THERMAL EXPANSION
The dimensions of most materials increase
when the material is heated; the material expands.
Design engineers and architects recognize the expansion
of material with temperature and design accordingly.
This is the reason that expansion joints are designed
into concrete roads, bridges, etc.
For the cables, the problem is more
severe.
- The temperature range is greater
- from -40C to 130C compared to -40C to 60C for
building materials.
- Cables are made of materials with
different properties. Some expand on heating but
recover to approximately original dimensions on
cooling, some expand but do not re- cover. Expansion
rates differ. These differences can cause rupture
or permanent deformation of components.
- Should voids in stress relief systems
or splices form or loose connections occur, corona
and hot spot or heat deterioration soon will lead
to cable failure.
For a long-lived cable, components
must be selected with their thermal expansion properties
in mind.
The following graph shows how two
different insulating materials can vary in expansion
characteristics over a range of temperature. XLP and
EP are used as examples. (For reference, copper expands
1.1% in volume, aluminum 1.6% from 25C to 250C. The
linear expansions are 0.4 and 0.53% for copper and
aluminum.)
THERMAL EXPANSION
Note: At normal 90C maximum operating
temperature, XLP expands approximately twice as much
as EP. At 130C emergency temperature, XLP expands
approximately three times as much as EP.
Excessive expansion of cable insulation’s
can initiate problems that can lead to eventual cable
failure, for example:
- Shielding tapes can rupture.
- Insulation’s can flow under
tightly fitting prefabricated slip-on accessories.
- Terminations can be damaged when
thermal expansion increases length as well as diameter.
- Expansion on heating and contraction
on cooling could result in voids at the stress relief
component and the insulation.
