Classification of hazardous areas Electrical equipment in hazardous areas

Technical information

Classification of hazardous areas

In each area in which there is a process that causes a raising of temperature of flammable substances and their evaporation, such as hydrocarbons, can produce a leak of gas or of vapor which, in contact with flames or sparks caused by electrical processes, may explode. This event can happen both during normal operations and in case of breakdown or accident.
The identification of hazardous area must be done by highly qualified personnel as process engineers.

Hazardous areas are defined by three main criteria:

  • The type of hazard
  • The likelihood of the hazard being present in flammable concentrations
  • The (auto) ignition temperature of the hazardous material

Gas Classification

The construction of explosion protected electrical equipment intended for use in potentially flammable atmospheres (hazardous areas) is defined clearly in two separate categories to suit the nature of the hazardous area application. The two categories recognised under both the CENELEC (European) Normatives and the IEC Standards are stated as being equipment groups or apparatus groups, and are defined as follows :-

Apparatus Group I - Electrical equipment for mining applications
Apparatus Group II - Electrical equipment for surface industry hazardous areas, other than mining applications

  • IIA - Propane
  • IIB - Ethylene
  • IIC - Hydrogen and asetylen

(The gases noted in the table are typical gases for each group.)

Gases and Vapours

Every dangerous place has to be classified into one of following three zones considering:

ZONE 0 Zone in which a mixture of explosive gas is always present or for long periods or frequently.
ZONE 1 Zone in which a mixture of explosive gas may be present occasionally during normal plant operation.
ZONE 2 Zone in which a mixture of explosive gas is not normally present, and if it is, only it is for short periods of time.

Zone 0 is the most DANGEROUS zone. It has the highest probability of a flammable atmosphere presence. Equipment for this zone must be very well protected against providing a source of ignition and have legal certification of this protection. Every other area of the plant is considered a SAFE AREA.

Ignition Temperatures

The ignition temperature of a flammable material is critical in determining the acceptability of equipment/instrumentation that operates in hazardous locations as the external surface of the enclosure can act as an ignition source if the surface temperature exceeds the ignition temperature of the flammable material. High surface temperatures can result from normal operation of the equipment; abnormal operation (i.e. overheating); or because of a fault condition.

Equipment destined for installation in hazardous areas is marked with a code representing the maximum external surface temperature. Equipment cannot be used in hazardous areas when the maximum surface temperature is greater than the ignition temperature of the gases/vapors present.

Protection Concepts

There are varying types of equipment that can be used within these zones to ensure that the potential for an explosion is removed or greatly reduced. This equipment must be designed and manufactured in accordance with particular construction parameters known as protection concepts. Essentially these concepts fall under four main methods. These methods are detailed below along with a brief description of some of the concepts:

ATEX Certified Electrical Equipment for Hazardous Areas

ATEX is the name given to a set of European Directives relating to Hazardous Area Installations (Flammable Atmospheres) that takes it's name from the French "Atmosphères Explosibles", and spells out a set of Essential Health & Safety Requirements (EHSR's) and conformity assessment procedures which when followed should enable the industry to operate safely & avoid accident or incident.

ATEX is a so-called "New Approach" Directive which must be applied to equipment within its scope before being placed on the European market.

There are two ATEX directives, 94/9/EC, commonly referred to as ATEX 95, and 99/92/EC, commonly referred to as ATEX 137.

The “New Approach” ATEX directive supersedes the “Old Approach” ATEX Directive, the Potentially Explosive Atmospheres Directive, 76/117/EEC – ATEX 94/9/EC (ATEX 95) facilitates free trade in the EC for hazardous area electrical equipment intended to be used in potentially explosive atmospheres. ATEX Directive 94/9/EC (ATEX 95) covers “Electrical Equipment”  used in hazardous areas with potentially explosive atmospheres created by presence of gases, vapours, mists or dusts. 

An example of Ex identification can be found below:

EEx…de…II…C…T&

  • EEx= Ex protection in accordance with European standards.
  • de = Type of protection
      d = Flameproof protection
      e = Increased safety
  • II = Apparatus Group:
      II = Electrical equipment for surface industry hazardous areas, other than mining applications
      C = is the most severe group. Hazards in this group can be ignited very easily indeed.
  • T = Temperature Classes
      T6 = Temperature group 85 C

All products or equipment intended for use in potentially explosive atmospheres that were not yet placed on the market in the EU had to comply with the ATEX 95 Directive, and the latest technological know how, or state of the art.

"Placing on the market" is considered to take place when finished product or equipment enters into the supply chain within the EC. This is accepted as being placed in the stock of the end user, the distributor or indeed the manufacturer, provided that his premises where the products or equipment are being placed is within EC boundaries.

Type of protection

To ensure safety in a given situation, equipment is placed into protection level categories according to manufacture method and suitability for different situations. Category 1 is the highest safety level and Category 3 the lowest. Although there are many types of protection, a few are detailed

  Ex Code Description Standard Location Use
Flameproof d Equipment construction is such that it can withstand an internal explosion and provide relief of the external pressure via flamegap(s) such as the labyrinth created by threaded fittings or machined flanges. The escaping (hot) gases must sufficiently cool down along the escape path that by the time they reach the outside of the enclosure not to be a source of ignition of the outside, potentially ignitable surroundings.

Equipment has flameproof gaps (max 0.006" propane/ethylene, 0.004" acetylene/hydrogen)

 IEC/EN 60079-1 Zone 1 if gas group & temp. class correct Motors, lighting, junction boxes, electronics
Increased Safety e Equipment is very robust and components are made to a high quality IEC/EN 60079-7 Zone 2 or Zone 1 Motors, lighting, junction boxes
Oil Filled o Equipment components are completely submerged in oil IEC/EN 60079-6 Zone 2 or Zone 1 Heavy current equipment
Sand/Powder/Quartz Filled q Equipment components are completely covered with a layer of Sand, powder or quartz IEC/EN 60079-5 Zone 2 or Zone 1 Electronics, telephones, chokes
Encapsulated m Equipment components of the equipment are usually encased in a resin type material IEC/EN 60079-18 Zone 1 (Ex mb) or Zone 0 (Ex ma) Electronics (no heat)
Pressurised/purged p Equipment is pressurised to a positive pressure in relative to the surrounding atmosphere with air or an inert gas, thus the surrounding ignitable atmosphere can not come in contact with energized parts of the apparatus. The overpressure is monitored, maintained and controlled. IEC/EN 60079-2 Zone 1 (px or py), or zone 2 (pz) Analysers, motors, control boxes, computers
Intrinsically safe i Any arcs or sparks in this equipment has insufficient energy (heat) to ignite a vapour

Equipment can be installed in ANY housing provided to IP54.
A 'Zener Barrier' or 'opto isol' or 'galvanic' unit may be used to assist with certification.
A special standard for instrumentation is IEC/EN 60079-27, describing requirements for Fieldbus Intrinsically Safe Concept (FISCO) (zone 0, 1 or 2)

 IEC/EN 60079-25
IEC/EN 60079-11
IEC/EN60079-27		 'ia': Zone 0 &
'ib': Zone 1
'ic: zone 2		 Instrumentation, measurement, control
Non Incendive n Equipment is non-incendive or non-sparking.

A special standard for instrumentation is IEC/EN 60079-27, describing requirements for Fieldbus Non-Incendive Concept (FNICO) (zone 2)

 IEC/EN 60079-15
IEC/EN 60079-27		 Zone 2 Motors, lighting, junction boxes, electronic equipment
Special Protection s This method, being by definition special, has no specific rules. In effect it is any method which can be shown to have the required degree of safety in use. Much early equipment having Ex s protection was designed with encapsulation and this has now been incorporated into IEC 60079-18 [Ex m]. Ex s is a coding referenced in IEC 60079-0. The use of EPL and ATEX Category directly is an alternative for “s” marking. The IEC standard EN 60079-33 is made public and is expected to become effective soon, so that the normal Ex certification will also be possible for Ex-s IEC/EN 60079-33 Zone depending upon Manufacturers Certification. As its certification states

The types of protection are subdivided into several sub classes, linked to EPL: ma and mb, px, py and pz, ia, ib and ic. The a subdivisions have the most stringent safety requirements, taking into account more the one independent component faults simultaneously.