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Derating is design process that can make significant contribution to reliability.The reliability of electrical/electronic component decrease when they operate at high stress level.These stresses are primarily temperature, voltage, current & power dissipation
In electrical derating means lowering the electrical capabilityof electrical apparatus'.In electronics derating is a operation of a device at less than its rated maximum power in order to prolong life.So it is necessary to know the derating factor for each component type related to one critical condition to run/operate in safe level.
Derating is a relative term. for cables ,drives and motors it is nomally related with term currrent w.r.to temperature etc. This data/details if given correctly allows user to use the same device with different rating(most of the times lower than rated) without having trouble to change it.
As an example most of the LV breakers referrecne temp is40 degree but for certain regions like Middle east where ambient is normally on much higher side,manufactuere either provide erating data for different ambient temperatures or they provide specially calibrated Brekers for55 degree ambient.
same concept also applies to cables(insulation consideration) drives(frequency) and motors(temp) .
In addition to this the age of the cable will have to be taken into account. As the insulation of the cable will degrade over time.
The current currying of cables will change depending upon the site temperature (location of site), type of run (it will run through duct, trench, buried etc.), number of tray, depth of trench, distance between cables. Considering this condition actual current currying capacity of cable reduce than current currying capacity (which given to cable Catalogue) this is called derating.
