Use of intrinsically safe electrical circuits

Until the 1950 s, the use of intrinsically safe electrical circuits had little application in other than just battery operated signaling devices. At this time due to advances in technology and an increase in the use of electrical equipment in hazardous locations, a new world-wide interest developed in the application of intrinsically safe electrical circuits beyond what had been its traditional role. 

In the United States, this new interest was recognized, and in 1956, the National Electrical Code (NEC) introduced the use of intrinsically safe electrical circuits. 

Circuits Not Device. During design, when considering the use of intrinsically safe electrical circuits, the whole electrical circuit must be considered. It is not enough just to consider the electrical apparatus employed in the hazardous environment. Consideration must be given to its associated apparatus located in the non-hazardous area. Therefore, it is not just the apparatuses which must be considered, but the whole circuit, both in the hazardous area and the non-hazardous area. 

The design requirements for intrinsically safe would seem to be demanding, and a review of NFPA 493 enforces this fact. Today s industrial environment imposes additional requirements not only on the use of intrinsically safe electrical circuits, but other hazardous electrical techniques as well. These requirements are due to the Occupational Safety and Health Act and the employer s increasing vulnerability for liability. 

The U.S. Army Defense Ammunition Center and School is employing the use of intrinsically safe electrical circuits in equipment designed to demilitarize and renovate munitions - from small arms to large projectiles. This is accomplished by using pneumatics and hydraulics to provide the power, while using position switches and solenoid valves linked to programmable controllers to direct the total machine process. 

In 1938, the United States Bureau of Mines began development of rules relating to the use of electricity for telephone and signaling equipment, which included application of intrinsically safe electrical circuits. 

Certification. It is a benefit to the employer to ensure that the intrinsically safe electrical circuit is certified. Certification can be achieved through the use of a third party, such as Underwriters Laboratories or Factory Mutual Research. Both of these organizations have their own standards for approval which are based on NFPA 493. The certification is accomplished in three steps ... 

The simplest method of using intrinsically safe electrical circuits is not to design and certify them yourself, but rather to take advantage of a clause contained in NFPA 493 which states ... 

Availability. Both Underwriters Labatories and Factory Mutual Research publish yearly guides to electrical equipment which they have certified and continue to certify as being rated for use in hazardous environments. Many of the items contained in these guides are rated as intrinsically safe or as associated equipment for use with intrinsically safe equipment. Further, the amount of equipment available should increase each year as the demand increases for intrinsically safe electrical circuits. 

Finally, intrinsically safe electrical circuits are an old idea, whose time has just begun. Tomorrow s world will see ever greater uses of programmable controllors, robotics, solid state circuits, and other low energy devices. This is the world in which intrinsically safe circuits belong. 

Due to the galvanic isolation lacking between intrinsically safe and non-intrinsically safe electrical circuits the power-limiting characteristic of components commonly used for it (e.g. small transformers, optocouplers, relays) is lacking as well. So, a robust safety barrier shall be constructed. 

Intrinsic Safety. Static electrical concepts such as minimum ignition energy do not directly apply when assessing the safety of electrical circuits such as radios, flashlights and instmmentation. Intrinsically safe electrical equipment is usually available which has been subjected to fault analysis and testing. The equipment must be certified for the flammable atmosphere in which it will be used (NFPA 497). Refer to texts on Intrinsic Safety such as [63]. 

Establishing a procurement policy, which includes a pre-procurement risk assessment, will ensure that correct selection of plant and equipment for use in hazardous areas (where a flammable or explosive atmosphere has the potential to cause a fire or explosion) will minimise the risk relating to such equipment. BS EN 50020 2002, Electrical apparatus for potentially explosive atmospheres - Intrinsic safety i , specifies the construction and testing of intrinsically safe apparatus, intended for use in potentially explosive atmospheres and for associated apparatus, which is intended for connection to intrinsically safe circuits which enter such atmospheres. This document may well form the basis from which an organisation establishes its procurement policy for such equipment. 

Physical Separation. In addition to providing electrical isolation, it is necessary to provide physical separation to ensure the non-hazardous circuits can not degrade the intrinsically safe portion of the circuits. This can be accomplished by planning the physical layout to incorporate the use of distance, enclosures, partitions, separate raceways, and insulation. The final physical layout selected should meet or exceed the requirements of Chapter 3, Sections 1, 2, 3, and 4 of NFPA 493. 

Intrinsic safety is based on the principal of restricting the electrical energy available in hazardous area circuits such that any sparks or hot surfaces that may occur as a result of electrical faults are too weak to cause an ignition. The useful power is about 1 watt, which is sufficient for most current instrumentation. It also provides a personnel safety factor since the voltages are low and it can allow field equipment to be maintained and calibrated "live" without the need for a gas free environment verification. Electrical components or equipment can be manufacturer as intrinsically safe and there readily usable in areas where combustible gases or vapors may be present. 

Personal electronic or electrical devices that may require control are pagers, cellular phones, personal digital assistants (PDAs), and personal radios or music players. Few, if any, of these devices are evaluated to determine if they may be safely used in hazardous areas. Typically, such devices do not claim to be "intrinsically safe" or of "nonincendive circuit" design (NFPA 70, Articles 500 and 504). 

Acceptable protection techniques for electrical and electronic valve accessories used in specific class and division locations include explosion-proof enclosures intrinsically safe circuits nonincendive circuits, equipment, and components dust-ignition-proof enclosures dusttight enclosures purged and pressurized enclosures oil immersion for current-interrupting contacts and hermetically sealed equipment. Details of these techniques can be found in the National Electrical Code Handbook, available from the National Fire Protection Association. 

The rule described above forms a basis for the secondary type of explosion protection, which results in the avoidance of ignition sources, e.g. by using either a flameproof enclosure of a commutator motor or the pressurized enclosure of a gas analyser, or by limiting the electrical values in intrinsically safe circuits. 

The description of the different types of protection in Chapter 6 indicates that there are two very different ways to solve this problem - if an electrical transmission is required at all. One way is to use intrinsically safe circuits, the other one applies industrial equipment as usual, additionally explosion protected by an enclosure as appropriate, e.g. flameproof housings for smallsized devices. In the history of process instrumentation, the appearance of semiconductors and integrated circuits has drastically reduced the power consumption of field devices. So, intrinsically safe circuits dominate this field today. 

Various protection techniques can be used to prevent electrical equipment from becoming an ignition source. Explosion-proof enclosures, pressurized and purged equipment, and intrinsically safe circuits can be used in Division 1 and Division 2 locations. Nonsparking equipment can be used in Division 2 locations. Details of the types of equipment permitted in each classified location are given in NFPA 70. 

Intrinsically safe type i protection is not applicable to electrical power equipment. It is mainly intended for electronic measuring and control circuits i.e. instrumentation and telemetry. The principle behind i protections is that a circuit and its devices do not have sufficient operating energy or stored energy to cause a spark that will ignite the gas-air mixture. A spark can occur but it must be inherently too weak to ignite the mixture. There are two sub-divisions of type i , namely l and it . The type ia has a more stringent specification than i, and is therefore allowed to be used in a Zone 0 area. Type ifc equipment cannot be used in a Zone 0 area. 

In a circuit where the amount of electrical energy available to cause a spark is below that necessary for igniting flammable vapour or gas, the equipment is considered to be intrinsically safe. It is particularly suitable for use in instrumentation, remote control etc. Intrinsically safe equipment to category IA may be used in any Zone except that in which there are contacts that could spark. It should not be used in Zone 0. Category IB equipment may be used in all zones except Zone 0, See BS5958 . ... 

In a circuit where the amoxmt of electrical energy available to cause a spark is below that necessary for igniting flammable vapour or gas, the equipment is considered to be intrinsically safe. It is particularly suitable for use in instrumentation, remote control etc. 

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