Customers often want to know what exactly is a Class I, Div. 2 industrial display. Or, what choices there are for HMI visualization and machine control in a Division 2 area, and if a purge system is required. The answers depend on what is connected to the display, and the type of HMI software. This information helps you decide whether to look at an industrial monitor, a panel PC, or a networked monitor via thin client or KVM back to a host in a control room.
There are several options for placing a monitor or HMI interface into the hazardous area. For Class I, Division 2, the industrial display requires a hazardous-rated LCD monitor and possibly a purged enclosure. Peripherals such as barcode scanners, keyboards, etc. must also be rated for the area.
To purge or not to purge?
The NFPA 496 standard for purged and pressurized enclosures for electrical equipment provides the answer. For Class I, Division 2, no purge is required if all the equipment within a protected enclosure is approved for Division 2. Check all your components for their ratings and use. For example, if the display has a split rating, with the front panel being Division 2, and the rear panel is general purpose, a type Z purge is required. Also, make sure the display can handle positive pressure so the touchscreen or LCD panel does not curve out.
- If a general-purpose PC, thin client, or KVM extender is mounted inside the enclosure, a purge system is required.
- The keyboard and barcode scanner require nonincendive barriers when connected to the USB ports of a PC, thin client, or KVM extender.
If any of these components do not meet the Division 2 requirement, a purge system is required, or the equipment cannot be used (i.e., general-purpose barcode scanner).
Wiring for hazardous areas
Division 2 wiring methods are followed for industrial displays located in Division 2 hazardous areas. Keep in mind that to qualify as Division 2, the hazard is only present under unusual circumstances and for short durations.
What about Class II, Division 2?
This is a different classification. Often, people will use a Class I Division 2 operator panel in a dusty environment for which it has no approval. We recommend that you select suitable equipment, and use a type Z purge and pressurization system rated for Class II environments. These systems skip the purge cycle per NFPA 496, so as not to disturb any existing dust. Wiping and cleaning the dust is the preferred method for removing dust, per NFPA 496.