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 confusion points so you will be better informed when making purchasing decisions

The concept of standard development begins with considering the specific hazard or hazards the standard is being developed to address. In the case of hazardous locations, explosion was the primary hazard of concern; however, the wide variation in explosive materials, ignition temperatures and materials concentrations caused standards writers to classify hazardous locations based on empirically derived data that combined the various environmental factors.

In North America, the National Electrical Code (NEC) has traditionally used a Class/Division/Group designation to identify the potential of the atmosphere, and conversely, the level of protection that must be employed by electrical devices installed in or introduced to that location. Under the NEC, there are three classes, two divisions per class and a total of seven groups (four groups in Class I and three groups in Class II). These break down as shown in the accompanying chart.

In addition, there are 14 temperature classifications for approved devices in the U.S. under NEC 500. Temperature classifications indicate the highest allowable operating temperature for any individual component used within the device. For example, a device may have been tested to a Class II, Division 1, Groups E, F, G, T3. T3 indicates the maximum component surface temperature of 200 degrees Celsius (392 degrees Fahrenheit) for the device when operating within its approved ambient temperature rating (if no ambient temperature range is indicated, it is understood to be from -20oC to 40oC). This temperature classification indicates the maximum thermal hazard temperature for its rated gas or dust groups. Under certain circumstances, coal dust has a minimum ignition temperature as low as 160oC (320oF), so this device would not be suitable for this particular explosive atmosphere. (See http://www.msha.gov/S&HINFO/TECHRPT/P&T/COALDUST.pdf.)

What this means is that hundreds of combinations of designators are used to describe the hazardous location; thus, an infinite number of levels of electrical safety certifications. Generic terms such as “Intrinsically Safe” or “Non-Incendive” are insufficient to describe the appropriateness of certification to environment.

 

Assessing risk

To understand what protection level is necessary and what safety margin is provided, one must first perform a risk assessment. This does not need to be complicated or difficult; however, consideration is necessary as costs can rise and performance can decrease as you move up the spectrum.

Classes are fairly straightforward. The NEC, National Fire Protection Association (NFPA) and Canadian Electrical Code (CEC) all define classes by the ignitable substance that is or can become present. What is the primary hazard from which you expect protection? Usually, those of us in the fire service are concerned about gases, so this places the focus on Class I locations.

Divisions are where confusion enters the picture. In a Division 1 location, the substance or hazard is present or can become present under normal operating conditions within the location. An example may be a metal coating or treatment facility where metal objects are placed in an open container or vat containing flammable chemicals. In this case, the potential for atmosphere to reach the lower explosive limit (LEL) for those particular chemicals is normally present. In a Division 2 location, the worker is typically isolated from the dangerous chemicals and would only be exposed should there be a rupture of containment, failure of equipment or other abnormal condition.

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.

Adding to the confusion is the fact that the Class/Division/Group designations are valid only in North America. Elsewhere, the International Electrotechnical Commission (IEC), European Committee for Electrotechnical Standardization (CENELEC) and others have moved to a system that consists of Zones, Gas Groups, Equipment Protection Level and Temperature Classification to describe hazards, environments and equipment. While there are some direct correlations between the IEC and NEC standards, there are also IEC approvals for which there is no direct NEC approval. The push is on for harmonization of these two codes, and it appears that the IEC system will triumph, signaling that the end of the NEC system is likely in sight. In support of harmonization, North American standards will allow for NEC or IEC classifications of locations and certifications of devices.

To further complicate matters, under the CENELEC standard, “Intrinsically Safe” refers to any device that is Class I, Division 2 and above. Several manufacturers of electronic and battery-powered devices advertise them as “Intrinsically Safe,” but in some cases they are combining the technical accuracy of a European approval with the common vernacular of the North American user. In this case, the customer is thinking Class I, Division 1 and the actual certification is technically equivalent to a Class I, Division 2 rating.

 

Conclusion

Caveat emptor! You must ask the technical detail of any stated rating. The buyer must decide what level of protection is desired. To what environments will the firefighter be exposed? To what environmental conditions will the firefighter be exposed?

The topic is technical and confusing, but if I was a hazmat technician preparing to enter a “hot zone” to address a leaking pressurized gas cylinder, I would want to know what my devices are certified for. Generic terms would not cut it for me and they should not cut it for you. n

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