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Wire & Cable ASIA – September/October 2007 104 13
To provide for electrical safety, methods that determine
the level of insulation resistance to transmission of
electricity through various substrates have been
developed(7). One measure is volts per mil determined
by ASTM D149(8). This method is a measure of the
dielectric breakdown voltage and dielectric strength of
the insulation. The value is high for a good insulation
material.
Another measure is dielectric constant (DC) or
dissipation factor (DF) measured by fluid displacement
using ASTM D1531(9). Electrical resistance to
breakdown by treeing in insulation is measured by
ASTM D3756(10). The value is low for a good insulation
material. ASTM D4872(11) is a test method for
dielectric testing of wire and cable filling compounds.
The inclined plane tracking and erosion test, ASTM
D2303(12), or the dust and fog tracking and erosion
resistance test, ASTM D2132(13), evaluate tracking
resistance on the contaminated surface of an insulation
material.
b Physical Properties and Chemical Resistance –
Tensile strength, elongation low temperature and room
temperature impact, crush resistance, and additionally,
oil, gasoline, and ozone resistance can be evaluated for
the operating environment. Examples of performance
standards that contain these performance properties
are ASTM D470(14) , UL 44(15) and UL 83 (16).
c Flame and Smoke performance – Various levels are
attained depending on the required level of safety.
For cable, UL 44 contains FT1 – dripping particles,
FT2 – horizontal and FT4 vertical tray fire tests for
example. Additional examples of W&C fire and smoke
performance tests are shown in Table 1.
Electrical cable product fire safety performance has
been evolving. Fire safety is partly based upon control
of ignition, the rate of heat released, and flame spread
or smoke released during a fire. These are critical
measures and are evaluated by UL 1685(25) and ASTM
D5537(23), for example. The amount of heat released
determines how quickly a fire can spread. Greater
and more rapid heat release will overcome some fire
resistant additives. More heat released creates a
greater fire fighting challenge for firefighters who arrive
on the scene.
Controlling the amount and intensity of the heat and
smoke released will allow trapped people more time
to escape safely and property damage may be less
extensive.
The level of smoke can obscure vision and inhibit
or prevent people from escaping from a fire scene.
There are standards designed to measure the level of
smoke through standards such as UL1685(25) , ASTM
E662(24) , or smoke obscuration measured by ASTM
D5424(21). ASTM Standard D5485(22), first issued in
1994, addresses the corrosion of electronic systems as
a result of fire damage. Loss of electronic systems can
affect alarms and other critical systems in buildings.
These revised and newer standards reflect the
evolution toward better fire safety.
d Temperature performance – 75 or 90ºC rating
defines performance of cable for overload capability, or
operating temperatures for example.
Problems: there were unfinished fire escapes, no
extinguishers, sprinklers, alarms, telephones, or water
connections, and blocked exits. The theater attendance
exceeded 2,000 and the building was over capacity and at
SRO. This tragedy contributed to produce better fire codes
and standards.
Some results and improvements: this tragedy led to
panic bars, a sheet metal screen to be raised and lowered
between the audience and stage (not universally adopted
by Codes & Standards) and doors of public buildings that
must open in the direction of egress.
Wire and cable and electrical equipment standards were
not implemented until later in the century. Standards
support the minimum properties required for safety and
performance.
Every three years the NEC is significantly revised to keep
pace with technology and further enhance protection
against electrical fire and shock hazards. The absence of,
or poorly written, standards would significantly reduce the
quality of life on earth. Good standards serve as a barrier
to sub-standard products. Standards should bring value,
be effective and not act as a barrier to free trade. You can
help this process.
2.1.2 Value of Standards Development:
Standards are developed by a consensus reached
based upon input from a variety of interests. Producers,
users, testing companies and customers each bring their
perspective to the standards development process. This
process adds value because it develops standards that are
effective, relevant, and credible based upon the integrity of
the developers(5).
Advances in technology influence standards. They evolve
as wire and cable (W&C) technology changes. This makes
all of us, as producers and users, important, with our input
being vital for good standards to bring value and benefits.
Wire and cable products meet demanding electrical test,
physical, mechanical and chemical property performance
criteria for a variety of rated temperatures.
Standards development revolves around generation of
performance data and information to permit safe practices
during installation, testing and usage of wire and cable
products. We expect these products to last for 30 or more
years.
ASTM was cited by The 1947 President’s Conference
on Fire Prevention(6) for its recognition that performance
rather than specific materials should decide the future
course of fire safety. Performance allows for new materials
developments to raise safety standards to the next level.
2.1.3 Benefits of Standards Development:
a Safety – The NEC and ASTM, among others, provide
practical guidance to help safeguard employees
and the public from hazards during the installation,
operation, and maintenance of electric supply and
communication lines and associated equipment. The
NEC covers a broad range of electrotechnical areas,
including storage batteries, transformers, conductors,
switchgear, circuit breakers, physical clearances, cable
terminations, safety warning signs, and protective
clothing for workers installing electrical equipment.