Differences Between Division and Zone Ratings

What Are the Differences Between Division and Zone Ratings for Hazard Locations?

Industrial manufacturing, petrochemical, and mining operations have inherent dangers. That's one reason safety managers and engineering departments must work hand-in-hand to ensure worker safety.

While fall protection, respiratory protection, and other forms of life-health issues are readily addressed, don't forget the hazard locations and their problems, which can sometimes be complex. With classified hazard locations, your decisions affect fire and explosion dangers.

An understanding of the hazard location system is crucial when designing and engineering workplace areas. The type of electrical equipment used in these locations is governed by the hazard zone location information. We'll touch more on that in a minute.

There are two hazard zone location systems used in industry:

  • The Class/Division/Group system is used in many United States and Canadian industries
  • The Zone system is used by other countries internationally.

In both systems, hazard locations are defined or classified by the flammable materials existing in the workplace area's atmosphere. The Class system used by the United States and Canada is based on the NEC (National Electrical Code) and CEC (Canadian Electrical Code) respectively.

The Zone system was developed by the IEC (International Electrochemical Commission).

The Classes are divided into three types of flammable/explosive hazards:

  • Areas with flammable gases and vapors are Class I locations in sufficient amounts to produce explosive or ignitable mixtures.
  • In a Class II location, the atmosphere contains sufficient amounts of combustible dust that may contribute to fire or explosion.
  • A Class III location contains easily ignitable fibers or flyings.

In the North American-based Hazard Location standards, the Classes are further defined using a Division structure, followed by a Group designation.

Groups are defined or listed by NFPA 70E (NEC®) by gases, vapors, and dust as pertaining to their ignitable or combustible properties when mixed with air. The group definition also includes the MESG or maximum experimental safe gap properties.

MESG is defined as a standardized measurement of how easily a gas flame will pass through a narrow gap that is bordered by a heat-absorbing metal.

Examples for this are:

  • Class I Group A – acetylene
  • Class I Group B – hydrogen
  • Class I Group C – ethylene
  • Class I Group D – propane

It's evident that while the Class/Division/Group system is precise, it is also somewhat complex in its structure. Even so, electrical components must be designed to the classifications for the atmospheres in which they will be used.

So, how is the Zone system different?

Zones Standards (IEC, NEC, CEC)

Here is a chart with a quick comparison of the differences between the Class/Division system (NEC/CEC) and the Zone system (IEC, NEC, CEC). Note that the Zone system is addressed in the North American standards as more industries continue to overlap geographically. Divisions and zones address the frequency, duration, or likelihood of a flammable or explosive mixture.

Division 1
Where ignitable concentrations of flammable gases, vapors, or liquids are frequently found.

According to NEC article 500, CEC section 18:
"In which ignitable concentrations of hazard exist under normal operating conditions and/or where hazard is caused by frequent maintenance or repair work, or frequent equipment failure."

Zone 0
Where ignitable concentrations of flammable gases, vapors, or liquids are present continuously or for long durations under normal working conditions.

Zone 1
Where ignitable concentrations of flammable gases, vapors, or liquids:

  • are intermittently or periodically present under normal operating
  • conditions exist frequently due to repair, maintenance, or leakage

Division 2
Where ignitable concentrations of gases, vapors, or liquids are nor frequently an issue. According to NEC article 500, CEC section 18: "In which ignitable concentrations of hazards are handled, processed or used, but which are normally in closed containers or closed systems from which they can only escape through accidental rupture or breakdown of such containers or systems."

Zone 2 
Where ignitable concentrations of flammable gases, vapors, or liquids:

  • are present under normal operation conditions
  • occur for short durations
  • can cause hazards in case of and accident or abnormal operations

Further differences between the Class/Division system and Zone system are found in the means of preventing fire and explosions, particularly related to electrical equipment design.

First, let's look at some definitions and concepts.

Article 100 of the NEC defines explosion-proof apparatus:

Apparatus is enclosed in a case that is capable of withstanding an explosion of a specified gas or vapor that may occur within it and of preventing the ignition of a specified gas or vapor surrounding the enclosure by sparks, flashes, or explosion of the gas or vapor within, and which operates at such an external temperature that a surrounding flammable atmosphere will not be ignited thereby.

In short, the enclosure of the equipment contains any internal explosions, keeping it from igniting potentially flammable or explosive atmospheres external to the unit. This means there may be gases or vapors within the enclosure itself.

Intrinsically safe design limits the potential of equipment or wiring to release sufficient electrical or thermal energy under normal or abnormal conditions. This does not allow ignition of specific hazardous atmospheric mixtures, even in the most easily ignited concentrations.

Changing the equipment to less ignitable, non-electrical types is an option. For example, pneumatics can be used in some cases of machine power, eliminating electrical hazards. Additionally, fiber optics can be used to eliminate electrical energy for communications and control.

The Division and Zone systems have different means of controlling the energy sources. Here is a comparison of Class I protection techniques.

Division 1

  • explosion proof
  • intrinsically safe
  • purged/pressurized

Division 2

  • hermetically sealed
  • non-incendive
  • non-sparking
  • Oil immersion
  • Sealed device
  • Purged/pressurized.

Zone 0

  • Intrinsically safe

Zone 1

  • Encapsulation
  • Flame proof
  • Increased safety
  • Intrinsically safe
  • Oil immersion
  • Sand/powder-filled
  • Purged/pressurized

Zone 2

  • Energy limiting
  • Hermetically sealed
  • Non-incendive
  • Non-sparking

What Divisions and Zones Mean For Workplace Design And Safety

First, you must do a workplace hazard analysis to determine if and where any hazardous atmospheres are possible. This should include the type of hazard and the frequency of the hazardous atmosphere.

Once determined, you then begin to specify electrical/electronic components that are rated for those conditions. These devices and enclosures must have the correct rating for the intended use. They should be rated by the manufacturer and labeled accordingly.

If using prefab buildings for your worksite, be sure to consult with the building or room manufacturer to ensure the criteria are met. Their design engineers can assist you in making the appropriate, code required choices to make sure your worksite is safe for both personnel and equipment.

When it comes to hazardous locations and designations, Specialist Services RedGuard, has a number of subject matter experts who can guide you in finding the appropriate modules or units for your offshore operation. Contact us today to learn more.