WITH R. STAHL
Stop sparks from flying
Where combustible substances, oxygen and a source of ignition meet, there is a risk that they will cause an explosion. The intrinsic safety type of protection is designed to prevent this exact scenario by eliminating one element of this explosive trio. In an intrinsically safe circuit, the amount of energy is reduced to a non-ignitable extent, which therefore eliminates the source of ignition in the form of sparks and thermal effects. This prevents ignition without interrupting operation or, depending on the safety level, in the event of a fault.
Over 50 years ago, the first intrinsically safe field devices and associated safety barriers were used for the first time. Since then, the variety and versatility of applications has continued to grow. Now, the intrinsic safety degree of protection (Ex i) is widely used – explosion protection is unimaginable without it. This type of protection covers a wide range of applications and is currently used in systems ranging from simple point-to-point wiring to digital automation for Industry 4.0. Ex i-protected solutions enable direct process data capture in hazardous areas, reduce installation work and can be tested and maintained during ongoing operation.
For years, intrinsically safe fieldbus versions have been primarily used to network remote I/O devices, operating terminals and analysis equipment. In addition, Ex i-protected fieldbus installations with PROFIBUS PA or Foundation Fieldbus H1 have proven to be useful for digital communication up to field devices. These developments didn't stop at wireless technology, either – in this sector, intrinsic safety allows more flexible use of antennae. Currently, the trend is heading towards intrinsically safe solutions for Industrial Ethernet. To drive the development of internationally accepted intrinsically safe Ethernet concepts, R. STAHL also actively participates in the Advanced Physical Layer (APL) project and the Intrinsically Safe Ethernet Working Group. Here, you can read more about intrinsic safety, seminars and services in this sector, and R. STAHL's Ex i interface solutions.
THE ADVANTAGE OF INTRINSIC SAFETY
Compared to other common types of protection, such as "Ex d" flameproof enclosures and "Ex p" pressurized enclosures, the intrinsic safety type of protection does not require any special enclosures that prevent an explosion that occurs inside the enclosure from being transmitted to the outside atmosphere. In this sense, this type of protection always comprises a combination of current/voltage sources, cables and consumers, which are typically used in hazardous areas. This achieves very high levels of safety, allowing corresponding designs to be implemented even in extremely hazardous areas in Zone 0.
Ex i-protected devices are extremely practical in terms of their handling during ongoing operation. Extensions and repairs can be performed on live equipment in hazardous areas (hot work) and devices can be added or separated without having to shut down the system or parts of the system entirely (hot swap). This is especially beneficial when troubleshooting problems or swapping out devices.
Thanks to its simple handling, extremely high level of protection and versatile installation options, intrinsic safety is an essential type of protection for process automation and data communication in hazardous areas.
The ignition limit curve – the measure for intrinsically safe circuits
The maximum permissible values for ensuring intrinsic safety for voltage (U) and current (I) mentioned previously have been determined experimentally. They are determined using ignition limit curves – a curve exists for each explosion group. An electrical circuit is considered to be intrinsically safe if the pair of values made up of the voltage and current are underneath the ignition limit curve for the relevant explosive atmosphere. For use in Zone 1 or Zone 0, the permissible value is reduced by an additional safety factor of 1.5.
Example: In Zone 1 or 0, the permissible value must be reduced by the safety factor of 1.5. If equipment is running in Ex group IIC and in Zone 1, it can be supplied with a maximum of 102 mA.
For use in Zone 0, a device with intrinsically safe circuits must comply with these current and voltage values even if two errors occur. For circuits in Zone 1, this applies if one error occurs. For use in Zone 2, it is sufficient for these values to be complied with during normal operation.
Depending on the intended application range (Zone 0, 1 or 2), the letter a, b or c is added to the Ex i designation. The same addition also indicates the Equipment Protection Level (EPL) for equipment approved for use in Zone 0, 1 or 2. The EPL, which is written as Ga, Gb or Gc, is indicated in the EC Type Examination Certificate and on the device's rating plate.
|Application area||EPL||Safety level||Without ignition||Intrinsic safety|
|Zone 2||Gc||normal||during normal operation||ic|
|Zone 1||Gb||high||during normal operation and with one fault||ib|
|Zone 0||Ga||very high||during normal operation and with two faults||ia|