Device protection in the danger zone

By Ian Verhappen

INTRINSIC SAFETY still is not widely understood in North America. Until recently, explosion-proof practices were commonly used in areas classified as requiring protection. The need for that protection is based on the likelihood of a potentially flammable atmosphere being present which, in turn, determines the class in the North American area classification system.

The experience of North American industrial machine builders that sell into hazardous environment markets overwhelmingly has been based on explosion-proof methods.

Instrument manufacturers for industries with these environments, typically hydrocarbon processing-related industries such as refining and chemicals, design their instruments to be both explosion-proof and intrinsically safe (IS). This allows manufacturers to sell the same device anywhere in the world, regardless of the area classification and protection system used by the facility.

Regardless of the method used to prevent fires or explosions in a facility, all methods are designed to remove one of the sides of the “fire triangle” shown in Figure 1.

FIGURE 1: BREAK UP THE TEAM

At least one side of the well-known “fire triangle” must be removed to prevent fires or explosions.

Explosion-proof and intrinsic safety systems remove (more correctly, manage) or limit the energy level released to the environment. Encapsulation and potting, on the other hand, keep oxygen away from the energy source.

Each gas has its own range of concentrations over which its stoichiometric ratio allows it to burn. Outside this range, combustion and hence an explosion will not occur. The extreme example of this: If a device is placed in a 100% methane environment, it will not burn or explode because there is no oxygen present to complete the reaction.

Similarly, every gas has a different temperature at which it ignites. The concept of divisions is based on the type of gas present, while the “T” or temperature rating is based on gas ignition temperatures.

All these chemical factors must be kept in mind when selecting equipment to be used to prevent explosions.

Prevent or Disperse Explosions?
As indicated, both explosion-proof enclosures and intrinsic safety prevent explosions by limiting the amount of energy in the explosive environment. An explosion-proof enclosure uses its mass and design to disperse the energy to a low level before it escapes the enclosure. Intrinsic safety systems are designed to prevent the energy level in the hazardous area from being above the explosive limit conditions.

ADVERTISEMENT "The standard industry practice of grounding instrument circuits at only one point is critical to the success of intrinsically safe circuits."

It is important to realize that intrinsic safety is a system. All the components of the system need to be considered in the design, including not only the IS device used to limit the energy available to the hazardous area, but cable and remote device(s) as well. Passive devices that do not store energy such as terminal blocks normally are not an issue and need not be included in the consideration. The capacitance of a cable, which is used when calculating the energy stored in a cable, is considerably affected by the presence of a screen or shield. It is important to use the correct capacitance value for the cable type installed.

IS devices, the key component in intrinsic safety systems, are available in two formats: safety barriers and galvanic isolators.

Safety barriers use zener diodes and current-limiting resistors to limit the current and voltage available at the hazardous area terminals. A fuse, if used in the barrier, restricts the fault power; the zeners restrict the voltage; and the resistor restricts the current. Figure 2 is a simplified schematic of a safety barrier.

FIGURE 2: SIMPLIFIED SAFETY BARRIER

In this simplified schematic of a safety barrier, resistors limit the current and zener diodes restrict voltage available at terminals in the hazardous area.

The excess energy from a barrier is routed to ground, normally through a low-impedance bus bar.

On the other hand, a galvanic isolator (Figure 3), as the name implies, breaks any direct connection between the safe and hazardous area circuits by interposing a layer of insulation between the two areas. The power transfer to the field--important to maintain loop-powered devices--normally is via some form of transformer, while the return signal from the device in the hazardous area is transmitted across the hazardous area/safe boundary via an optocoupler, transformer or relay.