Ex-pert Electrical Technologies

Dynamic elaboration on the Art & Science of Hazardous Area Protection


A haunting question. Is it a dilemma  or a  complete solution


….does it require the NEC-IEC vision…

Let us see… We will use the following picture as a symbolic representation of this situation. NEC IEC Joining in Ex

Two tracks , of different colors ,operated by two different protocols but leading to the same destination, which in our case is the Safe Operation of the Electrical Equipments in the Hazardous Area .By same analogy Explosion Proof & Flameproof are the two most commonly used  Hazardous Area Protection Techniques.

Historically, the first safety technique which was  applied to the electrical components was ENCASEMENT. Putting the electrical components , capable of electrical spark or arc in a sturdy metallic box and sealing it up with  a heavy metallic lid,  was the most logical and practical solution. The object was two fold.

1. To keep the insides isolated from the outer Hazardous  Atmosphere

2. If there is an ingress of outer explosive atmosphere resulting in an explosion inside the box , the products of the explosion , namely, the high pressure, the fire,the smoke the hot gases and the debris remain inside and is allowed to escape through (Not the over-pressure value ) the joints .(The joints between the lid and the enclosure , the conduits and the enclosure )

This was being done (at least) at two places in the world .

Europe  & America

We will bypass the normal flow of history and land in 1957.

IEC 79 appears for flameproof enclosures of electrical apparatus. The term Ex d appears many years later.

With the term Ex d comes the definition , as follows:

Flameproof Enclosure “d” . Enclosure in which the parts which can ignite an explosive atmosphere are placed and which can withstand the pressure developed during an internal explosion of an explosive mixture,and which prevents the transmission of the explosion to the explosive atmosphere surrounding the enclosure. (IEC 60079-1    7th edition)

Now consider the definition of the Explosionproof 

Article 100 of the National Electrical Code defines “explosionproof equipment” as equipment enclosed in a case that: (1) Is capable of withstanding an explosion of a gas or vapor that may occur within the case, (2) Is capable of preventing the ignition of a gas or vapor surrounding the enclosure by sparks, flashes, or explosion of the gas or vapor within the enclosure, and (3) Operates at such an external temperature that a surrounding flammable atmosphere will not be ignited by the outside of the enclosure.

In designing explosionproof equipment, it is assumed that the flammable gas or vapor will enter the equipment enclosure and be ignited by a spark or arc within the enclosure.  The threaded joints provided by rigid metal conduit and intermediate metal conduit are not gas-tight, so the gas can enter around these threads.  In addition, all electrical equipment intended to be connected to a wiring system has a removable cover of some kind to permit wiring and facilitate repair of the electrical parts. Some equipment has external shafts to permit operation of internal switches or, as in motors, to transmit mechanical motion through the enclosure.  The gases can enter through the interface between the removable cover and the remainder of the enclosure, or through the shaft and body interface.  These interfaces are called “joint surfaces,” or just “joints” and are commonly referred to as “flame paths”.

(KILLARK-2011 NEC code reviews)

The following common parameters are obvious:

1. STRONG Enclosure . Strong enough to withstand the internal pressure of the explosion.

2.Special Design of the enclosure to isolate the effects of the internal explosion and not to transmit any of the effects to the outer surrounding atmosphere.

3. The JOINTS  or the interfaces are , however, specially designed to control the expanding hot gases in such a way so as to cool them and quench the flame before letting some of the them escape through the “FLAME PATHS”. A flame paths is  always long in proportion to its width.

IEC 60079-1  & NEC 501-1 describe the design principles of these joints and we find a very elaborate description in IEC 60079-1.

One of the basic design parameter is the MESG -Maximum Experimental Safe Gap (for an explosive mixture)

It is the Maximum gap of a joint of 25mm in width which prevents any transmission of a explosion during TEN  (10) tests made the conditions specified in IEC 60079-1-1.

At this point let us examine the commercially available “Explosionproof ” & “Flameproof” enclosures.


Both the enclosures are made of the same alloyed material and by the same manufacturer.


Gas Explosion Protection Class I, Division 1, Groups B, C &D

Gas explosion protection  Ex d IIB + H2 , Europe (ATEX): Explosion protection Ex d … IIB + H2 T6…T4, Ex.

SO……what & where  is the difference ?? .

Now let us use the NEC-IEC vision to finally get the clarity.


There is no difference between the Explosionproof & Flameproof as far as following points  are concerned:

a. The underlying Philosophy

b. The magnitude of the embedded safety

c. commercial availability and choice of material

d. technical library

The differences:

1. Specification protocols

Explosionproof  follows the NEC Protocol

Flameproof follows the IEC Protocol

2. Testing parameters

Explosionproof  are constructed to withstand 4 times the explosive pressure of the gases Flameproof enclosures are tested for over-pressure : 1.5 times the reference pressure , with a minimum of 3.5 bars  or 4 times the reference pressure for enclosures not subjected to routine overpressure testing Conclusion:


Designing my own Ex d (Explosion Proof / Flame Proof) terminal box or a control station.

Empty Ex d Enclosures are available in the market.

An enthusiastic (not-so-Ex-PERT)  person (like me) may have this temptation of purchasing an empty Ex d Enclosure and ‘making ‘ a junction box or a control-station by installing industrial type electrical components for operating it in a classified Hazardous Area.


(See the details below )

·         You can do the Purchasing of an Empty Ex d box.

(After all you have purchased a branded Certified Empty Box and you have downloaded the certificate from relevant web site)

·         You can carry out the installation of components

BUT You CANNOT take your designed terminal BOX into a Hazardous Area and ENERGIZE it.

 Because you are NOT ALLOWED by the Hazardous Area Protection laws

 (For you own safety,safety of the concerned staff and the safety of the plant)

Just check the Certificate; it will have the following form

IECEx                                    IECEx KEM —— U

Europe (ATEX)                     KEMA — ATEX —– U

 The letter U clearly indicates that the empty enclosure is a component and NOT a COMPLETE ASSEMBLY and it requires testing for the Hazardous Area once it takes the form of an assembly (of components inside)

IEC 60079-0           Sec 3.28

Ex Component: Part of electrical equipment or a module , marked with the Symbol “U”, which is not intended to be used alone and requires additional consideration when incorporated into electrical equipment or systems for use in explosive atmospheres.

And CERTAINLY your designed assembly of components has never been tested in any laboratory of the world and thus the Protection Level of the Junction Box / Control Station is not known and thus DANGEROUS

Furthermore the Ex d enclosures are expected to withstand the explosion-generated pressure of the Assembly of the components installed in it. This scenario can only be tested inside the factory or the certifying body.

The Official way is to give details, to the manufacturer, of all the components to be installed inside the enclosure and let them do the designing , manufacturing and testing for you.

The purpose  of an Ex Component ENCLOSURE certificate for empty enclosures is to enable the manufacturer of  FLAMEPROOF enclosures to obtain a certificate without the internal equipment being defined, so as to enable the empty enclosure to be made available to third parties for incorporation into a FULL EQUIPMENT Certificate without the need for repetition of all the type tests for flameproof enclosures ‘d’ as required by IEC 60079-1 and IEC 60079-0.

 (Ref: IEC 60079-1  (2014)   Annex D)

So what is the use of the U certified enclosure?
Actually these empty enclosures with component certificates are not intended for the end -users , in fact they are for other manufacturers to install their products and then apply for  full equipment certificate , by-passing some or most of the type tests.

Furthermore , since the production process is already being monitored for the empty enclosures for quality assessment, the third party is assured of the safety of the purchased enclosure

Reference to IECEx OD 017 is worthwhile here.This Operational Documents lists the documents & drawings to be submitted by the manufacturers to the Ex testing laboratories when applying for certification.

For the Flameproof Enclosures d (Flameproof because we are using IEC) , some of the details to be shown in the submitted drawings includes :

Area in which cable entries , switch operators , windows, receptacles etc may be fitted

Layout and heat dissipation of internal component showing location and approximate dimensions of each component, including clearances between components and the nearest sidewall. ( The purpose is to control)

  • Location of sources of heat, for temperature classification purposes, for temperature withstand of plastic parts , window cement, other potting materials, cable entries etc.
  • Cross check the pressure piling effects of the dimensions of the enclosure.

♦ be safe & consult the ex-perts ♦

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About Asif Mirza

electrical engineer artist
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