Intrinsic Safety: a barrier to harmony?

06 October 2015

Peter Gardner looks at the different approaches taken to preventing ignitions caused by electrical equipment in hazardous areas.

Differences between Europe and the US abound in every aspect of our lives and culture. Sometimes this is also apparent in industry, where our approach and philosophy often differs. Take, for example, the subject of intrinsic safety, where the immediate question is why is it so popular throughout Europe while in the US it has never really caught on? 

To better understand the reasons behind this it is first necessary to examine the concept of ‘Intrinsic Safety’ and clarify where it differs from Explosion Proof.

The purpose of both Intrinsic Safety (IS) and Explosion Proof is to prevent a malfunction in a piece of electrical process equipment from initiating an explosion or fire through ignition of gases that may be present in the surrounding atmosphere. Both systems do this by keeping the potential energy level below that necessary to start the ignition process. 

Intrinsic Safety manages the amount of energy available to a level below which ignition can occur. Explosion Proof on the other hand contains the energy of any possible explosion within an enclosure that contains the possible ignition source. This containment is achieved through careful design of the enclosure so that the resulting energy is not only contained but is dissipated through the large surface of the flanges or threads of the enclosure. Consequently, if the integrity of the enclosure is compromised, either because of a scratch across the flange face or threads or incomplete tightening of the cover, the result is a significant increase in the risk of an explosion. The net result is that Explosion Proof protection has a higher level of required maintenance than an Intrinsically Safe system. Furthermore, Explosion Proof enclosures are big and bulky which can be an issue in limited space applications.

Looking back at the origins of Intrinsic Safety it is not surprising to find that its rise in popularity in Europe is closely linked to the availability of transistorised equipment and the emergence of computer control.
A possible key factor for the different approach was the area classification. The existence of Zone O, where the flameproof technique was not acceptable, required that intrinsic safety had to be used for some sensors, and since it had to be used for this purpose it became a recognised practice. There was no corresponding requirement imposed by Divisional classification. 

Class 1, Division 1 (Zone 0) areas are defined as having combustible mixtures present routinely or all the time. The probability of a combustible mixture in Div. 1 is defined as 1 to 10-1. This type of probability is a very high risk if arcing devices are used, so Div.1 electrical design is very conservative.  Div. 1electrical designs allow for multiple simultaneous failures without the possibility of igniting a flammable mixture. 

Generally arcing devices for Div.1 areas are sealed and contained in explosion proof containers. A practical alternative to this approach is to limit power delivery to the extent that it is impossible to generate a spark. This alternative is Intrinsic Safety. 

Class 1, Div. 2 (Zone 2) areas are defined as areas where the likelihood of a flammable mixture is low, but it is significant enough that normal electrical activity should not present an ignition source. The probability of a flammable mixture in Div. 2 is defined as less than 10-5. Single failure tolerance with normal activities is much cheaper to design, construct, and maintain than the more redundant designs required for Div. 1. Div. 2 design is the most commonly found area classification in the US. Unclassified areas have a negligible probability of hydrocarbons. This type of area classification might be found in an office building or control centre. 

Driven by cost
The question of cost is a prime factor in the final system choice.  With intrinsically safe circuitry there is a common misconception that the cost is higher and that it is more complicated. However, the cost of ownership is generally much lower taking into account the fact that customers want to have higher system availability and be able to produce product without shutting down, in addition to running the process safely. 

With Explosion Proof equipment the electronics must be locked out and turned off. Unfortunately, with fieldbus systems this is not possible as they must be worked on live and connected to the network.
Reputable companies, well versed in this field, have carried out careful analysis of the two methods and, based on their analysis, the installed cost of an IS analogue loop is 16% less than Explosion proof, while for a discrete signal it is 17% less expensive. 

Intrinsic Safe installations allow live maintenance, although in practice they, like Explosion Proof techniques require the use of hot work permits because many of the maintenance tools used are not IS and the work itself has the potential to create a spark of sufficient energy to initiate a fire. 

In North America most hazardous locations are considered Class I, Div. 2 which very closely resembles the Zone 2 specification of Europe. Many European factories are located very close to large populations. Even though Zone 2 classifications are relatively safe, there has never been an accident on record with an intrinsically safe installation. Insurance companies like to eliminate risks rather than mitigate risks. North American engineers are reluctant to use something that is still perceived as ‘new’ technology even though it is now over 50 years old. 

The debate about Intrinsic Safety is dynamic and ongoing and will continue back and forth as technology evolves and legislation changes. In the meantime Europe must agree to disagree with its counterparts across the Atlantic.

Peter Gardner is managing director of UK-based Turck Banner.

Interface series approved for worldwide use 
A study of the development of interface technology by Turck revealed that customers increasingly have three key requirements – safety, especially with regard to Ex separation and functional safety (SIL); the space requirement on the DIN rail; and performance, particularly in relation to speed and accuracy of the devices. Although existing interface devices can meet these requirements to a certain extent, the possibility of their further development is normally limited due to the original circuit design and construction.

The company decided that a completely new electronic platform was required to provide its customers with a product to meet all these needs. The result is the IMX series The range was developed in compliance with the requirements of IEC 61508.

The devices have been comprehensively approved for use in Europe, North America, South America, China and Asia and are provided with UL, FM, ATEX, Nepsi, Kosha, Imetro and IEC-Ex certification. This allows customers to operate the devices reliably at different locations worldwide.

The new IMX series only requires 6.25mm per temperature signal. A narrow 12.5mm housing width, and up to four terminal banks per side, allows the devices achieve impressive channel densities. The space requirement on the DIN rail for the isolating switching amplifiers with a relay output (2-channel 4-wire resistance temperature sensors) has been reduced to half of that required by previous interfaces. Four separate removable terminal banks facilitate ease of connection as only actively used terminals need to be unplugged.

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