Intrinsic Safety 101

Intrinsic safety is an explosion-prevention design technique applied to the electrical equipment and wiring installed in hazardous locations; it limits the electrical and thermal energy levels well below those required to ignite a specific hazardous atmospheric mixture.

 

An oil refinery. Image used courtesy of Pixabay

 

The Ignition Triangle

Oxidation, combustion, and explosion are exothermic reactions with diverse speeds from a chemical standpoint. Such reactions require the presence of three components in proper proportions:

• Fuel: gas, vapor, or powder.
• Oxidizer: typically, air or oxygen.
• Ignition energy: thermal or electrical. A precise energy level transferred to the fuel draws the trigger – combustion or explosion will not be below this minimum energy amount.

The vertices or corners of a triangle – the ignition triangle – typically represent, graphically, these three factors. All the protection techniques used in electric devices and wiring to reduce the risk of fire or explosion to an acceptable level eliminate one or more triangle components.

After igniting the chemical reaction, the result can be controlled combustion, a flame wave, or an explosion, depending on the exothermic energy speed.

 

The Birth of Intrinsic Safety

Coal mines have methane gas and coal dust. If methane ignites, a much more violent dust explosion follows. Beginning the 20th century, the hoists in British mines operated with low voltage signaling bells. This scheme comprised six Leclanche-type batteries – a 12 V power supply – two bare wires, and a bell. The workers inside the mine, a hazardous area, short-circuited the wires using their shovels or hands to ring the bell, advising the surface workers that the coal cars were ready for hoisting to the surface. This 12 V circuit was not considered dangerous, even though there were sparks during the short-circuiting action.

Two disastrous mine explosions occurred in British mines in1912 and 1913. Later investigations showed that the explosions originated from the sparks produced by the mine’s signaling bells.

Such searches revealed that the energy stored in the circuit´s electric and magnetic fields generates sparks to ignite the air/gas mixture.

The first regulation for testing and certifying signaling systems for mines followed these findings. The intrinsically safe concept was born.

This concept consists of designing electric equipment and circuits in such a way as to render them incapable of producing enough electrical and thermal energy levels to ignite specific hazardous atmospheric mixtures under normal and abnormal or fault conditions – an explosion-prevention design technique.

Later on, the surface industries, handling even more dangerous gases – like hydrogen and acetylene – applied the intrinsic safety concept.

 

Energy Storage in an Electric Circuit

Figure 1 shows an elementary RLC circuit.

 

Figure 1. Elementary RLC circuit. Image used courtesy of Lorenzo Mari

 

Wiring always has inductance and capacitance associated with it – these elements store energy.

The capacitor will charge when the switch is open, storing electric energy. If there is a short circuit between conductors or between a conductor and ground, it is equivalent to closing the switch. During this event, the capacitor discharges and produces an arc or spark. Too much capacitance could result in enough energy to ignite the hazardous atmosphere.

The inductor will charge when the contact is closed, storing magnetic energy. The inductor discharges when the switch opens, producing an arc or spark. Similarly, too much inductance could result in the release of enough energy to ignite the hazardous atmosphere.

The danger of explosion exists when the arc or spark energy exceeds the Minimum Ignition Level (MIL) of the material involved. The IS system must be designed and wired to assure that it will not exceed the MIE of the hazardous material.

The circuit’s inductance and capacitance depend on conductor spacing, size, and length.

 

The Meaning of “Intrinsic Safety”

The terms intrinsic and intrinsically safe reveal that safety comes from the system’s design, not by adding protective measures – like the explosion-proof technique.

Safety exists throughout the system’s life, during maintenance, and despite inadequate care.

Lack of care may cause the loss of safety in an explosion-proof housing – the improper installation of the housing cover after maintenance, corrosion, and mechanical damage, will compromise safety. IS focuses on the source of the problem, not providing the energy needed to cause an explosion – intrinsic protection.

 

Principle of Operation of an IS System

Figure 2 shows a basic diagram of an IS system.

 

Figure 2. Basic diagram of an IS system. Image used courtesy of Lorenzo Mari

 

The system in figure 2 shows an electrical apparatus located in a hazardous location (the IS apparatus), an electrical apparatus located in a non-hazardous location (the associated apparatus), and the wiring between the two.

An IS apparatus is typically part of a system where the certified components assure the system’s safety for the application. All circuits in an IS apparatus are intrinsically safe.

An associated apparatus is not intrinsically safe but affects the IS circuits’ energy and must maintain intrinsic safety. A complete IS circuit must not ignite the material in a hazardous location.

Also important is the concept of the simple apparatus (not shown in figure 2). A simple apparatus is a device that cannot generate more than 1.5 V, 0.1 A, 25 mW, or a passive component, compatible with the intrinsic safety of the circuit, that does not dissipate more than 1.3 watts.

A simple apparatus poses no hazard as it will not ignite hazardous mixtures. Examples are simple probes and devices such as thermocouples, RTDs, LEDs, switches, and transducers for measuring and transmitting process data, and current-to-air pressure transformers for servo-control purposes. They often do not require certification for IS location use.

Article 100 of the National Electrical Code (NEC) provides detailed definitions.

 

Power-limiting Arrangements

Limiting the current or voltage levels in a circuit section is vital to control the power transferred between the non-hazardous and hazardous locations.

There are three basic arrangements of elements to limit the current or voltage levels:

1. Series arrangement: series resistors limit the AC and DC amperes flowing into an IS circuit. Series capacitors are helpful in high-frequency, low-power AC circuits. Fuses and circuit breakers are not current-limiting devices, but fuses are satisfactory for protecting other energy-limiting devices.
2. Shunt arrangement: linear or non-linear shunt resistors dissipate the energy stored in the magnetic field. A diode in parallel to the inductor dissipates its stored energy. Current doesn’t flow through the diode under normal conditions. If the circuit breaks, the voltage generated by the collapsing magnetic field in the inductor activates the diode, making it conduct current to the ground. The diodes require testing to demonstrate their effectiveness. Shunt capacitors may work, but testing is mandatory to prove successful for the particular application, as under some circumstances, they may boost the hazard.
3. Series-shunt arrangement: combines series and shunt devices. The series element limits the current, while a non-linear shunt element limits the voltage. Nowadays, the most common voltage-limiting device employed is the Zener diode.

 

Fields of Application

The IS technique applies only to equipment and circuits requiring low energy levels for operation. 

Process control instrumentation is an excellent field for IS equipment since these electrical systems use low energy levels. For this reason, IS finds many applications in the instrument industry – 4 to 20 ma signal circuits, instruments to measure pressure, flow, and temperature, as well as radios and other equipment.

IS techniques and low voltage solid-state technology evolve together at a fast pace. Nowadays, more electrical devices are designed and certified for IS than for any other type of explosion prevention method.

 

Economics

The cost of an IS system may seem high. However, since there is no requirement for explosion-proof fittings and conduits in the hazardous location, the price of the installed system can be much lower than other technologies. Additional savings arise because IS technology permits conventional instrumentation wires and allows maintenance on a running plant – no need for a plant shutdown.

 

Applicable Standards

There are two standards in the United States for construction and performance requirements for intrinsically safe systems:

• ANSI/UL 913, Standard for Intrinsically Safe Apparatus and Associated Apparatus for Use in Class I, II, and III, Division 1, Hazardous (Classified) Locations (formerly NFPA 493), and
• ANSI/UL 60079-11, Electrical Apparatus for Explosive Gas Atmospheres – Part 11: Intrinsic Safety “I,” based on the IEC 60079-11 standard

The standards employed for installation rules are:

• ANSI/ISA-RP 12.6, Recommended Practice for Wiring Methods for Hazardous (Classified) Locations Instrumentation Part 1: Intrinsic Safety
• NFPA 70, National Electrical Code (NEC)

The 1990 NEC Edition introduced Article 504, “Intrinsically Safe Systems.” Safe installations – free from hazards – will be attained by carefully interpreting and applying this article.

ISA’s Recommended Practice:

• ISA-RP12.2.02, Recommendations for the Preparation, Content, and Organization of Intrinsic Safety Control Drawings

It guides and promotes the uniformity of manufacturers’ control drawings for intrinsically safe apparatus, associated apparatus, and intrinsically safe systems. This RP does not cover the design or installation of intrinsically safe equipment or systems.

 

Certifications

Several diverging regulations and requirements worldwide direct how to design, develop, manufacture, and certify IS products. Some examples are:

In the USA:

• Underwriters Laboratory (UL), an independent organization for testing, inspection, and verification of products for safety. UL is on the Occupational Safety and Health Administration (OSHA) list of nationally recognized testing laboratories.
• FM Approvals, third-party testing and certification services.

In Canada:

• CSA, also on the OSHA list of nationally recognized testing laboratories.

In the European Union:

• ATEX, a commonly known synonym for the ATEX guidelines.

Internationally:

• IECEx certifies electrical equipment for hazardous locations.

 

About Intrinsic Safety

Intrinsic safety is a technique applied to electrical equipment in hazardous locations.

The inductance and capacitance in electric apparatus and wiring may store enough energy to ignite a hazardous atmosphere.

Two significant accidents in British mines in 1912 and 1913 triggered intrinsic safety technology.

The IS technique limits the electrical and thermal energy levels transferred to a hazardous location under normal and abnormal conditions, preventing explosions rather than confining them.

There are many – and different – certification regulations around the world. NFPA 70 (the National Electrical Code) sets installation rules in the USA.

 

Feature image used courtesy of Pixabay