When installing a purged control panel or industrial HMI in a hazardous location, power supplied to the control panel must be regulated so that components are not energized before the panel is protected by a purge cycle.
In a Type Y or Z purge system, power may be manually controlled by closing a remote switch (energizing the contents of the enclosure) after the purge cycle is completed, and opening it (removing power to the contents of the enclosure) before depressurizing or opening the control panel. In a Type X purge, power is controlled automatically by the purge system.
Power is not the only factor to consider when designing your control panel or HMI for a hazardous location. Consideration should be given to design of the network communication to the panel, such as a wired Ethernet connection over a CAT5e cable. In normal use, this signal is low-voltage and not dangerous – but it is not without hazard. If the panel were not yet purged and there happened to be a fault or surge in the control room, it could travel through the CAT5e cable to the panel within the hazardous area and cause a spark or explosion if an ignitable gas or dust is present.
There are several options to prevent network communication signals from compromising the protection of your panel or industrial HMI in a hazardous area:
- Shut off the network switch that the panel is wired to. Opening this network switch will prevent any signal from passing to the panel. As is the case with the power supply, controlling the switch can either be done manually or automatically. Type X purge systems require automation. For Type Y and Z purge units, you can simply turn off the network switch until the panel is purged and can then be safely energized. For Type X purge systems, the automated power contactors are utilized to control a relay that powers the network switch. De-energizing the network switch is generally the least costly option, but it may not be desirable if there are other panels or control stations connected to the same switch.
- Communicate over a fiber optic connection using a pair of copper-to-fiber media converters. This isolates the panel electrically, making it impossible for a fault to induce an electrical voltage within the panel when it is not powered. The advantage of this option is that it provides a high-speed connection over a long distance. This option does require specialized knowledge and equipment for installation and connection.
- Make the Ethernet signal intrinsically safe. This can be done with a pair of intrinsically safe Ethernet barriers. Place one inside the panel within the hazardous area and the other located near the network switch. These barriers limit the amount of power that can pass through the CAT5e cable to the enclosure using a circuit consisting of Zener diodes and other electrical components. If there is a surge or fault condition, the intrinsic safety barriers shunt the current toward ground, preventing dangerous voltage from entering the unpurged cabinet. In addition, because the signal in the cable is intrinsically safe, less expensive wiring installation methods can be used.