Intrinsically Safe Circuits

The intent of this whitepaper is to determine what defines a nonincendive fire protection initiating circuit and when intrinsic barriers are not required.  Read the article below or download the PDF whitepaper.

RE: Nonincendive Fire Protection System Circuits

Purpose The intent of this document is to determine what defines a nonincendive fire protection initiating circuit and when intrinsic barriers are not required. Codes, Standards and Recommendations ANSI/ISA
  • ANSI/ISA-12.12.01, Nonincendive Electrical Equipment
National Fire Protection Association (NFPA)
  • NFPA 70, National Electric Code (Article 300, 500, 502, 760, and 800)
  • NFPA 72, National Fire Alarm Code
  • NFPA 499, Classified of Combustible Dusts
  • NFPA 850, Fire Protection for Electric Generating Plants and High Voltage Direct Current Converter Stations
Definitions (NFPA 70) Class II, Division 1, Group Locations. Class II. A class two locations is one that contains combustible dust. Division 1. Is a location in which combustible dust is in the air under normal operating conditions in quantities sufficient to produce explosive or ignitable mixtures, or (2) where mechanical failure or abnormal operation of machinery or equipment might cause such explosive or ignitable mixtures to be produced, and might also provide a source of ignition through simultaneous failure of electric equipment, through operation of protection devices, or from other causes, or (3) in which combustible dusts of an electrically conductive nature may be present in hazardous quantities. Division 2. Is a location where combustible dust is not normally in the air quantities sufficient to produce explosive or ignitable mixtures, and dust accumulations are normally insufficient to interfere with the normal operation of electrical equipment or other apparatus, but combustible dust may be in suspension in the air as a result of infrequent malfunctioning of handling or processing equipment and (2) where combustible dust accumulations on, in, or in the vicinity of the electrical equipment may be sufficient to interfere with the safe dissipation of heat from electrical equipment or may be ignitable by abnormal operation or failure of electrical equipment. Nonincendive Circuit. A circuit, other than field wiring, energy on the wiring under conditions such as opening, in which any arc or thermal effect produced under intended operating conditions of the equipment, is not capable, under specified test conditions, of igniting the flammable gas-air, vapor-air, or dust-air mixture. Nonincendive Field Wiring. Wiring that enters or leaves an equipment enclosure and, under normal operating conditions of the equipment, is not capable, due to arcing or thermal effects, of igniting the flammable gas-air, vapor-air, or dust-air mixture. Normal operation includes opening, shorting, or grounding the field wiring. Nonincendive Equipment. Equipment having electrical/ electronic circuitry that is incapable, under normal operating conditions, of causing ignition of a specified flammable gas–air, vapor–air, or dust–air mixture due to arcing or thermal means. Encapsulation. Type of protection where electrical parts that could ignite an explosive atmosphere by either sparking or heating are enclosed in a compound in such a way that this explosive atmosphere cannot be ignited. Evaluation Should we or should we not provide Intrinsically Safe Technologies or is it enough to say we meet the requirements for Nonincendive circuits and devices; much research has gone into the question. The National Electric Code as well as many other codes many times can be difficult documents to interpret and when it comes to topics of discussions such as Hazardous Environments, things can get out of hand from a cost perspective. This is due to the lack of knowledge within the industry of risk mitigation from and insurance and liability standpoint. Many times when we are asked to provide a design that mitigates the potential for any possibility of an explosion, we as Life Safety and Proper Protection professional will utilize every check and balance that is afforded to us, to eliminate any potential harm to another human or when the cost of down time and the protection of million dollar equipment is so great we can and usually go overboard. In further research into codes and manufacturers equipment from a fire protection perspective has greatly help our industry to understand. One example of this is after several attempts of trying to get multiple manufacture’s intrinsic barriers and galvanic isolators to work appropriately with fire alarm/protection systems it has been determined that either we would have to sacrifice system functional that is both required from a code and rather costly from and end-users daily operational stand point. The majority of modern fire protection systems are power limited because they have an internal power source and operate under the low voltage guidelines of the National Electric Code and therefore afford us the opportunity to utilize the ANSI/ISA-12.12.01 requirement for nonincendive circuits. When applying figures 1 through 8 to the electrical characteristics of the fire protection equipment you will see that these systems fall far below the areas that would require us to provide any additional protection. However due to the nature of the industrial environment of these types of projects we inherently are providing yet another means of explosion mitigation, because of the necessity of ridged conduit and encapsulated detection via linear heat detection and NEMA rated enclosures for all of the termination points with seal offs. With a completely sealed device and wiring system there is no real potential for the ingress of coal dust. Fire protection systems that operate at 24vdc are not required to have any additional insulation per the NEC article 502.25 because they do not operate above 30 volts. Conclusions Due to its physical and electrical characteristics, an intrinsically safe circuit does not develop sufficient electrical energy (millijoules) in an arc or spark to cause ignition or sufficient thermal energy resulting from an overload condition to cause the temperature of the installed circuit to exceed the ignition temperature of a specified explosive dust, gas or vapor under normal or abnormal operating conditions. An abnormal condition may be due to accidental damage, failure of electrical components, excessive voltage, or improper adjustment or maintenance of the equipment. Nonincendive circuits and devices are permitted by NFPA codes as an alternative way of ensuring protection in place of intrinsically safe devices, as they provide the same level of protection and risk mitigation. Reasons why intrinsic protections are not warranted:
  • The circuits are either total encapsulated and installed in screwed rigid conduit with NEMA 4x junction boxes, which make it dust proof. NEC article 500.7 (b)
  • The Circuits and Devices are considered Nonincendive per the NEC and ANSI/ISA-12.12.01.
  • The operating voltage is not over 30vdc per the NEC and do not pose a great enough threat during normal operations. 502.25
In light of the above referenced code requirements and provisions no further protection is required for the hazards presented by Coal Dust in the specific process of coal transport via conveyers. Sincerely, Ryan McCorkendale, SET Engineering Design Project Manager NICET # 106251