ex d motor would be the least of my worries.

I hope this clarifies rather than confuses.

By Tarun Garg on 22 July, 2010 - 1:29 am

Hi all,

I am working as Engineer with one of India's Largest Power Generation Company NTPC Ltd. I am having a query regarding material of junction box that is to be installed in HFO and LDO Pump House Area. There are two options available with us- One is made of FRP (Fiber Glass Reinforced Polyester) and other is made of Cast Aluminium Alloy LM6.

As per information provided by vendors and other sources, Cast Aluminum made Junction Boxes provide Ex-d protection whereas FRP Junction Boxes can provide only Ex-e protection.

However, We are not able to find any authenticated 3rd party document which which substantiates materials that can be used for construction of Exd protected Junction Boxes. In short we want to ensure that Exd Junction Boxes cannot be made with FRP.

Also, from all the answers above, I could not make out what is the advantage of Exd over Exe if these are to be used in HFO and LDO Pump House Area.

Please help.

Hi there,

You are making things to complicated. First look at the area classification (Zone 1, 2, 3) of the pump stations to see what is the minimum requirements. Exd or only exe. Then find a manufacturer for exd or exe junction boxes. If you need to install exd boxes the manufacturer will rate these boxes so you do not need to concern yourself with the material it is made of. The same is true for the exe boxes. Your concern is only with the rating of the box. If the manufacturer made the box out of a certain material and give it a exd rating then you can be sure it is exd, since he have to comply with various regulations in order to approve these boxes as exd or exe.
Anyway I have personally never seen a exd rated JB made out of anything else than thick Aluminum. Boxes with exe or IP rating is normally made from FRP.

I will try to give you some insight into the subject.

Since you are working in a power plant which basically doesn't deal with hazardous area, your engineering department may not have people who knows how to classify a hazardous area. If you don't have any information then just assume that your area is Zone-1 hazardous area.
Now for Zone-1 hazardous area both Ex'd' and Ex'e' protection techniques are suitable.

If you are using Ex'd' junction box then normal material is cast aluminum. The reason is for Ex'd' protection, the housing has to retain the explosion within the box and that's why it has to be strong thick metal. If you make an Ex'd' box with carbon steel or stainless steel that will be very heavy. This is the normal reason Ex'd' boxes are generally built with lightweight strong Aluminum material. Now it should be clear to you why FRP can't be used as a material for Ex'd' box.

FRP or normal carbon/ stainless steel is generally used for Ex'e' junction boxes. Ex'e' is a protection technique where basically protection is achieved by "increased safety" design. For an Ex'e' box all the items inside the box has also to be Ex'e' certified. For a junction box there are only terminals inside and this is very easy to achieve.

For a junction box we normally prefer Ex'e' because of few reasons:

1. Lower cost than Ex'd' box;

2. For an Ex'd' box before opening the box, you need to shutdown the power. No such limitation for Ex'e' box;

3. For an Ex'd' box if you want to make additional cable entry at site you can't do that. That will nulify the certificate of the box.
Therefore, for Ex'd' box you have to be very careful. You need to define correctly no. of cable entry inside the box and size of the cable glands before buying the box;

For Ex'e' box if your cable size changes or you want to make additional cable entry - that is possible at site. This is one of the most important reason people normally prefers Ex'e' over Ex'd' junction box.

However, there are situations where Ex'd' is the only choice. For example if you have to install an equipment inside the box which is not Ex'e' certified (e.g. a converter or an isolater), then you have to use Ex'd' box only.

Material wise FRP is a good corrosion resistant material. But you have to consider UV resistant material in India. FRP is not as durable as SS316. Therefore, in offshore industry (salt laden atmosphere) many people just use SS316 for longer life.
For your application - FRP Ex'e' is technically absolutely suitable. Only take care that the terminals inside JB must be Ex'e certified and the cable glands also must be Ex'e' certified.

Asok Kumar Hait

Hi guys,

I would like to know whether we can modify non-ex compressor to ex-type by simply modifying the termination and terminal box material.

No - for any form of explosion protection you need to have the equipment certified which is quite a process
If you want to get it rated as flameproof Ex d you need to meet restrictions on flange sizes and clearances, and the equipment has to undergo a series of tests where the flammable atmosphere inside the enclosure is deliberately ignited. For increased safety Ex e things are not quite as bad but you do need to meet requirements about the termination materials and clearances, as well as ensure that any motors have suitable protection.


By Zacharia, Tomy on 4 March, 2011 - 11:39 pm

Bruce is right, No equipment is permitted to be modified or components substituted. If the equipment is already certified, it automatically loses certification, and unless tested as per code in an approved laboratory, equipment cannot be modified and be compliant.

Tomy Zacharia

Thanks for all for the information.

Learned something

By S,VENKATARAMAN on 4 May, 2015 - 3:42 am


I am S VENKATARAMAN AEE(E) from ONGC and it is 5 years since you posted your query. By now you must have some clear ideas on zones, groups and Ex d, Ex e or other equipment for hazardous locations. If you still have any doubts or queries please contact me through my e mail.

My e mail-

By Zahid Hussain on 25 March, 2011 - 6:56 am

This makes clear to me about the haz area classification. Thanks to all.

Very good technical information I got.

Thanks guys.

By Nathan Lamont on 30 June, 2011 - 11:46 pm

I also have to thank you for helping to understand the zoning and requirements... I know this is a bit left field but does anyone know how the ratings work with the belts between the Motor and Pump?

As I have arranged to get made for me a Zone 1 Gas Group IIc T6 compliment motor (Ex d) but I have had several ask me about the drive belt, but I cant find anything on it to meet the heat specification or any other rating!

The current international rating system (Ex d, Ex e, etc) refers only to electrical equipment. However, the European ATEX rules require that all equipment that is capable of ignition has to be certified.
I'm not familiar with any codes or standards relating to mechanical equipment but overheating belts are certainly a possible source of ignition - as a minimum I would suggest you apply the same temperature rating to the belt as you would to the motor surface. If you are asking for a T6 motor, this means that the maximum surface temperature of the belts under worst-case conditions should be below 85 deg C.

Another possible cause of ignition from belts could be electrostatic discharge.

I've just googled "ATEX drive belts" and got quite a few hits - it appears that certified belts are certainly available. Whether or not the ATEX requirements for mechanical equipment are required in your area, it would be good practice to use them if they are suitable.

By Anar Guliyev on 9 August, 2011 - 2:57 am

Please tell me somebody when I should use Ex d or Ex i? For example, I use Ex ia device, do we need the additional Ex d certification or vice-versa?

If you have an Ex ia device, you can use it in Zone 0 - an Ex d device can be used only in Zones 1 and 2. So you don't need Ex d certification if you have Ex ia.


By Nick Thompson on 9 November, 2011 - 7:25 am

if i have ex d junction boxes can i use blu points to join cables inside? the enclosures have compound filled glands

What are the five main Construction requirements of the field Junction box in non-hazardous areas?


Please be careful following zone 1 and zone 2 certifications. They are designed to protect the person who developed them and the manufacturer of the equipment. I have had 20 years experience in the EEX and IS equipment use and sales. For your own protection zone 2 is non reliable as a safety standard if you feel an explosive environment may be present consider it a zone 1 application. Having a zone 2 piece of equipment in operation when the zone changes means you have a problem. Understand also the weight of gases compared to our O2 environment, some are lighter some heavier. This means in containment during operation a gas pocket can be caught, even with venting, causing an area of concern with the zone classification.