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Calibration in hazardous areas

14 October 2014

This article discusses many of the issues surrounding calibration in hazardous areas, highlighting some important issues that need to be addressed before entering a hazardous area with calibration equipment. 

A hazardous area is any area that contains, or may contain, flammable substances. The flammable substance can be a liquid, gas, vapour or dust. The area may contain a flammable substance all of the time, most of the time, or only in specific situations – such as during shutdowns or accidents. In such a hazardous area, an explosion or fire is possible if all three conditions of the ‘explosion triangle’ are met. These three conditions are fuel (a flammable substance), source of ignition (or heat) and oxygen (air).  

Keeping in mind the explosion triangle, it is important to eliminate one or more of the three elements. In many situations eliminating the flammable substance is not possible, and therefore the oxygen (air) or source of ignition has to be eliminated. However, it is also often impossible to eliminate the air. The most practical solution is, therefore, to eliminate the source of ignition, spark or heat. 

Electrical calibration equipment can be specially designed for use in hazardous areas. Often it is designed in such a way that it cannot provide enough energy to cause the source of ignition, spark or heat.

There is a great deal of discussion about flammable and combustible liquids. Generally speaking, they are liquids that can burn. They may be gasoline, diesel fuel, solvents, cleaners, paints and chemicals. Flashpoint and autoignition temperatures are also often discussed.  Flashpoint is the lowest temperature of a liquid at which it produces sufficient vapour to form an ignitable mixture with air. With a spark or enough heat, it will ignite. Autoignition temperature is the lowest temperature at which a liquid will ignite even without an external source of ignition. Most commonly, flammable and combustible liquids have autoignition temperatures in the range of 300°C to 550°C. However, there are liquids that have an autoignition temperature as low as 200°C or less. 

Based on their flashpoint, liquids are classified as flammable or combustible. Flammable liquids may ignite at normal working temperatures, while combustible liquids burn at higher temperatures.  Often 37.8°C is considered as the temperature limit. Flammable liquids have a flashpoint below this point and combustible liquids above. 

Flammable and combustible liquids themselves do not burn, it is the mixture of their vapours and air that burns. There are also limits of the concentration within which the mixture can burn. If the concentration of the mixture is too low it will not burn; the same is true if the concentration is too high. The limits are known as lower and upper explosive limits (LEL and UEL).
It is useful to remember that some liquids may have a rather low flashpoint. For example, gasoline has a flashpoint as low as -40°C. It produces enough vapours in normal environmental conditions to make a burnable mixture with air. Combustible liquids have a flashpoint way above normal environmental conditions, and they have to be heated before they will ignite.

Preventing explosion
As mentioned earlier, to prevent an explosion, one of the three elements of the explosion triangle needs to be eliminated. In practice, eliminating the source of ignition would be the most sensible. 

There are various techniques in electrical equipment that make them safer for hazardous areas. These fall into two main categories – eliminate the source of ignition (Exe, Exi) or isolate the source of ignition (Exd, Exp, Exq, Exo, Exm). Figure 1 briefly describes some of these different techniques:

The Exi ‘Intrinsically Safe’ technique is the most commonly used and is the most suitable protective technique for electrical calibration equipment. Intrinsically safe equipment is designed for any situation; it will not provide enough energy to generate sparks and excessively high surface temperatures, even in the case of a faulty device.  The equipment is designed to be intrinsically safe. Inside an Exi device, the Exm (‘Encapsulated’) technique may also be used for certain parts of the equipment (as in a battery pack).

Using non-Ex calibration equipment in a hazardous area is possible, but requires special approval from the safety personnel in the factory. It also often involves the use of safety devices – such as personal portable gas detectors – to be carried in the field while working. However, the use of Ex-rated equipment is an easier solution as it does not require any special approvals. 

There are two different standardisations specifying hazardous areas and classification of the equipment used in hazardous areas. One is the International IEC standard and the ATEX directive used in International and European legislation. The second is the North American legislation.
 
The international standard family of IEC 60079 defines the different standards for related regulations. The IECEx scheme involves international co-operation based on the IEC standards. The objective of the IECEx system is to facilitate international trade in equipment and services for use in explosive atmospheres, while maintaining the required level of safety. Today, there are approximately 30 member countries in the IECEx, including the USA.

The ATEX directive was introduced to unify hazardous equipment and work environments within the European Union. The zone classification specifies how likely it is for a certain flammable substance to occur in the atmosphere in a certain area. The classification has been developed to specify the different hazardous areas (Zones) as shown in Figure 2.

In ATEX-directive Group II, equipment is divided into product categories specifying the use of the equipment in different zones. In the IEC standards, the same thing is expressed using EPLs (Equipment Protection Levels) using very similar the categories:

A product category 1/EPL a device, for example, is safe even in the event of two simultaneous faults in the device. This means that all protective safety circuits are tripled. A category 2/EPL b device has doubled safety circuits and can be used in Zones 1 and 2. Category 3/EPL c devices have single safety circuits and can be used in Zone 2 only.  It is important to know the zones where the calibration equipment will be used and select the equipment accordingly.

Electrical equipment for explosive atmospheres according to the IEC 60079-0 standard is divided into the following groups:
Group I – Electrical equipment in Group I is intended for use in mines susceptible to firedamp.
Group II – Electrical equipment in Group II is intended for use in places with an explosive gas atmosphere other than mines susceptible to firedamp. Electrical equipment in Group II is subdivided according to the nature of the explosive gas atmosphere for which it is intended.
Group II subdivisions include:
   • IIA, a typical gas is propane
   • IIB, a typical gas is ethylene
   • IIC, a typical gas is hydrogen
This subdivision is based on the maximum experimental safe gap (MESG) or the minimum ignition current ratio (MIC ratio) of the explosive gas atmosphere in which the equipment may be installed (see IEC 60079-20-1).
Equipment marked IIB is suitable for applications requiring Group IIA equipment. Similarly, equipment marked IIC is suitable for applications requiring Group IIA or Group IIB equipment.
Group III – Electrical equipment in Group III is intended for use in places with an explosive dust atmosphere other than mines susceptible to firedamp. Electrical equipment in Group III is subdivided according to the nature of the explosive dust atmosphere for which it is intended.
Group III subdivisions:
   • IIIA: combustible flyings
   • IIIB: non-conductive dust
   • IIIC: conductive dust

Equipment marked IIIB is suitable for applications requiring Group IIIA equipment. Similarly, equipment marked IIIC is suitable for applications requiring Group IIIA or Group IIIB equipment.

Temperature class
The temperature class specifies the maximum surface temperature in the equipment. It is important that it matches the flammable gas that may be present in the plant’s own hazardous area. 
T1 - 450°C
T2 - 300°C
T3 - 200°C
T4 - 135°C
T5 - 100°C
T6 - 85°C
Some equipment may also have a maximum surface temperature specified as a certain temperature being in between the classes. Depending on the type of flammable substance in a certain area, the flashpoint and auto-ignition temperatures will be different. The equipment selected to be used in that hazardous area must have a temperature classification that suits the substances in question. The temperature class of a device will be included in its marking – for example ‘T4’.

North American differences
While in the IEC standard the hazardous areas are divided into zones, the North American system divides them into divisions. While numbers 0 to 2 are used in zones, numbers 1 and 2 are used in divisions. Zones 0 and 1 both are covered by Division 1. 
Zone 0  - Divison 1  - Area in which an explosive substance in the atmosphere is present all the time.
Zone 1 – Divison 1 - Area in which an explosive substance in the atmosphere is present in normal operation.
Zone 2 – Division 2 - Area in which an explosive substance in the atmosphere is present only in abnormal operation.

Environmental conditions
It is also important to ensure that the equipment is suitable for the environmental conditions in which it will be used. For example, the safe operating temperature of the device must match the temperature in which the equipment is used in a plant. In wet and dusty conditions, the protection rating of the equipment casing needs to be considered; this can be classified IP (Ingress Protection) or NEMA. Different protective techniques may require different classification on the casing.

It is also important to remember that the casing of some Ex equipment is made out of non-static (semi-conducting) material to avoid accumulation of any static electricity. Depending on the classification, there are limits on the size (static) of labels that can be put onto the device. For example, Group I equipment, for Zone 0, with gas Group IIC, may have a label sizing an area of maximum 4 cm2. It is important to keep this in mind before attaching any identification labels on Ex equipment. 


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