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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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.

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