Defining Misperceptions on Intrinsically Safe and Nonincendive Equipment

Voodoo Science Nonincendive Circuits are better than Intrinsically Safe Circuits (NOT TRUE)

This misconception is mainly because nonincendive circuits employ the same basic rationale for reducing the risk of ignition of a hazardous atmosphere both rely on energy limitation.  Also the sound of the word itself nonincendive is part of the voodoo.  The word nonincendive strongly sounds to a layman like something simple that will not (Lite, Ignite or set A Fire) any atmosphere. On the other hand the word Intrinsic itself which sounds foreign, mysterious, microscopically looked at in detail.  The word Intrinsic sounds complicated must be thoroughly looked at in detail if not it will (Lite Ignite or set A Fire) any atmosphere at the smallest quantity.  This is reversed rationale and logic otherwise we can call it voodoo science.

The important differences nonincendive circuits are permitted in Division 2 and Zone 2 classified locations only. Intrinsically safe circuits on the other hand, are suitable for use in Division 1 and 2, and Zone 0, 1 and 2 classified locations. The main difference between the two types of equipment is that nonincendive circuits are evaluated for ignition capability under normal operating conditions, while intrinsically safe circuits are evaluated under fault conditions. During an evaluation of an intrinsically safe circuit, it is allowed to introduce up to two independent faults per the requirements in the applicable product standard in an attempt to increase the energy available in the circuit. Under these conditions, the circuit must still be incapable of causing ignition of the specified gas either by introduction of a spark (shorting of two points in the circuit) or by thermal effects of individual components within the circuit.

Therefore Intrinsically Safe Circuits are better than Nonincendive circuits (THIS IS TRUE)

Intrinsic safety and nonincendive equipment and wiring methods are practices where apparatus is designed with low power levels and low stored energy, so that an arc produced during normal functioning of the equipment or as the result of equipment failure has insufficient energy to initiate ignition of the explosive mixture. Equipment enclosures can be pressurized with clean air or inert gas and designed with various controls to remove power or provide notification in case of supply or pressure loss of such gases. Arc-producing elements of the equipment can also be isolated from the surrounding atmosphere by encapsulation, immersion in oil, sand, etc. Heat producing elements such as motor winding, electrical heaters, including heat tracing and lighting fixtures are often designed to limit their maximum temperature below the auto-ignition temperature of the material involved. Both external and internal temperatures are taken into consideration.

Section 500.7(F) of the National Electrical Code explains that it is a protection technique for equipment installed in Class I, Division 2, hazardous (classified) locations. With the advent of more stringent environmental regulations, intentional release of flammable materials is greatly restricted and, with the resulting changes in containment techniques, many areas that previously were classified as Division 1 (continuously flammable) have been reclassified as Division 2 (infrequently within the flammable range).

This allows the installation of nonincendive equipment in applications previously limited to Division 1 protection techniques, such as explosion proof or intrinsic safety. Nonincendive equipment tends to be less costly and less complicated to maintain than Division 1 equipment. The obvious economic benefits will mean that this protection technique will see an expanded usage in the years to come.

The same concept can be applied in Class II and Class III, Division 2, locations; but, in those locations, the enclosures of the nonincendive equipment must also prevent the entrance of dust.

In an industrial plant such as a refinery or chemical process plant, handling of large quantities of flammable liquids and gases creates a risk of leaks. In some cases the gas, ignitable vapor or dust is present all the time or for long periods. Other areas would have a dangerous concentration of flammable substances only during process upsets, equipment deterioration between maintenance periods, or during an incident. Refineries and chemical plants are then divided into areas of risk of release of gas, vapor or dust known as divisions or zones. The process of determining the type and size of these hazardous areas is called area classification. Guidance on assessing the extent of the hazard is given in the NFPA 497 Standard, or API 500 and according to their adaptation by other areas gas zones is given in the current edition of IEC 60079.10. For hazardous dusts, the guiding standard is IEC 61421.10.

Typical gas hazards are from hydrocarbon compounds.

Safe area

An area such as a residence or office would be classed as safe area, where the only risk of a release of explosive or flammable gas would be such things as the propellant in an aerosol spray. The only explosive or flammable liquid would be paint and brush cleaner. These are classed as very low risk of causing an explosion and are more of a fire risk (although gas explosions in residential buildings do occur). Safe area on chemical and other plant are present where the hazardous gas is diluted to a concentration below 25% of its lower flammability limit (or lower explosive limit (LEL)).

      Division 2 or Zone 2 area

This is a step up from the safe area. In this zone the gas, vapor or mist would only be present under abnormal conditions (most often leaks under abnormal conditions). As a general guide, unwanted substances should only be present under 10 hours/year or 0–0.1% of the time.

Division 1 or Zone 1 area

Gas, vapor or mist will be present or expected to be present for long periods of time under normal running. As a guide this can be defined as 10–1000 hours/year or 0.1–10% of the time.

Zone 0 area

Gas or vapor is present all of the time. An example of this would be the vapor space above the liquid in the top of a tank or drum. The ANSI/NEC classification method considers this environment a Division 1 area. As a guide this can be defined as over 1000 hours/year or >10% of the time.

In the case of dusts there is still a chance of explosion. An old system of area classification to a British standard used a system of letters to designate the zones. This has been replaced by a European numerical system, as set out in directive 1999/92/EU implemented in the UK as the Dangerous Substances and Explosives Atmospheres Regulations 2002

The boundaries and extent of these three dimensional zones should be decided by a competent person. There must be a site plan drawn up of the factory with the zones marked on.

The zone definitions are:

            Zone 20

A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is present continuously, or for long periods or frequently.

      Zone 21

A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is likely to occur, occasionally, in normal operation.

Zone 22

A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is not likely to occur in normal operation but, if it does occur, will persist for a short period only.