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from the Confused in simple issues department...
Basic difference between Zone-2 and Zone-1 equipment
Application Questions and Problems topic
Posted by A.K.Hait on 12 July, 2007 - 10:42 pm
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.

Posted by Gerry on 15 July, 2007 - 12:23 pm
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

Posted by A.K.Hait on 17 July, 2007 - 4:13 pm

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?

Posted by Gerry on 17 July, 2007 - 10:43 pm
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,


Posted by Bruce Durdle on 18 July, 2007 - 10:55 pm
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).



Posted 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.


Posted by Gerry on 28 August, 2007 - 11:42 pm

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

Posted by Hamid on 26 September, 2007 - 11:30 pm

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).


Posted by Phill Wood on 18 December, 2008 - 12:15 am

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?


Posted 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

Posted 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.

Posted 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

Posted by Bruce Durdle on 17 July, 2007 - 4:48 pm
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.



Posted 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.

Posted by Bruce Durdle on 20 September, 2007 - 12:36 am

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

Posted by A.K.Hait on 20 September, 2007 - 10:36 pm
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!

Posted by Bruce Durdle on 21 September, 2007 - 11:28 pm
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.


Posted by Steve YATES on 21 September, 2007 - 11:37 pm
>1. What is the exact difference between
>an Exd equipment suitable for zone-2 and
>an Exd equipment suitable for Zone-1? <>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. < class="more" style="padding-top: 5px; padding-right: 5px; padding-bottom: 10px; padding-left: 5px; ">Reply to this post...

Posted by A.K.Hait on 3 October, 2007 - 12:51 pm

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.


Posted by Bruce Durdle on 3 October, 2007 - 11:12 pm
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.



Posted 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).


Posted by Steve Yates on 4 October, 2007 - 9:02 pm
Mr. Hait,

I don't think I understand your question.


Posted 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.


Posted 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?

Posted by NIK on 19 December, 2008 - 5:10 am
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

Posted 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 (<

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