Technical Specifications for ABS Plastic and Polycarbonate Electrical Enclosures

BriefOverview

It's common to find metal electrical enclosures installed nearly everywhere you turn and used throughout a multitude of industries and applications. However, it suprises us even today, that many of our customers have only known metal to be the answer to many of their projects when in fact, Polycarbonate or Acrylnitrile-butadiene-styrene (ABS) enclosures are suprising contenders if not better for their applications.

Both ABS Plastic and Polycarbonate electrical enclosure material offer excellent performance at both a material and application level and can be a suitable substitue over stainless steel, metal or even fiberglass enclosures depending on the application. Check out some of their higher-level benefits and drawbacks below before we get to the nitty-gritty.

*This is a general guide. The specific environment should be taken into consideration when selecting an enclosure.

*This is a general guide. The specific environment should be taken into consideration when selecting an enclosure.

Properties & Resistance of Gasket Material

Gaskets play an essential role in enclosure protection. In the long run, the IP rating and its reliability depend mainly on the properties of the gasket used in the enclosure. Key performance factors of a gasket are its compression set and proper fit in the gasket groove. The best general purpose gasket material is polyurethane which has an exceptionally low compression set. When accurately foam injected, the polyurethane gaskets stay tightly in the right place. EPDM material provides the best protection against the most popular industrial chemicals. However, it is wise to check the effect of a chemical on each gasket material since the chemical resistance of gasket materials varies greatly.

In some cases the performance of an enclosure can be modified by changing its gasket. The degree of protection depends on the material and the cross-section profile of the gasket. In addition to the cross-section profile of the gasket, there is the cross section of the gasket's contact with the surfaces of the enclosure's bases and covers. If the cross-section structures and manufacturing accuracy of an enclosure are not top quality, the IP rating will remain low even when equipped with a good gasket. When choosing gasket material, compare the elasticity properties of different materials and how these properties are affected by cold and heat, or by contact with various chemicals. An important factor is that both the enclosure and gasket must withstand the same chemicals in order to reach a safe resistivity.

*Physical Property Table includes some commonly-used physical properties of the gaskets.

*Chemical Performance Table contains only rough generalizations to give you some clues to gasket material behavior. The resistance of each chemical should be checked separately.

Flammability & Fire Safety UL 94 Classification

UL 94 is the standard applied by the American Underwriters Laboratories for the testing of the flammability and fire safety of plastic materials used in devices and appliances. The UL 94 HB (Horizontal Burning) tests the burning of a horizontal plastic specimen and the most demanding UL 94V (Vertical Burning) tests the burning of a vertical plastic specimen. Precise details of the equipment, conditions and test systems to be used can be found in the UL 94 standard.

UL 94 HB: Horizontal Burning Test

The length of the test specimen is 5" (127 mm) and width 0.5" (12.7 mm). The test sheet cannot be more than 0.5 (12.7 mm) thick. Lines are marked on the sheet at the 1" and 4" locations. The sheet is fixed horizontally at one end and tilted 45 degrees in the direction of the width, as shown in the diagram. The burner used is adjusted so that it forms a 1" long blue flame. The flame is directed at an angle of 45 degrees to the front edge of the plastic sheet so that about 1/4 of the test specimen’s front edge is in the flame. The flame is applied for 30 seconds, after which it is removed. If the test specimen burns to the 1" line before 30 seconds have passed the flame is removed immediately. The test is carried out on three test sheets.

The material under investigation is classified as UL 94 HB if, after the removal of the flame either condition below is true:

• The speed of burning does not exceed 1.5" (38.1 mm) per minute when the test specimen thickness is .120-.500" (3.05-12.7 mm)
• The speed of burning does not exceed 3.0" (76.2 mm) per minute when the thickness of the specimen is less than .120" (3.05 mm)
• Burning finishes before the flame reaches the 4" line

UL 94 V-0, V-1 and V-2 Vertical Burning Test

The test specimen length is 5" (127 mm) and width 0.5" (12.7 mm). The sheet to be tested can not be more than 0.5" (12.7mm) thick. The sheet is fixed 1/4" from the upper end in the vertical position. A metal mesh covered with surgical cotton is positioned 12" (305 mm) beneath the test sheet. The flame of the burner used is adjusted so that is forms a 3/4" blue flame. The flame is directed from below at the lower edge of the plastic sheet at a distance of 3/8" (9.5 mm). The flame is applied for 10 seconds, after which it is removed. The burning time of the test specimen is recorded. Immediately the burning stops the flame is reapplied for 10 seconds. After removal of the flame the burning time and afterglow time is again noted. The entire test is carried out on five different test specimens.

The material tested is UL 94 V-O classified if:

• None of the five test specimen burn for over 10 seconds at any time when the burner flame is removed.
• The total burning time of the 10 ignition test does not exceed 50 seconds.
• No test specimen burns either with a flame or afterglow to the clamp.
• No burning drops should fall which would cause the cotton underneath to ignite from any test specimen.
• The afterglow burning of no test specimen exceeds 30 seconds.

The material tested is UL 94 V-1 classified if:

• None of the five test specimen burn for over 30 seconds at any time when the burner flame is removed.
• The total burning time of the 10 ignition test does not exceed 250 seconds.
• No test specimen burns either with a flame or afterglow to the clamp.
• No burning drops should fall which would cause the cotton underneath to ignite from any test specimen.
• The afterglow burning of no test specimen exceeds 60 seconds.

The material tested is UL 94 V-2 classified if:

• None of the five test specimen burn for over 30 seconds at any time when the burner flame is removed.
• The total burning time of the 10 ignition test does not exceed 250 seconds.
• No test specimen burns either with a flame or afterglow to the clamp.
• Only such burning pieces may fall from the test specimen, which burn only momentarily, and of which some ignite the cotton underneath.
• The afterglow burning of no test specimen exceeds 60 seconds.

UL 94 5V Vertical Burning Test

The length of the test specimen is 5" (127 mm) and the width 0.5 (12.7 mm). The test sheet shall not be more than 0.5" (12.7 mm) thick. Flammability classification is granted for specific thickness intervals, for which both the maximum thickness and minimum thickness sheet must be tested. The test sheet is fixed 1/4" from the upper end in the vertical position. The flame of the burner used is adjusted so that it forms a 5" flame whose blue heart is 1,57" (40mm) long. The flame is directed from below at the corner of the plastic sheet at a distance of 1". The flame is applied for 5 seconds, after which it is removed for 5 seconds. This is repeated five times. The entire test is carried out on five test specimen.

The material tested is VI 94-5V classified if:

• No test specimen burns with a flame or afterglow for more than 60 seconds after the fifth ignition.
• The total burning time of the 10 ignition test does not exceed 250 seconds.
• No material drops from the test specimen.

Electrical Enclosure NEMA Ratings

The National Electrical Manufacturers Association (NEMA) is a U.S. Manufacturers Organization that actively promotes standardized product specifications for electrical apparatus. NEMA performance criteria and test methods are used by Underwriters' Laboratories as guidelines for investigation and listing of electrical enclosures.

*Approximate IP equivalents in parentheses.

Electrical Enclosure IP Ratings

Ingress protection class of enclosures is given in form of IP classification, a two digit coding which is shown below. The latter requires the second digit to be tested from class 6 upwards separately to each level of class, thus the double marking IP 66 / IP 67 indicates that the actual tests have been made for both levels.