In last month’s installment we looked into the different types of explosion protection techniques which can be applied to electrical systems and electronic devices to make them safe for use in hazardous areas (explosive atmospheres).

This month we’ll be diving into Intrinsic safety (IS). Intrinsic Safety is a protection technique used in hazardous areas to prevent ignition of flammable gases, vapors, dusts, or fibers by limiting the electrical and thermal energy in a circuit to levels below what can cause ignition. There is some misconception in the industry as some people refer to areas of plant as intrinsically safe. This is technically inaccurate as intrinsic safety is just one of the various different types of explosion protection which allows for electronics and electrical systems to be used in hazardous areas.

There are several "types" or more accurately "levels" of intrinsic safety defined by IEC 60079-11 and adopted worldwide (ATEX, NEC/CEC, etc.). These varying levels are referred to as “protection levels (ia, ib, ic).”

The below table provides more information about the different IS protection levels:

Intrinsic Safety Protection Levels (ia, ib, ic)

As intrinsic safety is all about the limitation of electrical energy in the circuits of a product or system, there are certain components that must be present in order to provide the necessary protection. These include voltage clamping zener diodes, current limiting resistors, fuses and even transformers in order to provide galvanic isolation between non-IS and IS circuits.

Within process control systems, instrumentation is often certified as intrinsically safe, in order to meet the requirements and have the necessary energy limiting protection in place, additional devices know as IS barriers or galvanic isolators are needed. These are devices that are installed in typically a safe (non-hazardous) area and include the energy limiting components which limit how much electrical energy can possibly pass out into the hazardous area and thus onto the intrinsically safe field instruments.

Here is an example of an IS instrumentation system typically used within process control systems:

Intrinsic safety (IS) is widely used wherever electrical/electronic equipment must operate in hazardous (explosive) areas but there are certain use cases or applications where IS provides significant advantages over other forms of explosion protection.

Some of the advantages of IS are as follows:

• Ignition is physically impossible - Ex ia is the only technique that guarantees the circuit cannot release enough energy to ignite the explosive atmosphere even when any two independent faults occur simultaneously (e.g., short-circuit + open-circuit + component failure). All other methods allow ignition or explosion to occur and then try to contain or prevent propagation.
• No Contained Explosion Ever Occurs. Explosion Proof or Flameproof (Ex d) boxes are designed to withstand an internal explosion and cool the flame so to prevent the external flammable atmosphere from igniting, this means a lot of caution and maintenance is involved with ensuring the mechanical structural integrity remains in tact. A small scratch to the flame path on the flange of an explosion proof enclosure creates a serious safety issue. Intrinsic safety: no explosion, no hot gases, no pressure wave — nothing to contain.
• Safe Even When the Enclosure Is Opened (Live Working Permitted) With Ex ia, you can open the cover, connect a HART communicator, or replace a sensor while powered in the explosive atmosphere without a gas-clearance or hot-work permit (in most jurisdictions). No other method allows live working in Zone 0 or 1 without special procedures.
• No Single Point of Failure Can Cause Catastrophe. Flameproof: one cracked conduit seal or loose bolt → catastrophic failure possible. Pressurization: one pressure switch failure → entire enclosure becomes unsafe. IS: even if the barrier is damaged and multiple field faults occur, energy is still too low to ignite.

Image below shows a scratch to the flame path of an explosion proof enclosure creating serious safety issue: