A term commonly used in the hazardous area industry is “intrinsic safety” or, when used in reference to a piece of equipment, “intrinsically safe”. But what does it mean? Is intrinsically safe equipment the same as hazardous area equipment? In this article, we explore what intrinsic safety means and why it’s important.
Intrinsically safe – understanding what it means
Intrinsically Safe is a technique used for the monitor and control of low power instrumentation, valves, solenoids, switches and LED’s located in hazardous areas. The purpose in the design of intrinsic safety is to provide low power circuits to the hazardous area that does not contain enough energy to ignite the hazardous atmosphere, whether containing gases or dusts. It is the only technique that can be maintained live unlike other technologies used for high power equipment, such as Exe & Exd. With these, complete removal of energy sources are required before accessing any equipment located in the hazardous area.
Different gas groups have different levels of volatility and, with oxygen, can ignite via a heated surface or energy spark. Hazardous area zones can use different technologies for gasses and dusts and are categorised by the level of risk as Zone 0/1/2 (Gas) or Zone 21/22 (Dust), with 0 being the highest danger and 2 the lowest. Always attempt to remove the potential hazard by design or construction techniques first.
Hazardous area zones
Zone 0 is an area in which an explosive atmosphere is present continuously for long periods of time or will frequently occur.
Zone 1 is an area in which an explosive atmosphere is likely to occur occasionally in normal operation. It may exist because of repair, maintenance operations, or leakage.
Zone 2 is a place in which an explosive atmosphere is not likely to occur in normal operation but, if it does occur, will persist for a short period only. These areas only become hazardous in case of an accident or some unusual operating condition.
Group IIC such as Hydrogen has the lowest energy ignition level, while Group 1 gas such as Methane has a higher energy ignition level so there is different products designed for differing gas groups.
Intrinsically safe components
Common parts used in intrinsically safe circuits are transformer isolated barriers. These are normally mounted in the safe area with the I.S (intrinsically safe) side wiring from the blue terminals to the field. They provide isolation between the safe and hazardous areas and are designed for the purpose of their function e.g.: A digital input isolator will accept digital signals from the field and repeat the digital status in the safe area via a relay or transistor output, depending on switching speed. Others are Digital Outputs (DO), Analogue Inputs (AI), Analogue Outputs (AO) and others for temperature, RS485, and other purposely designed products.
Common Field sensors such as switches can be mechanical, whereas all electronic devices such as proximity switches, transmitters, valves and solenoids should have an IECEx approval for their use in a hazardous area. Proximity sensors for use in hazardous areas most likely will have a Namur output following the Namur standard for 2 wire operation. This, used in conjunction with a DI TIB (Digital Input Transformer Isolated Barrier), can also detect lead breakage and short circuits in the field. Analogue sensors such as transmitters for measuring temperature, flow and pressure will either use a 2 wire or 3 wire method and require a TIB to repeat the analogue signal on the safe side.
When matching a field device to a TIB, the field device’s maximum parameters such as voltage, current and power should be higher than the maximum parameters of the TIB. If not, the TIB could overload the field device and cause a hazard.