A Guide to Intrinsic Safety
When selecting equipment to use in potentially hazardous locations, things can get complicated quickly. Today we will be discussing the different standards for intrinsic safety as well as many terms that are common to the industry. The scope of this application note will cover:
Methods of Intrinsic Safety
North American vs. International Standards
1. Methods of Intrinsic Safety
To prevent the ignition of vapors in a hazardous location, there are three different strategies used by the industry.
- Containment (Explosion-Proof)
Containment is the only method where an explosion is permitted. The source of ignition is surrounded by an enclosure designed to contain the explosion to a defined area. This is what is meant by an “explosion-proof” enclosure. The safety of this method is entirely dependent on the mechanical integrity of the enclosure, so periodic inspections are necessary. Some handheld test instrument manufacturers may use the term “explosion-proof” when they mean “intrinsically-safe”. Strictly by definition, this would insinuate that the instrument itself could explode internally instead of being designed not to ignite an explosion!
Segregation is a method used to separate the ignition source from hazardous material using isolation techniques. Some of these techniques include using pressure differences to remove the hazardous material from the ignition source, or by submerging the ignition source in an insulative oil, powder, or resin.
Prevention is the method where intrinsically safe practices and engineering are used to design devices to be used in the hazardous environment so that these devices are virtually incapable of having enough energy stored-up that could cause a spark that would ignite the hazardous material. This method is the preferred one due to the ignition source being omitted completely and Prevention will be the focus of the remainder of this paper.
2. NEC vs. ATEX
There are currently two standards in use worldwide that address intrinsically safe practices. In the United States and Canada, this standard is defined by the NEC (Section 500). For the EU, the standard is defined by ATEX (2014/34/EU). For the rest of the world, the standard is defined by IECEx (IEC 60079) which shares most of its terminology with the ATEX directive.
The NEC standard uses a system of classes and divisions to define hazardous areas while the ATEX standard uses a zone system.
The ATEX/IECEx standard was added to the NEC as Section 505 in 1996 for international compatibility. The main difference between the standards is how they define the hazardous areas.
3. Class/Division System (NEC Section 500)
NEC Section 500 organizes hazardous areas into classes, divisions, groups, and the temperature of the electrical device.
Class – This refers to the properties of the environment, specifically if it contains a hazardous vapor, dust, or fiber.
Class I – Flammable gases or vapors are present
Class II – Dust with the potential to combust is present
Class III – ignitable fibers are present
Division – This refers to the likelihood of the hazardous substance producing an explosion if ignited.
Division 1 – There is a high probability of an explosion in the area if ignited
Division 2 – There is a low probability of an explosion in the area if ignited
Group – This refers to the flammability properties of hazardous material in the environment.
The groups are organized in accordance to their maximum experimental safe gap (MESG) and the minimum igniting current ratio (MIC).
MESG is a standardized measurement of how easily a gas flame will pass through a narrow gap.
MIC is the ratio of the minimum current required from an inductive spark to ignite the hazardous material, divided by the minimum current required from an inductive spark to ignite methane.
Group A – The environment contains acetylene
Group B – The environment has a mixture of hazardous gas or vapors with an MESG of less than 0.45 mm (excluding acetylene) or a MIC ratio of 0.40
Group C – The environment has a mixture of hazardous gas or vapors with an MESG greater than 0.45 mm but less than or equal to 0.75, or a MIC ratio greater than 0.40 but less than or equal to 0.80
Group D – The environment has a mixture of hazardous gas or vapors with an MESG greater than 0.75 mm or a MIC ratio greater than 0.80
The following chart illustrates how the maximum experimental safe gap (MESG) and the minimum igniting current ratio (MIC) relate to the gas group:
Group E – The environment contains combustible metal dusts
Group F – The environment contains combustible carbonaceous dusts
Group G– The environment contains combustible dusts not mentioned in Group E or Group F
Temperature – This refers to the maximum surface temperature of any parts of the electrical equipment to be used in the hazardous environment.
T1 – 450 °C
T2 – 300 °C
T3 – 200 °C
T4 – 135 °C
T5 – 100 °C
T6 – 85 °C
4. Zone System (IECEx/ATEX/NEC Section 505)
The IECEx/ATEX systems, also defined in NEC Section 505, organize hazardous areas into zones and groups of electrical devices.
Zone – This refers to the likelihood of the hazardous substance to be concentrated in the environment enough that it is explosive.
Zone 0 – The explosive concentration is present in the environment constantly or for long periods of time
Zone 1 – The explosive concentration is present in the environment during normal operating conditions
Zone 2 – The explosive concentration is present in the environment but not likely to occur during normal operating condition and if it does, it is for a short amount of time
Zone 20 – The explosive dust or fiber is present in the environment constantly or for long periods of time
Zone 21 – The explosive dust or fiber is present in the environment during normal operating conditions
Zone 22 – The explosive dust or fiber is present in the environment but not likely to occur during the normal operating condition and if it does, it is for a short amount of time
Group – This refers to electrical equipment, which is organized into three categories.
Group I – Equipment for use in mines
Group II – Equipment for use in explosive environments (other than mines) that contain flammable gas
Group IIA – Environment contains propane or flammable gas with similar properties
Group IIB – Environment contains ethylene or flammable gas with similar properties
Group IIC – Environment contains acetylene, hydrogen, or flammable gas with similar properties
Group III – Equipment for use in explosive environments that contain flammable dust
Group IIIA – Environment contains flammable flyings
Group IIIB – Environment contains non-conductive dust
Group IIIC – Environment contains conductive dust
Protection Concept – This terminology refers to the high-level protection classification of the electrical instrument used in the environment. Intrinsically safe equipment will be marked with “Ex i” but be aware that there are many different types of high-level protection concepts outside the scope of this paper. Some examples of protection codes used for electronics are listed below.
(Click table to enlarge)
Each system specifies a way to mark equipment in order to designate what type of hazardous environment the equipment is designed to be operated within.
Approved equipment is marked with the class, division, group, and the temperature code it is rated for.
Class I, Division 1, Group C and D, T4
CL I Div 1 GP CD T4
Approved Equipment is marked with the high-level protection concept the equipment was designed for, its group, and the temperature code.
Ex ia IIB T4
Class I Zone 0 iEx IIB T4
6. For More Information
TEGAM offers a variety of intrinsically safe test & measurement equipment solutions for various measurement applications within aerospace, industrial, and research scenarios – including our single-channel intrinsically safe thermocouple thermometer, a dual-channel I.S. thermocouple thermometer, an I.S. air/gas temperature probe, I.S. high-temperature surface probe, and our intrinsically safe milli-ohmmeter & bond meter.
You can also reach-out and contact TEGAM for more individual assistance as needed via phone or form – we look forward to being of assistance. Thanks for reading!
Richard Steiner, Applications Engineer, TEGAM