Intrinsically safe electrical circuits

Intrinsically Safe Electrical Circuits in Explosives Facilities... 

This task was assigned to what is now called the Safety in Mines Research Establishment. It was at this organization where the concept of intrinsically safe electrical circuits was first defined after continued research into the ignition of methane-air mixtures. 

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. 

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. 

Intrinsically safe electrical circuits are now recognized around the world as an additional technique for providing electrical energy in hazardous locations. 

Intrinsically Safe and the Explosive Environment. If the evaluation concludes that the environment is in fact a Class II, Group G, Division 1 location, then the intrinsically safe electrical circuits must be designed as dust-tight and meet the... 

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. 

Safety Barriers. Figure 1 illustrates an application employing intrinsically safe electrical circuits for the demilitarization of ammunition. Three separate areas are required for this application - one area, classified as non-hazardous, to serve as the control and loading area a second area, classified as hazardous, where the actual demilitarization is accomplished and a third area, classified as non-hazardous, is required for the hydraulic pump due to the level of noise produced. 

Figure 1. Application employing intrinsically safe electrical circuits.

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. 

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 ... 

First, once designed, evaluated, and installed, the safety of the system cannot easily be degraded because the safety is in the design, not protection added afterward. In fact, the intrinsically safe electrical circuit will cease to fulfill the function for which it was designed long before it can become a hazard. This is due to the consideration which must be given to fault conditions. The only possible way for the circuit to become hazardous is if an unapproved or unauthorized component is substituted into the circuit. 

Fourthly, intrinsically safe electrical circuits are the easiest to maintain. Since intrinsically safe circuits by their nature are incapable of causing ignition, they can be maintained without regard to shutting down operations, nor are hot permits required, or is lengthly disassembly, assembly and recertification of added protection required. 

Finally, due to the requirements for intrinsically safe circuits being the most conservative of hazardous location circuits requirements, intrinsically safe electrical circuits offer the maximum in safety. Not only do they control the conditions which can lead to initiation of energetic materials, by their very nature - they eliminate it. 

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. 

Due to the concept of low energy, intrinsically safe electrical circuits do not provide the energy necessary to drive motors or high powered electrical equipment. Nevertheless, this does not limit or restrict their application in the real world. 

Intrinsically safe electrical circuits provide the capability to combine the strengths of pneumatic and hydraulic systems with the sophistication of the programmable controllor and robotics, and to do so with the maximum safety and flexibility. 

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. 

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. 

Fig u re 6.10 Power supply for intrinsically safe electric circuits. 

Figure 6.10 shows a typical example for a q-apparatus. It is a power supply for intrinsically safe electric circuits to be installed in zone 1 (and zone 2). The power input (at 24 V AC/DC level) is fed via flameproof plug-and-socket connectors to the apparatus. So, the power supply can be replaced under load without conflicting with the requirements of explosion protection. 

To ensure correct cooling conditions, the water flow and/or the temperature difference between water outlet and water inlet shall be monitored continuously, acting on an electrical interlock with the motor s power supply, e.g. by intrinsically safe electric circuits. 

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