EPC School

What is Purging?
Purging/pressurization systems permit the safe operation of electrical/electronic equipment under hazardous conditions when approved hazardous location equipment is not part of the application or the equipment
is rated but to a lesser hazardous location than is present.Purging is defined as a process of supplying a flow of clean air or an inert gas to the inside of an enclosure or motor to reduce hazardous internal airborne impurities to an acceptable safe level.

Which are the standards which describe the Purging Principals?
There are a few standards regarding purge and pressurization. Within North
America these are:
• NFPA 70 – National Electrical Code, Article 500
• NFPA 496 – Purged and Pressurized Enclosures for Electrical Equipment 1996 Edition
• ISA 12.4-1996 -- Pressurized Enclosures
• UL 698 – Industrial Control Equipment for use in Hazardous Locations, Class I, Gr. A, B, C, D and Class II, Gr. E, F, G
• CSA C22.1 Part 1 – Canadian Electrical Code

In Europe the following standards are to be considered:
• IEC 79-2
• EN 50016


What are the various types of Purge/Pressurization?
Purging of enclosures is not required before pressurization in some cases. For Class I, purging is required to remove flammable gases and vapors from the enclosure interior. Purging is not required for Class II although
combustible materials must be manually removed before starting pressurization.

The standards differ between three types of pressurization:
• Type X – Reduces classification within the enclosure from Class I or
II, Division 1 to non-hazardous.
• Type Y – Reduces classification within the enclosure from Class I,
Division 1 to Class I, Division 2.
• Type Z – Reduces classification within the enclosure from Class I,
Division 2 to non-hazardous.

• Type X purge/pressurization

This type of pressurization is defined to reduce classification within the enclosure from Class I or II, Division 1 to non-hazardous. The internal equipment can be rated for general-purpose, non-hazardous locations. The
purge/pressurization system and the electrical power source must be interlocked since a power failure to the pressure system and not the internal equipment could result in an explosion due to the continuous presence of a hazardous atmosphere in Division I areas. The enclosure or motor must exchange 4 or 10 volumes  espectively before power can be applied for Class I timing systems. The internal pressure must be aintained at 0.10” or 0.50” H2O, depending on whether the external hazardous atmosphere is Class I or II.

• Type Y purge/pressurization

This type is defined to reduce classification within the enclosure from Class I, Division 1 to Class I, Division 2. All internal protected equipment must be rated for Class I, Division 2 and an initial purge is required to ensure internal atmosphere is at a minimum, Division 2. The interlock between power system and the pressurization system is optional, but an audible alarm or visual indicators must be used to notify low pressure. The pressure must be maintained at 0.10”/0.50” H2O during the pressurization process.

• Type Z purge/pressurization

Type Z is defined to reduce classification within the enclosure from Class I, Division 2 to non-hazardous. The internal equipment can be rated for general-purpose, non-hazardous locations. An initial purge is required to
ensure internal atmosphere is non-hazardous. The interlock between power system and the pressurization is also optional, but an audible alarm or visual indicators must be used to notify low pressure. The pressure must be maintained at 0.10”/0.50” H2O during pressurization process.

• Differences between X, Y, Z

For Type X and Z systems, the internal equipment can be rated as generalpurpose equipment, but Type Y systems, Division 2 rated equipment is to be used. The purging time for Type Y and Z is based on a measured inlet airflow rate to the enclosure. Type X purging time is based on a measured airflow rate to the outlet. The removal of power to the enclosure is not required on loss of pressure for Type Y and Z systems. Since a Type X systems may have non-hazardous rated internal equipment in a Division 1 area, an interlock between the power and pressurization system is required to cut off power to this internal equipment in case of lost internal pressure. Type X systems also require electronic timers for full automatic control. In principle it can be said that Type X systems are more costly than Type Y and Z systems.