What shortens the lifetime of Resistance heaters?
 
Aside from mechanical abuse and manufacturing defects, there are three basic causes of earlier heater failure.
 
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.
       
Causes of failure due to mechanical abuse :
  • Pulling on the heater leads -

  • Over tightening of post terminals - 

  • Using only one nut ( or tightening the bottom nut ) on post terminals - 

  • Lead wire insulation being rubbed off by motion of the leads ( can occur inside metal hose)