| REASON |
CONTAMINATION |
OVER-TEMPERATURE |
EXCESSIVE CYCLING |
| What it is ? |
Any foreign
material that is or becomes electrically conductive.
|
Anything that
causes the temperature of the resistance wire in the heater or
the lead wire to exceed its temperature rating. |
Turning the
heater on & off on a cycle that allows the resistance wire in
the heater to cool of by several hundred degrees below the
temperature reached when "on". |
| What it does? |
Makes a
conductive path between the resistance or lead wire and any
"grounded" object ( such as the heater sheath or metal lead
protection ) thus causing an electrical short. |
If the insulation rating is very high such as a ceramic, as
used in cartridge heaters , the resistance wire itself may
oxidize or melt; otherwise, the insulation breaks down, allowing
the resistance wire to short through. |
Any temperature cycle, even one, shortens heater life
because it causes more rapid oxidation on the surface of the
resistance wire, then tends to break off the oxide coat when
cooling. The greater the temperature difference in the cycle
over time, the shorter the lifetime. |
| Typical causes ? |
1 ) Water,
when soaked into the insulation of either the lead wire or the
element water makes a good short. Even high humidity can cause a
problem.
2 ) Oil, plastic or other hydrocarbons. These materials are
insulators at room temperatures, but conductors at higher
temperatures. It only requires trace amounts to cause a short. |
The basic cause of over - temperature is that the heat is
not being removed from the heating element fast enough (for
example wattage too high for application ). If the wattage of
the heater was selected well for the application, then these
factors are typically responsible
1) Heater not in good contact with the surface of the object
being heated.
2) Higher than design voltage applied to heater.
3 ) Heater not being used as directed (for example, strap not
tight on band heaters, or reduced fuel flow rate on immersion
heaters ).
4 ) Thermocouple used to monitor the temperature of the part
being heated is not in good contact with the part. |
Wattage of the
heater is much larger than needed and being used with a
temperature controller that runs the heater on long enough for
the element to get hot, then keeps it off long enough to become
cool. |
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