How Nema Electrical Ratings For Enclosures Affect Cathodic Protection Design And Installation

The National Electrical Manufactures Association (NEMA) has designated electrical enclosures into different categories or types depending on the level of protection required. These enclosure ratings affect a cathodic protection (CP) design or installation.?

Design Purpose

To determine the appropriate rating and enclosure for a CP design, an engineer first considers the purpose.?It can include:

• Protection from the external environment (rain, dripping water, water spray, salt air, dirt, dust)
• Security of the internal components from physical damage or unauthorized tampering
• Convenient access to the internal components by the intended user
• Proper enclosure size to contain needed equipment and allow adequate room to work within the enclosure for cable routing, etc.

Enclosure Ratings

• Type 1 – Indoor use only (to keep out solid objects such as fingers, falling dirt and similar)
• Type 2 - Same as Type 1 and prevents ingress of dripping or lightly splashed water
• Type 3 - Can be used indoors or outdoors and provides same protections as Type 1 & 2 with additional protection from windblown dust, ingress of water (rain, sleet, snow) - allowing it to be unaffected by external formation of ice on the enclosure
• Type 3R - Same as Type 3 but does NOT provide protection from windblown dust (Most air-cooled CP rectifier enclosures comply with this rating)
• Other Type 3 Ratings - 3S, 3X, 3RX, 3SX (only minor differences)
• Type 4 - Same as Type 3 and includes protection from hose directed water
• Type 4X - Same as Type 4 and adds a level of protection against corrosion via the use of stainless steel, aluminum or non-metallic materials

NOTE: Additional NEMA types include 5, 6, 6P, 11, 12, 12K, and 13.?Alternatively, types 7, 8, 9, 10 are used in hazardous locations. A complete listing of NEMA types can be found at:?https://en.wikipedia.org/wiki/NEMA_enclosure_types .

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Classifications

All enclosures will ultimately be installed in one of two AREA classifications:??

1. Unclassified Area– Any enclosure type is allowed if it meets the physical and environmental requirements and complies with specific company electrical specifications.
2. Classified Area– These are for locations considered “Hazardous” or has a "Hazardous Area Classification".?There is typically some type of product or substance within the physical area that would be flammable or explosive in certain concentrations.?This could include flammable gases, liquids, dust and lint.

To define a hazardous area, there are Classes, Divisions and Groups:

Class Definition

• Class I - Locations in which flammable vapors and gases?may?be present
• Class II - Locations in which combustible dust may be found
• Class III - Locations that have easily ignitable fibers or “flyings”

Division Definition

• Division 1 - Ignitable concentration of hazards exist under normal operation conditions and/or where hazards are caused by frequent maintenance or repair work or frequent equipment failure
• Division 2 - Ignitable concentrations of hazards are handled, process or used, but which are normally in closed containers or closed systems from which they can only escape through accidental rupture or breakdown of such containers or systems.

Group Definition

• Group A - Acetylene
• Group B- Hydrogen, Ethylene Oxide, Propylene Oxide
• Group C - Ethylene, Ethyl Ether
• Group D- Propane, Acetone Benzene Butane, Ethanol

In most refineries and/or process facilities, the most common hazardous area classifications include:?????

• Class 1 Div.1 Group D - Hazardous area and all enclosures must be “explosion proof”
• Class 1 Div. 2 Group D - NEMA 3R enclosures MAY be permissible

Area classifications are normally assigned when the facility is designed. A classification, within an area, can change within a few feet from a classified area to an unclassified area, or vice versa.?Therefore, having knowledge of where the classified area change occurs is important to ensure that the proper NEMA enclosure rating is specified.

It may seem odd but there may be occasions where a NEMA 3R rated enclosure can be used in a Class 1 Div. 2 area. According to National Electrical Code (NEC) Article 501-3, Para (b),(1),c., NEMA 3R rated enclosures are allowed for the use of "non-incendive” circuits that, under normal conditions, do not release sufficient energy to ignite a specific ignitable atmospheric mixture.?

Interpretation of Article 501-3 of the NEC permits general-purpose enclosures in a Class 1 Div. 2 area for some applications if the equipment does not constitute a source of ignition under normal operating conditions. Therefore, the use of a NEMA 3R enclosure for a simple anode shunt box or structure junction box would be permitted, as there is no spark or incendive equipment that exists under normal operations.

In the case of CP rectifiers in a Class 1 Div. 2 area, the rectifier MUST be oil cooled and all conduit fitting MUST be “explosion proof”.

As a designer of cathodic protection, it is important to know the environment, physical requirements and area classification prior to specifying an enclosure or rectifier type.