One of the acceptable “protection techniques” for electrical enclosures within a Hazardous Location is to “pressurize the enclosure.” Here are the three (3) Types of Pressurized Enclosures and the HAZLOC they are permitted to be used, and their design basis:
| NFPA 70, 500.7, (D) Purged and Pressurized. This protection technique shall be permitted for equipment in any hazardous (classified) location for which it is identified. |
In Class I locations, purged and pressurized enclosures are used to eliminate or reduce the infiltration of the surrounding hazardous atmosphere from entering the enclosure. In Class II locations, pressurized enclosures prevent the entrance of dusts into an enclosure. Purged and pressurized enclosures make it possible for equipment that is not otherwise acceptable for Class I and Class II locations to be used in those locations.
The types of pressurizing are as follows:
Type X Pressurizing
Enables use of equipment suitable for unclassified locations within the protected enclosure where the equipment would otherwise be required to be suitable for Division 1 or Zone 1 locations.
Type Y Pressurizing
Enables use of equipment suitable for Division 2 or Zone 2 locations within the protected enclosure where the equipment would otherwise be required to be suitable for Division 1 or Zone 1 locations.
Type Z Pressurizing
Enables use of equipment suitable for unclassified locations within the protected enclosure where the equipment would otherwise be required to be suitable for Division 2 or Zone 2 locations.
Type X Design
- Detection shall be provided to indicate failure to maintain positive pressure within a protected enclosure.
- Failure to maintain the positive pressure within a protected enclosure shall be communicated by an alarm or an indicator.
- It shall not be required to de-energize the protected equipment upon detection of the failure to maintain positive pressure within a protected enclosure.
- Any protected enclosure that can be isolated from the protective gas supply shall be equipped with an alarm, except as permitted below
- The protected enclosure shall be permitted to be equipped with an indicator where the isolation is done with a valve(s) that complies with the following:
- The valve(s) is immediately adjacent to the protected enclosure.
- The valve(s) is intended for use only during servicing of the protected enclosure.
- The valve(s) is marked as required in 4.12.5.
Where an alarm is used, the following shall apply:
(NOTE: An “alarm” is a piece of equipment that generates a visual or audible signal that is intended to attract attention.)
- The alarm shall be located at a constantly attended location.
- The alarm actuator shall take its signal from the protected enclosure and shall not be installed between the enclosure and the protective gas supply.
- The alarm actuator shall be mechanical, pneumatic, or electrical.
- Electrical alarm actuators shall be identified for a Division 2 or Zone 2 location as applicable.
- No valves shall be permitted between the alarm actuator and the enclosure.
- The alarm shall be permitted to satisfy the requirement in 4.4.2 to provide an alarm on the protected gas supply.
Where an indicator is used, the following shall apply:
(NOTE: An “indicator” is a piece of equipment that shows flows or pressure and is monitored periodically, consistent with the requirement of the application.)
- The indicator shall be located for convenient viewing.
- The indicator shall not be installed between the enclosure and the protective gas supply.
- The indicator shall indicate either pressure or flow.
- No valves shall be permitted between the indicator and the enclosure.
- The protective gas supply shall have an alarm that is located at a constantly attended location to fulfill the requirement in 4.4.2.
Type Y Pressurizing
- All of the requirements for TYPE X shall apply.
- Equipment within a protected enclosure shall be approved for Division 2 or Zone 2 locations.
- Electrical alarm actuators shall be identified for a Division 1 or Zone 1 location as applicable.
- Ventilated equipment that would develop temperatures higher than the marked temperature class (T Code) upon failure of the ventilation shall be automatically de-energized when the flow of protective gas stops.
Type Z Pressurizing
Except as permitted below, a device shall be provided to automatically de-energize ALL circuits and equipment within the protected enclosure that are NOT IDENTIFIED for Division 1 or Zone 1 upon failure of the protective gas supply to maintain positive pressure, per the following:
- The device provided to de-energize power upon failure of the protective gas supply to maintain positive pressure shall be actuated by flow or pressure sensors.
- The flow or pressure sensor shall be identified for the location in which it is installed.
- No valves shall be permitted between the flow or pressure sensor and the protected enclosure.
- The sensor shall take its signal from the protected enclosure and shall not be installed between the enclosure and the protective gas supply.
Power shall be permitted to be continued for a short period if immediate loss of power would result in a more hazardous condition, and if both audible and visual alarms are provided at a constantly attended location.
Except as permitted below, equipment that can be overloaded shall be provided with devices to detect any increase in temperature of the equipment beyond its design limits and shall de-energize the equipment automatically.
Power to the circuits shall be permitted to be continued for a short period if immediate loss of power would result in a more hazardous condition, and if both audible and visual alarms are provided at a constantly attended location.
Except as permitted below, the flow of protective gas shall provide sufficient cooling for ventilated equipment, even during overload conditions, or the equipment subject to overloading shall be provided with devices to detect any increase in temperature beyond its design limits and to de-energize that equipment automatically.
Power to the circuits shall be permitted to be continued for a short period if immediate loss of power would result in a more hazardous condition and if both audible and visual alarms are provided at a constantly attended location.