EX Technology – Designing equipment to be used in hazardous areas
When designing electronic equipment for industrial environments such as oil rigs or chemical production facilities, the design has to be safe to use within potentially explosive atmospheres. What are the requirements and how do we meet them?
What are “Hazardous Areas”?
Hazardous areas are defined as places where concentrations of flammable gases, vapors, or dusts may occur. Electrical equipment that is installed in such locations must be specifically designed, tested and certified to ensure it does not ignite an explosion.
Enterprises handling flammable gases, liquids or dusts, are obliged to consider the explosion risk and perform a zone classification according to the Norwegian guidelines of ATEX-directive 99/92/EC. The potentially hazardous areas are divided into 3 different zones, determined by the likelihood of the presence of an explosive atmosphere. Each zone has its own rules for applying protection methods and precautions.
One distinguishes between two types of explosions: gas explosion and dust explosions. For gas explosions, the zone classification is as follows:
An area where an explosive gas atmosphere is present continuously, or for long periods.
An area where an explosive gas atmosphere is likely to occur in normal operation.
An area where an explosive gas atmosphere is not likely to occur in normal operation, and if it occurs, will only exist for a short amount of time.
For gas explosions, there are three different gas groups (IIA, IIB and IIC), dependent on the spark energy sufficient for ignition. There are also six different temperature classes (T1-T6), dependent on the surface temperatures sufficient to cause an explosion in the flammable atmosphere.
How to avoid explosion
In general, there are three different methods to prevent explosions:
Prevent an explosive atmosphere from being formed. Prevent flames from expanding.Prevent sparks, discharges and self-ignition.
These three methods represent three different design approaches. The first approach is an environmental approach, for example by assuring sufficient ventilation or by adding an inert gas to the atmosphere of the installation. The second approach is to find appropriate ways to encapsulate the equipment in the installation, preventing it from interacting with the explosive atmosphere. The third and last approach is to construct equipment that is designed not to behave in a hazardous way.
For electronics designers, the most important ignition sources to be aware of are surface heating and electrical sparks. However, there are also other ignition sources to be aware of, like mechanical sparks (from moving parts coming into contact), static electricity, ground currents, electromagnetic fields and electromagnetic radiation (both ionizing and non-ionizing).
Intrinsically Safe Design
Intrinsically safe equipment is composed of intrinsically safe circuits that neither give sparks energetic enough to ignite an explosion, nor do they consume power high enough to heat surfaces to a dangerous level. An intrinsically safe circuit is safe by design, and in principle it does not need any additional protection.
Intrinsically safe circuits must also be provided with clearly defined input and output parameters, allowing the user to calculate which combinations of other circuits are acceptable, and which are not.
When designing intrinsically safe circuits, power consumption must be kept to a minimum to prevent surface heating. The power consumption of a device is typically kept under control by using current limiting Zener barriers. However, even with a very low power consumption, components like inductors and capacitors may store energy sufficient to give sparks. All nodes in the schematics therefore have to be analyzed using their worst-case values plus safety factors. Capacitors and inductors storing too much energy must either be reduced in value, or a resistor must be inserted in series to increase the discharge time constant of the node, which reduces the effective capacitance or inductance.
The required level of intrinsic safety, also called Equipment Protection Level, is determined by the zone where the electronics are to be installed. Installations in Zone 0 are required to be explosion proof even when there are two failures at the same time (Intrinsic level ‘ia’). Zone 1 installations must remain explosion proof during a single failure (Intrinsic level ‘ib’), and Zone 2 installations must remain explosion proof during normal operation (Intrinsic level ‘ic’). Zone 0 and Zone 1 requirements can be fulfilled by inserting redundant circuits, for example by using three zener diodes in parallel for an Intrinsic level ‘ia’ Zener barrier.
Ex certificationEquipment to be used in potentially explosive areas need to be certified according to the relevant standards. In North America, the Ex standards are defined in ANSI/NFPA 70 NEC Article 500 or Article 505. In Europe, the Ex standards are defined by IEC 60079. The ATEX directive 94/9/EC obliges all EU member states to incorporate this standard in their own national standards, and Norsk Elektroteknisk Komite has harmonized the IEC 60079 into NEK EN 60079. Equipment that is to be installed in hazardous areas must comply with the general standard IEC 60079-0. Depending on the Equipment Protection Level and methods used, equipment shall in addition comply with one or more sub-standards (i.e. IEC 60079-11 for intrinsic safety).