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Electrical Measurements in Explosive Environments (Ex)

April 17, 2018

What is an explosive (Ex) atmosphere?

An explosive atmosphere contains a mixture of dangerous substances in many forms such as gas, vapor, mist or dust, which, under certain conditions of temperature and pressure, has the potential to catch fire and explode. An explosive atmosphere only needs a source of ignition to cause an explosion.

Preventing releases of dangerous substances, which can create explosive atmospheres, and preventing sources of ignition, are two common ways of reducing the risk of danger in these hazardous locations. Several different standards define acceptable actions in both areas.

Those standards are promulgated by three different bodies, separated into geographic regions, which certify suitability for Ex. UL provides certification for the North American Region, while ATEX applies to and is a compulsory requirement of European Union (EU). IECEx targets internationally to the world and all countries.

Underwriters Lab (UL)

Most North Americans are familiar with the ubiquitous UL label on their appliances but many do not realize that UL also certifies compliance for industrial applications, including Ex environments. OSI transducers for Ex environments (e.g. ISC-201X5) are certified to UL-913, 7th Edition.


ATEX is mandatory in the EU only, and relates to the two European Directives for controlling explosive atmospheres:

  1. Directive 99/92/EC (also known as 'ATEX 137' or the 'ATEX Workplace Directive') focuses on requirements in improving the health and safety protection of workers potentially at risk from explosive atmospheres.
  2. Directive 94/9/EC (also known as 'ATEX 95' or 'the ATEX Equipment Directive') describes the laws of Members States regarding equipment and protective systems intended for use in potentially explosive atmospheres.

As a manufacturer, OSI focuses on delivering equipment to meet the applicable provisions of ATEX 95.


International standards IEC 60079 (Ex Manufacture) and ISO/IEC 80079-34 (Ex Quality), which are harmonized with EN European Directives, are hazardous area standards that are intended to operate globally across many national and regional jurisdictions. These requirements shape OSI’s design, quality and manufacturing practices of IECEx listed devices.

What do all those numbers and letters mean?

The markings define:

  • Type of protection (d, ia, e, p, nA etc.)
  • Group for which the equipment is approved. (e.g., IIA, IIB, IIC)
  • Temperature code or maximum surface temperature (T1,T2,...T6)
  • Equipment Protection Levels. (Ga, Gb or Gc)

We won’t bore you by repeating all the categories, especially when you can find a visual at: PreSafe’s site

Consider OSI’s certifications and markings as an example:

OSI transducers for ATEX are marked: CE2460II 1G Ex ia IIC T4 Ga

OSI transducers for IECEx are marked: Ex ia IIC T4 G

  • CE – Complies with the European Directive
  • 2460 – Notified Body number
  • – Specific CE marking for Explosion Protection
  • II – This is the equipment group and permits all other explosive areas (except mining)
  • 1 – can be used in zone 0.This is an area in which, under normal operation, a potentially explosive atmosphere is continually present. Zones 1 and 2 are included by the Zone 0 designation.
  • G – Gas (as opposed to D – Dust)
  • ia – Intrinsic Safety. This means the equipment is permitted for zone 0, 1 and 2. Development of inadmissibly high temperatures, ignition sparks and arcs are avoided by restricting the circuit energy.
  • IIC – Explosion group. This equipment is suitable for application at the surface and other locations with substances such as Methane, Propane and Hydrogen. Mine applications are excluded.
  • T4 – 275°F / 135°C Maximum Temperature (shown as “T135°C” for dust).
  • Ga – Equipment Protection Level (EPL) = Zone 0

Here are some additional guidelines for understanding the markings

  • Higher certification also allows for use in lower categories. For example, Zone 0 with Hydrogen also includes Zone 2 with Propane.
  • Temperature value is maximum surface temperature of device calculated as environment plus added heat from device operation.
  • Gas and dust are separate certificates.
  • No human should be permitted to work in Zone 0. These areas frequently attempt to reduce the explosion potential by reducing the oxygen level.
  • A higher explosion group (e.g. Hydrogen, IIC) has higher ignitability potential. The explosion is usually very rapid.
  • Most industrial damage is caused by the lower explosion groups, which typically have a slower explosion and burn.
  • Anything that can burn can typically also explode if milled to fine enough dust. Grain is an excellent example of this phenomenon.

NOTE: This is only an overview with an abbreviated explanation. It cannot be a substitute for knowledgeable engineering practice. If you are uncertain of the operating environment or unclear on its requirements, please consult with an Ex credentialed expert.

NOTE: Certifying Bodies require that OSI reviews every customer’s stated requirement(s) compatibility to the Ex certificate of the Ex product. Please do not be offended when we ask.

View our existing line of ATEX Torque Sensors here.