Thinking about the box: Determining the appropriate level of protection for an enclosure

Though enclosures are not normally considered to be part of a fieldbus system, they are an important component of a successful implementation because practically every field termination is placed in some form of “box.”

The most common use of an enclosure is to protect the equipment inside from the elements, as well as mechanical damage from accidental contact with other equipment. But how do you determine the appropriate level of protection for an enclosure?

The answer exists in the enclosure rating – normally a NEMA or IP number. (See Tables 1 and 2)

Table 1
Ingress protection (IP) codes

First numerical protection
against solid bodies

No protection 0
Objects greater than 50 mm 1
Objects greater than 12 mm 2
Objects greater than 2.5 mm 3
Objects greater than 1.0 mm 4
Dust protected 5
Dust tight 6

Second numerical protection
Against liquid

No protection 0
Vertically dripping water 1
Angled dripping water,
from 75 to 90 degree angles 2
Sprayed water 3
Splashed water 4
Water jets 5
Heavy seas 6
Effects of immersion 7
Indefinite immersion 8

Table 2
NEMA enclosure rating (From NEMA 250-1997)

1 Ideal for indoor use to provide a degree of protection to personnel against contact with the enclosed equipment, and to provide a degree of protection against dirt. – IP Equivalent: IP10

2 Type 1; provides a degree of protection against dripping and light splashing of liquids. – IP Equivalent: IP11

3 PC cabinet for indoor or outdoor use to provide a degree of protection to personnel against contact with the enclosed equipment; provides a degree of protection against dirt, rain, sleet, snow and dust; ensures that the equipment will not be damaged by the external formation of ice on the enclosure. – IP Equivalent: IP54

3R Cabinet for indoor or outdoor use to provide a degree of protection to personnel against contact with the enclosed equipment; provides a degree of protection against dirt, rain, sleet, snow and dust; ensures that the equipment will not be damaged by the external formation of ice on the enclosure. – IP Equivalent: IP14

3S Type 3; external mechanism(s) remain operable when ice laden. – IP Equivalent: IP54

4 Computer enclosures constructed for indoor or outdoor use to provide a degree of protection to personnel against contact with the enclosed equipment; provides a degree of protection against dirt, rain, sleet, snow, dust, splashing water and hose-directed water; ensures that the equipment will not be damaged by the external formation of ice on the enclosure. – IP Equivalent: IP56

4X Constructed for indoor or outdoor use to provide a degree of protection to personnel against contact with the enclosed equipment; provides a degree of protection against dirt, rain, sleet, snow, windblown dust, splashing water, hose-directed water and corrosion; ensures that the equipment will not be damaged by the external formation of ice on the enclosure. – IP Equivalent: IP56

5 Computer enclosure constructed for indoor use to provide a degree of protection to personnel against contact with the enclosed equipment; provides a degree of protection against dirt, dust, lint and fibres, as well as dripping and light splashing of liquids. – IP Equivalent: IP52

6 PC enclosures constructed for indoor or outdoor use to provide a degree of protection to personnel against contact with the enclosed equipment; provides a degree of protection against dirt, hose-directed water and the entry of water during occasional temporary submersion at a limited depth; ensures that the equipment will not be damaged by the external formation of ice on the enclosure. – IP Equivalent: IP67

6P Cabinet constructed for indoor or outdoor use to provide a degree of protection to personnel against contact with the enclosed equipment; provides a degree of protection against dirt, hose-directed water and the entry of water during prolonged submersion at a limited depth; ensures that the equipment will not be damaged by the external formation of ice on the enclosure. – IP Equivalent: IP67

12 Computer cabinet constructed (without knockouts) for indoor use to provide a degree of protection to personnel against contact with the enclosed equipment; provides a degree of protection against dirt, dust, lint and fibres, as well as dripping and light splashing of liquids. – IP Equivalent: IP52

12K Constructed (with knockouts) for indoor use to provide a degree of protection to personnel against contact with the enclosed equipment; provides a degree of protection against dirt, dust, lint and fibres, as well as dripping and light splashing of liquids. – IP Equivalent: IP52

13 Constructed for indoor use to provide a degree of protection to personnel against contact with the enclosed equipment; provides a degree of protection against dirt, dust, lint and fibres, as well as the spraying, splashing and seepage of water, oil and non-corrosive coolants. – IP Equivalent: IP54

Once you have identified the type and size of the enclosure, keeping in mind room for maintenance and future expansion, you need to consider the necessary accessories, such as: seals; breathers and vents; enclosure material; support (How will the enclosure be mounted?); security; additional internal components; and what materials should be used for internal and external nameplates.

When making these decisions, don’t forget to consider maintenance, accessibility of the components in the enclosure, and unit wash down. You should also consider what you are going to do with the cables entering the enclosure. Will you terminate them on terminals or coil the spares in the bottom of the cabinet?

Selecting the appropriate enclosure is an important decision. Getting a reliable signal from the field requires proper selection of such accessories as the enclosure to make sure the entire system works.