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Certification and Classification 

 

 
 

 

The North American system for the certification, installation, and inspection of hazardous locations equipment includes the following elements:

Installation Codes

(e.g. NEC, CEC)

Standard Developing Organizations (SDOs)

Austria
Belgium
Cyprus
Czech Republic
Denmark
Estonia
Finland
France
Germany
Greece
Hungary
Iceland
Ireland
Italy
Latvia
Lithuania
Luxembourg
Malta
Netherlands
Norway
Poland
Portugal
Slovakia
Slovenia
Spain
Sweden
Switzerland
United Kingdom

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ATEX
ATEX = ATmospheres EXplosibles

There are two European Directives that have been law since July 2003 that detail the manufacturers and users obligations regarding the design and use of apparatus in hazardous atmospheres.


The ATEX directives set the MINIMUM standards for both the employer and Manufacturer regarding explosive atmospheres. It is the responsibility of the employer to conduct an assessment of explosive risk and to take necessary measures to eliminate or reduce the risk.

ATEX Directive 94/9/EC article 100a
Article 100a describes the manufacturers responsibilities:

  • The requirements of equipment and protective systems intended for use in potentially explosive atmospheres (e.g. Gas Detectors) 
  • The requirements of safety and controlling devices intended for use outside of potentially explosive atmospheres but required for the safe functioning of equipment and protective systems (e.g. Controllers)
  • The Classification of Equipment Groups into Categories
  • The Essential Health and Safety Requirements (EHSRs). Relating to the design and construction of the equipment / systems 
In order to comply with the ATEX directive the equipment must:
  • display a CE mark
  • have the necessary hazardous area certification
  • meet a recognised performance standard e.g. EN 61779-1:2000 for flammable gas detectors

The classification of hazardous areas has been re-defined in the ATEX directive.

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Equipment Markings

 

 

 

 

 

 

 

 

 

 

ATEX Directive 1992/92/EC article 137
Article 137 describes the responsibilities of the employer. New plant must comply from July 2003. Existing plants must comply from July 2006. In the UK, this directive (also known as the ‘Use’ Directive) is implemented by the Health and Safety Executive (HSE) as The Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR).

 It sets out to:

 

 

 

 

 

 

 

 

 

Assessment of Explosion Risks
The employer must conduct a risk assessment including: 

1.  Probability of explosive atmosphere
     Zone Area classification

2.  Probability of ignition source
     Equipment Categories

3.  Nature of flammable materials
     Gas groups, ignition temperature (T rating), gas, vapour, mists
     and dusts

4.  Scale of effect of explosion
     Personnel, plant, environment

ATEX Additional Markings 

Explosive Atmospheres Warning Sign
The employer must mark points of entry to places where explosive atmospheres may occur with distinctive signs:

In carrying out the assessment of explosion risk the employer shall draw up an Explosion Protection Document that demonstrates:

  • explosion risks have been determined and assessed
  • measures will be taken to attain the aims of the directive
  • those places that have been classified into zones
  • those places where the minimum requirements will apply
  • that workplace and equipment are designed, operated and maintained with due regard for safety

 

 

 

 

 

 

 

The employer may combine existing explosion risk assessments, documents or equivalent reports produced under other community acts. This document must be revised with significant changes, extensions or conversions. 

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Area Classification
Not all areas of an industrial plant or site are considered to be equally hazardous. For instance, an underground coal mine is considered at all times to be an area of maximum risk, because some Methane gas can always be present. On the other hand, a factory where Methane is occasionally kept on site in storage tanks, would only be considered potentially hazardous in the area surrounding the tanks or any connecting pipework. In this case, it is only necessary to take precautions in those areas where a gas leakage could reasonably be expected to occur. 

In order to bring some regulatory control into the industry, therefore, certain areas (or ‘zones’) have been classified according to their perceived likelihood of hazard. The three zones are classified as:

ZONE 0
In which an explosive gas/ air mixture is continuously present, or present for long periods

ZONE 1 
In which an explosive gas/ air mixture is likely to occur in the normal operation of the plant 

ZONE 2 
In which an explosive gas/ air mixture is not likely to occur in normal operation In North America the classification most often used (NEC 500) includes only two classes, known as ‘divisions’.

Division 1 is equivalent to the two European Zones 0 and 1 combined, whilst Division 2 is approximately equivalent to Zone 2. 

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Apparatus Design


To ensure the safe operation of electrical equipment in flammable atmospheres, several design standards have now been introduced. These design standards have to be followed by the manufacturer of apparatus sold for use in a hazardous area and must be certified as meeting the standard appropriate to its use. Equally, the user is responsible for ensuring that only correctly designed equipment is used in the hazardous area. 

For gas detection equipment, the two most widely used classes of electrical safety design are ‘flameproof’ (sometimes known as ‘explosion-proof’ and with an identification symbol Ex d) and ‘intrinsically safe’ with the symbol Ex i.

Flameproof apparatus is designed so that its enclosure is sufficiently rugged to withstand an internal explosion of flammable gas without suffering damage. This could possibly result from the accidental ignition of an explosive fuel/ air mixture inside the equipment. The dimensions of any gaps in the flameproof case or box (e.g. a flange joint) must therefore be calculated so that a flame can not propagate through to the outside atmosphere. 

Intrinsically safe apparatus is designed so that the maximum internal energy of the apparatus and interconnecting wiring is kept below that which would be required to cause ignition by sparking or heating effects if there was an internal fault or a fault in any connected equipment. There are two types of intrinsic safety protection. The highest is Ex ia which is suitable for use in zone 0, 1 and 2 areas, and Ex ib which is suitable for use in zone 1 and 2 areas. Flameproof apparatus can only be used in Zone 1 or 2 areas. 

Increased safety (Ex e) is a method of protection in which additional procedures are applied to give extra security to electrical apparatus. It is suitable for equipment in which no parts can produce sparking or arcs or exceed the limiting temperature in normal service. 

A further standard, Encapsulation (Ex m) is a means of achieving safety by the encapsulation of various components or complete circuits. Some products now available, achieve safety certification by virtue of using a combination of safety designs for discrete parts. Eg. Ex e for terminal chambers, Ex i for circuit housings, Ex m for encapsulated electronic components and Ex d for chambers that could contain a hazardous gas. 

Hazardous Area Design Standards


Ex s is not used in the latest standards but may be found on older equipment still in use.

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Apparatus Classification
As an aid to the selection of apparatus for safe use in different environmental conditions, two designations, apparatus group and temperature classification, are now widely used to define their limitations.

As defined by standard No EN50014 of the European Committee for Electrical Standards (i.e. Comite Europeen de Normalisation Electrotechnique or CENELEC), equipment for use in potentially explosive atmospheres is divided into two apparatus groups: 

Group I
- for mines which are susceptible to firedamp (Methane) 

Group II
- for places with a potentially explosive atmosphere, other than 
  Group I  mines
 

Group II clearly covers a wide range of potentially explosive atmospheres and includes many gases or vapours that constitute widely different degrees of hazard. Therefore, in order to separate more clearly the differing design features required when used in a particular gas or vapour, Group II gases are sub-divided as indicated in the table. Acetylene is often considered to be so unstable that it is listed separately, although still included in Group II gases. A more comprehensive listing of gases can be found in European Standard EN50014. 

The Temperature Class rating for safety equipment is also very important in the selection of devices to detect gas or mixture of gases. (In a mixture of gases, it is always advisable to take the ‘worst case’ of any of the gases in the mixture). Temperature classification relates to the maximum surface temperature which can be allowed for a piece of apparatus. This is to ensure that it does not exceed the ignition temperature of the gases or vapours with which it comes into contact.

The range varies from T1 (450°C) down to T6 (85°C). Certified apparatus is tested in accordance with the specified gases or vapours in which it can be used. Both the apparatus group and the temperature classification are then indicated on the safety certificate and on the apparatus itself. 

North America and the IEC are consistent in their temperature or T-Codes. However unlike the IEC, North America includes incremental values as shown opposite.

Temperature Class

 

Apparatus Group

 

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Ingress Protection of Enclosures
Coded classifications are now widely used to indicate the degree of protection given by an enclosure against entry of liquids and solid materials. This classification also covers the protection of persons against contact with any live or moving parts inside the enclosure. It should be remembered that this is supplementary to and not an alternative to the protection classifications for electrical equipment used in hazardous areas. 

In Europe the designation used to indicate the Ingress Protection consists of the letters IP followed by two ‘Characteristic Numbers’ which indicate the degree of protection. The first number indicates the degree of protection for persons against contact with live or moving parts inside, and the second number shows the enclosure’s protection against entry of water. For example, an enclosure with a rating of IP65 would give complete protection against touching live or moving parts, no ingress of dust, and would be protected against entry from water spray or jet. This would be suitable for use with gas detection equipment such as controllers, but care should be taken to ensure adequate cooling of the electronics. The two digit IP rating is a short form more commonly used in Britain. The full international version has three digits after the IP rather than two, e.g. “IP653”. The third digit is impact resistance. The meanings of the numbers are given in the following table. 

IP codes (IEC / EN 60529)

In North America enclosures are rated using the NEMA system. The table below provides an approximate comparison of NEMA ratings with IP ratings. 

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Safety Integrity Levels (SIL)
Certification has essentially been concerned with the safety of a product in its working environment i.e. that it won’t create a hazard in its own right. The certification process (particularly in Europe with the introduction of the ATEX standard pertaining to Safety Related Devices) has now moved on to also include the measurement/physical performance of the product. SIL adds a further dimension by being concerned with the safety of the product in terms of being able to carry out its safety function when called to do so (ref: IEC 61508 manufacturers requirement). This is increasingly being demanded as installation designers and operators are required to design and document their Safety Instrumented Systems (ref: IEC 61511 user’s requirement). 

Individual standards applicable to specific types of equipment are being developed from IEC61508. For gas detection equipment the relevant standard is EN50402:2005 Electrical apparatus for the detection and measurement of combustible or toxic gases or vapours or of Oxygen. 

Requirements on the functional safety of fixed gas detection systems
Managing safety is about risk reduction. All processes have a risk factor. The aim is to reduce the risk to 0%. Realistically, this is not possible so an acceptable risk level that is ‘As Low As Reasonably Practical’ (ALARP) is set. Safe plant design and specification is the major risk reduction factor. Safe operational procedures further reduce the risk as does a comprehensive maintenance regime. The E/E/PES (Electrical/Electronic/Programmable Electronic System) is the last line of defence in the prevention of accidents. SIL is a quantifiable measure of safety capability of the E/E/PES. In typical applications, this relates to the F&G systems- detectors, logic resolvers and safety actuation/annunciation. 

It is recognised that all equipment has failure modes. The key aspect is to be able to detect when the failures have occurred and take appropriate action. In some systems, redundancy can be applied to retain a function. In others, self checking can be employed to the same effect. The major design aim is to avoid a situation where a fault which prevents the system carrying out its safety function goes undetected. There is a critical distinction between reliability and safety. A product which appears to be reliable may have unrevealed failure modes whereas a piece of equipment which appears to declare a large number of faults may be safer as it is never/rarely in a condition where it is unable to do its function or has failed to annunciate its inability to do so.

There are 4 levels of SIL defined. In general, the higher the SIL, the greater the number of failure modes that are accommodated. For Fire and Gas systems the levels are defined in terms of “average probability of failure to perform the intended function on demand”. 

Many current fire and gas detection products were designed long before the introduction of SIL and therefore on individual assessment may only achieve a low or no SIL rating. This problem can be overcome by techniques such as decreasing the proof test intervals or combining systems with different technologies (and hence eliminating common mode failures) to increase the effective SIL rating. 

For a safety system to achieve a specified SIL, the sum of the PFDavg must be considered. 

For SIL 2 
PDF (Sensor) + PFD (Resolver) + PDF (Actuator) < 1x10-2  

The selection of SIL required for the installation must be made in conjunction with the level of safety management within the design of the process itself. The E/E/PES should not be considered the primary safety system. Design, operation and maintenance have the most significant combination to the safety of any industrial process.

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 Gas Detection Explained

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