Better intrinsically safe than sorry

Better intrinsically safe than sorry

by Greg Paula, Associate Editor

Intrinsically safe devices and systems are becoming more common at sites with a potential for fire and explosion.

At facilities where there are flammable or explosive materials, operating at low energy can prevent electrical devices from creating arcs, sparks, or heat (during normal or fault conditions) that could ignite the explosive substance. Such a system is termed "intrinsically safe." The alternative is a system in which something separate from the system itself--such as an explosion-proof enclosure--is used.

There are four basic ways to set up an interface between the safe and hazardous areas, from simple zener diode barriers to bus-compatible flexible I/O systems

Explosion-proof enclosures are effective but have several disadvantages, apart from cost. Since the enclosure is not explosion-proof when it is open, performing any maintenance requires the entire area to be temporarily decommissioned, which is not always possible. Also, permanent enclosures make it difficult to reconfigure equipment and processes.

The number of devices that can be made intrinsically safe has increased significantly, and solid-state equipment can handle almost all types of devices. A handful of applications cannot be made intrinsically safe. General-purpose lighting needs to operate at relatively high power. Engines also cannot be made intrinsically safe, because of the sparks needed to ignite the fuel, but intrinsically safe engine controls can be connected to an explosion-proof engine.

An intrinsically safe circuit has three components: the field device, the intrinsically safe barrier, and the field wiring. Field devices may be either simple or complex. Simple devices--including contacts, thermocouples, light-emitting diodes, and resistors--do not store energy and do not need to be approved. Complex devices--such as transmitters, solenoids, relays, and transducers--can store excess energy and require independent certification. Contacts, transmitters, and temperature sensors are the most commonly used field devices in intrinsically safe applications.

Safe and hazardous areas may be interfaced with zener diode barriers, a motherboard cable assembly, a remote process interface, or an intrinsically safe remote process interface. Intrinsically safe circuits may be wired like comparable circuits, with two exceptions: separation and identification. U.S. organizations recently established wiring standards for intrinsically safe applications based on European ones.

The above was adapted from an article by Greg Paula, Associate Editor. The full text may be found in the June 1997 issue of Mechanical Engineering magazine. © 1997 ASME International.© To obtain a copy of this issue, click here.