Explosion Proof Devices and the Code – Why Not Use I.S?

In a different article, we looked at explosion-proof devices and the 
NEC®. In it, we briefly discussed some of the locations that require 
explosion-proof components and how the area classification impacts the 
devices design. Emerson

Steve Maurer, IME

In a different article, we looked at explosion-proof devices and the NEC®. In it, we briefly discussed some of the locations that require explosion-proof components and how the area classification impacts the devices design.

Additionally, I gave a brief description of the difference between explosion-proof and intrinsically safe designs. To recap:

  • An explosion-proof design contains any internal sparks or other ignition source, keeping them from igniting external flammable materials surrounding the device, such as vapors, gases, or dust. Containment is the key word when talking about explosion-proof. The containment applies to conduit and other fittings that connect the device to the system. This can be expensive to install and difficult to maintain.
  • An intrinsically safe design is much different. The key word here is prevention. The design of the component eliminates the possibility of explosion, either internally or externally.
So, the very legitimate question arises: Why not skip explosion-proof devices altogether and use only intrinsically safe components? That sounds like the safer, more efficient means of preventing explosions in hazardous atmospheres.

However, intrinsically safe design has important limitations. Limitations that decrease or even prevent some applications. Every component of the I.S. system must be carefully controlled. In particular, inductive and capacitive load must be designed and evaluated to ensure no spark or ignition is possible. This obviously means that the power range of intrinsically devices is limited. This often consigns intrinsically safe devices to monitoring circuits, operating at low voltage.

High power devices simply cannot be designed and built to meet I.S. standards. Even the accessories for an intrinsically safe device can degrade the rating if they are not I.S. rated as well.

Some of the devices that cannot be rated as intrinsically safe include:

  • High voltage switchgear
  • Electric motors
  • Most industrial lighting operating at normally available voltages
Even devices used for preventive maintenance must comply when used in hazardous atmospheres. Often that requires an explosion-proof rating, even though the device itself is intrinsically safe. Attachments may change the rating, as will other components in the system, even if not directly connected.

For example, a camera used to record physical conditions may be intrinsically safe. However, if a flash attachment is used, it may change the entire rating so as to require an explosion-proof design.
So, to answer the question about using intrinsically safe devices alone, it's not feasible... or even possible. While safer than explosion-proof devices, the limitations of the I.S. standards make explosion-proof devices the only option when installing devices or equipment used at high voltages and with inductive or reactive potential.


Photo courtesy of Emerson