When I do trainings on Explosion Protection methods, there is usually a moment when some people in the audience seems to enter into the twilight zone:
This usually happens when the presentation reaches the definition of the Ex “e” protection method. I usually start describing basic principles like the Explosion Triangle, the LEL and UEL concepts, the Flashpoint and MIE graphics, etc.
After that I get into the area classification methods according to ATEX, IECEx and NEC and things usually go smoothly, then I reach the description of the protection methods available for each standard and then I get to the EX “e” slide: Twilight Zone stare!
Let’s see the definition of Ex “e” according to IEC 60079-7:2007:
“This part of IEC 60079 specifies the requirements for the design, construction, testing and marking of electrical apparatus with type of protection increased safety “e” intended for use in explosive gas atmospheres. This standard applies to electrical apparatus where the rated voltage does not exceed 11 kV r.m.s. a.c. or d.c. Additional measures are applied to ensure that the apparatus does not produce arcs, sparks, or excessive temperatures in normal operation or under specified abnormal conditions. This standard supplements and modifies the general requirements of IEC 60079-0. Where a requirement of this standard conflicts with a requirement of IEC 60079-0, the requirement of this standard takes precedence.”
Looks simple?
Initially I thought so, but when you get into the innards of the standard, there are quite a lot of factors to be taken into account:
Take the paragraph 3.5, as an example:
3.5 increased safety “e”:
“type of protection applied to electrical apparatus in which additional measures are applied so as to give increased security against the possibility of excessive temperatures and of the occurrence of arcs and sparks in normal service or under specified abnormal conditions”
What this paragraphs means basically is that only non-sparking devices can be mounted into a Ex “e” enclosure, but the “additional measures” concept can be daunting to approach, because there are two notes below the definition that state the following:
“NOTE 1: This type of protection is denoted by “e”. The “additional measures” are those required for compliance with this standard.
NOTE 2: Apparatus producing arcs or sparks in normal service is excluded by this definition of increased safety.”
So suddenly things become tricky: the “additional measures” to be taken in order to ensure that something is Ex “e” are those described in the standard, which is 70+ pages long.
This is somewhat different from the other protection methods such as Ex “d”, Ex “‘p” or Ex “i”. Those methods can be explained in a very concise and compact way, because they are based on one idea: explosion containment for Ex “d, displacement of the explosive atmosphere by a clean or inert atmosphere using positive pressure in Ex “p” and energy limitation circuits in Ex “i”.
Ex “e” is a different kind of beast, its not possible to resume in a paragraph all the additional measures that have to be taken into account. You have to consider impact protection of the enclosure, temperature dissipation of all the devices contained by the enclosure, which also happen to be either non-sparking or intrinsically safe, care must be taken with connections and whether they are internal or external, there are grounding requirements that depend upon the components and the enclosure material and so on.
When you consider which are the usual applications of the Ex “e” method, which are electric motors, lighting devices and control panels, all those safety premises have been solved by the manufacturer. But there is one application where this is not usually the case: custom junction boxes and custom control cabinets.
So what approach should a integrator or end user follow in order to use Ex “e”?
The first answer that comes to my mind is that they have to go to a systems and solutions supplier who can provide them with a fully certified product. DIY Ex “e” enclosures have to be certified by a competent authority and that is not easy nor cheap.
The benefits and advantages of the Ex “e” concept are out of question: for several applications its quite more cost efficient when compared with flameproof enclosures, which are also heavier, have very rigorous maintenance procedures and require a gas clearance certificate in order to be opened in the field. Ex “e” enclosures have easy access by its opening doors and if they contain IS devices or simple apparatus, they can be connected or disconnected without stopping the process.
But Ex “e” protection is based on a combination of several concepts and it’s not easy to include them all in a single paragraph. Hence the note about “additional measures”.
The Ex “e” method was initially developed in Germany by the early 1950″s, and became a IEC standard (79-7) in 1967. So it’s not what you’d call a new idea.
Process Automation is a field well known by its conservative nature and rightly so, since any kind of innovation comes with new risks and additional complexity. A mistake in Process Automation can be quite expensive both in material and human cost.
That is why I still can do intrinsic safety trainings, or flameproof vs explosion proof comparisons or deal with the different approaches to install RIO systems and fieldbus infrastructure components in hazardous areas.
I was updating an explosion protection presentation when this issue came to my mind. It would be really interesting to see if there is a way to make Ex “e” easier to understand for people that do not want to get into the innards of standards. I like to do so, but I must confess that I’m a geek. Current trends show that the use of combined EX “d” and Ex “e” solutions will grow significantly in the following years: by combining these two methods of protection users wil get the robustness of flameproof enclosures with the easy maintenance requirements of Ex “e” cabinets. So it will be useful for PA people to get familiar with the increased safety protection method.
Europe and the IEC world are more receptive to the Ex “e” approach, they have been exposed to it for quite a long time, the rest of the world will surely follow.
By the way North American standards include the Ex “e” concept:
It’s mentioned in the NEC505 standard as AEx e (or AEx eb) and based on the ISA 60079.7 standard and also in the CSA standards (CSA 22.2 No. 60079-7).
It is listed as a valid protection method for applications in Class I Zone 1 or 2 environments.
Some people will say: Class I Zone 1 or 2?! Classes are supposed to go with Divisions, not Zones, that’s European stuff…
But that’s another story.
Mirko Torrez Contreras is a freelance Process Automation consultant with a almost obsessive need to learn and share knowledge. He’s also starting a freelance writing career, in order not to bore his family by continuously talking about tech stuff at home.