What is the correct purge system to use for a Class I, Div. 1 industrial display? It’s a common question asked of our technical support experts. Another variation: What choices do I have for HMI visualization and machine control in a Division 1 area and which purge system does it require?
The first two questions to answer are to what is the display connected and what is the HMI software. This information tells us whether we are looking at an industrial monitor, a panel PC, or a networked monitor via thin client or KVM back to a host in a safe area.
There a few simple ways to get the desired monitor or HMI interface into the hazardous area. For Class I, Division 1, the display requires a hazardous-rated LCD monitor and potentially a small purged enclosure. Peripherals such as barcode scanners, keyboards, etc., also need to be rated for the area.
X, Y, or Z purge
The NFPA 496 standard for purged and pressurized enclosures for electrical equipment provides the answer. For Class I, Division 1, there are two valid purge solutions: Type Y purge and type X purge. There is a fundamental difference in operation, thus safety level and risk.
According to NFPA 496 4.10 Type Y Pressurizing, the key point is in 4.10.2: All “Equipment within a protected enclosure shall be approved for Division 2 or Zone 2 locations.”
- This includes any PC, thin client or KVM component. While Division 2 rated PCs and thin clients are readily available, only Pepperl+Fuchs has UL listed Division 2 rated KVM extenders.
- Division 2 wiring methods must be followed.
- All cables must be mechanically held in place and require a tool for removal. A “tie wrap” around a USB cable is insufficient.
- The touchscreen shall be rated for Division 1 use (intrinsically safe). If the rear is general purpose, a Type X purge is required. Yes the display can have two ratings; one for the front, one for the rear.
- Operator terminals or operator panels do not have Division 1 rated touchscreens, nor function keys and therefore are not suitable for Division 1 use.
For Type X purge, general purpose equipment may be used within the protected enclosure. However, any externally mounted equipment, such as a touchscreen, must be rated for the area.
We mentioned safety level and risk. Another fundamental difference between Type Y purge systems and Type X purge systems is that a Type Y is only required to provide an alarm output, while a Type X is required to disable power from the protected enclosure. Many insurance company underwriters, safety directors, and risk assessment professionals require power be removed from the enclosure upon loss of air. Check with your safety director for their policy.
In any case, the entire solution is evaluated to Division 1 requirements. The certification on the purge system is insufficient for the entire enclosure, display, and components. However, using certified components makes getting system certification easier and satisfies the expectations of risk assessment personnel.