| All common metal parts are
subject to size variations due to a change in the ambient temperature.
This is referred to as its "Coefficient of Linear Expansion." With respect
to the cable used with our switches, this physical attribute is mostly
an insignificant number except when cable lengths are in the excess of
150 feet (approximately 50m). This is especially critical on a switch that
utilizes a taut cable for actuation. In actual dimensions, the coefficient
is 6.89x10^-6 inches per degree per inch for steel cable. This is indeed
a very small dimension (in fact, it is 400 to 500 times smaller than a
human hair), and many people will overlook its effect on the operation
of a switch, but it can and often does become the source of numerous nuisance
tripping.
As an example, let us consider an installation that
has a cable run of 150 feet and is subjected to a |
temperature range of 50 to
100 degrees Fahrenheit. When we calculate the effect of temperature based
on these numbers, we find the following:
[0.00000689 inches / degree / inch
x 50 x 150 feet
x 12 inches / foot = 0.62 inches]
This means that in the above installation the cable
will change it's length by 0.62 inches just from the effects of a temperature
change. Additionally, steel cable is known to stretch with use adding to
the length of the actuating cable. With a switch that only requires a linear
movement of its shaft in the range of 0.20" to 0.60", the temperature related
change is a significat figure and must be addressed. See the chart below
for calculated variations. As can be seen the range is from near zero to
over 1.65." |
