Page 62 - Unistrut-Australia Catalog
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           NEMA CABLE LADDER - GENERAL INFORMATION
THE NEMA STANDARDS
NEMA STANDARD VE 1 is published by the National Electrical Manufacturers Association in the U.S.A. The Standard provides for the technical requirements of construction, performance and testing of cable ladder tray systems. It is regularly revised by the Association in order to keep pace with technology and the ever changing requirements of the manufacturers, contractors, consultants and other users throughout the electrical industry.
There is presently no Australasian Standard governing cable support systems. Despite the existence of other versions from places such as Canada and Europe, the NEMA VE 1 is by far the most widely accepted and the best known Standard for cable supports in Australia. In recognition of this situation, and in order to produce cable ladders of known quality and load capacity, Unistrut® has adopted a policy of constructing and rating its cable support products in accordance with the VE 1 Standard wherever possible. This policy is reflected in the name and various class designation numbers now used by Unistrut® which are drawn directly from the Standard.
For example NEMA Class 12B, 16A or 20B. Please note that in most cases Unistrut® ladders exceed the minimum strength requirements of each ladder class and therefore the published load graphs should be consulted in order to find the actual safe load capacity for each ladder type.
The more important aspects of the NEMA Standard VE 1 which are relevant to Unistrut® products are described as follows:
1. LOAD CAPACITY AND SAFETY FACTOR
Safe working loads are required to be determined as a result of testing a series of sample ladders. Tests must be conducted as simple spans (i.e. the worst case for loading) and over various span lengths with a safety factor of 1.5 against the collapse load
of the ladder. In this way, loads are based on average performance of a number of samples and not just a single test or some calculations. The Standard does not permit working loads to be determined by calculation because it has proven to be too unreliable. Cable ladders are specialist products which are unconventional in the structural sense. That is, they have an unusual combination of slenderness, local buckling of thin material and overall lateral restraint elements which are not satisfactorily intepreted by normal design methods.
2. DEFLECTION
The NEMA Standard VE 1 does not specify any limitation on the deflection of cable support members. To do so, would inevitably result in an over-designed (and hence uneconomical) system. For further information on deflection please refer to notes under Guidelines for Ladder Selection.
3. ELECTRICAL CONTINUITY
The electrical resistance of connections is limited to a maximum of 330 micro-ohms. Representative samples of Unistrut® splice joints (both steel and aluminium) as well as the run to side-rail joint in aluminium ladders have been tested by an Independent Electrical Laboratory, and in all cases were found to comply with the NEMA Standard VE 1 specification.
EXPLANATION OF NEMA VE 1 LOAD/SPAN CLASS DESIGNATIONS
The NEMA VE 1 rating method is based on the Imperial system of measurement, as follows: 1. The numerals indicate the ladder span in feet.
8= 8ft (2.4m)
12= 12ft (3.6m)
16= 16ft (4.8m)
20= 20ft (6.0m)
2. The letter indicates the working load category.
A= 50lbs/lin.ft (75kg/m)
B= 75lbs/lin.ft (112kg/m
C= 100lbs/lin.ft (149kg/m)
Example: A 20B class ladder requires a minimum safe working load of 75lb/ft. over a 20ft.span. (ie.112kg/m over a 6.0m span)
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