Intrinsic Safety & Thermal Imaging Part 2

Last month, we reviewed the origins of the need for and the standards surrounding electrical safety ratings as well as some of the terminology related to the topic. This month, we delve into design considerations and address some of the most common...


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A key point of understanding is that Class, Division and Group all refer to the environment or location. They do not refer to the device carried into that environment or location. Quite simply, this part of the standard was intended to identify the level of threat posed by a particular environment or location only after a proper risk assessment has occurred. These designations are geographically specific and should not be generalized. You cannot say that all metal fabrication shops are Class I, Division 1 locations, since some may use non-volatile chemicals while others use a fully automated and contained treatment process. Many times, particularly in industrial settings, these locations will be placarded for easy identification.

 

Product design and electrical protection

Manufacturers must consider a multitude of design requirements based on the environment for which the device is intended, the use for which is intended and what may happen under failure mode.

Several design options can be used when designing for hazardous locations. Most commonly, the overriding concern in designing for these types of locations is reduction of sparks and thermal hazards. Thermal hazards are best described as heat generated by the components within the device itself under normal operating conditions. The easiest way to limit this potential is to limit the amount of energy available in the first place. In most battery-operated devices, this places design focus on the power source – the battery itself.

Increasing consumer demand for smaller, more powerful batteries that last longer has led to advancements in battery design and chemistry that yield large amounts of power in small packages. Lithium-ion and lithium-polymer are two examples. Consumers prefer lithium-based rechargeable batteries because they produce the same amount of energy as other battery types, but at a fraction of the size and weight. However, as a result of this improved performance, lithium-based batteries are capable of producing a large amount of energy. Should this power escape the circuit in any way, a spark or thermal hazard can be generated quickly.

Of secondary concern when designing for hazardous locations is the operating temperature of the device circuitry. There are times, however, when limiting the available energy is not an option. Large, petrochemical facilities use very large pumps that require very large amounts of electricity to operate. How do you make that safe? Many people are surprised to learn that explosion containment is perfectly acceptable for Class I, Division 1 environments. In this design approach, an explosion is allowed to occur, but the explosion itself must be contained within the device and any gases generated by the explosion must be cooled before being vented.

After these design considerations, attention must move to operating conditions of the device. In the same way that, under the NEC, each environmental classification contains two divisions (one for normally present and one for abnormally present), there are two equipment operating conditions that must be considered:

1. Normal operation

2. Failure mode or fault condition

Normal operation would mean the device is not capable of generating enough energy to ignite an explosive atmosphere under normal operating conditions. Power limitation, sealed switches, electrical componentry, safety circuits and many other strategies are employed to prevent a thermal hazard from occurring at any point within normal operation. Devices meeting this requirement are typically referred to as “Non-Incendive.”

Failure mode means the device is incapable of generating a spark or thermal hazard, even when the device itself has failed in a catastrophic manner. In battery-powered devices, this is commonly focused on ground fault. What happens when the power source is short-circuited? Devices meeting this requirement are typically referred to as “Intrinsically Safe.”

 

It gets confusing from there

Many things add to the confusion and blur the lines between reality and perception. First and foremost is terminology. In an effort to simplify an otherwise complicated topic, many people use generic terms like ”Intrinsically Safe” interchangeably between device and environment, as well as inclusively for various classes and divisions of locations.