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The Importance of EODL
As noted, t-rise in a contact depends on both resistance and current. As it ages, a contact's resistance will increase. The contact designer will specify a maximum resistance for the contact: this level is the end-of-design-life resistance. Before the contact is tested for current, AMP subjects it to a sequence of tests that exercises the major failure mechanisms and thereby simulates EODL conditions. Conditioning includes mating cycling, industrial mixed-flowing gases, humidity and temperature cycling, and vibration to sequentially introduce wear, corrosion, stress relaxation, and mechanical disturbances.
Presentation
The presentation of current-carrying capacity in AMP product specifications includes two parts. First is a base curve showing current levels versus t-rise for a single circuit and the largest wire size. This represents the maximum current capacity of the contact. The curve is usually flat up to 75 degrees C ambient and then drops off. Up to 75 degrees C, the 30 degrees C t-rise limits the amount of current, and above 75 degrees C the current must be reduced to keep the combination of ambient temperature and t-rise from exceeding the maximum operating temperature of 105 degreesC.
Second are rating factors, a table of multipliers to account for connector loading and for smaller wire sizes.
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Rating factors allow the base current to be adjusted for various connector loadings and wire sizes. |
The designer first determines the base current for the ambient conditions of his application. He then multiplies this base current by the rating factors to find the current level for the application's loading factor and wire size.
An Example
To demonstrate the method of specifying current, consider the following application conditions: an ambient temperature of 65 degrees C, a 50% loading of contacts in the housing, and 20 AWG wire.
- From the base rating curve shown in the article, the base current rating is 14 amps with 18 AWG wire
- From the rating factors table, the rating factor for 50% loading and 20 AWG wire is 0.68
- The specific rating for this application is the product of the base rating and the rating factor: 14 x 0.68 = 9.5 amps
- Each of the contacts can carry 9.5 amps
- Notice, however, if the ambient temperature is 80 degrees C, the allowable t-rise becomes 25 degrees C. The base current must be lowered to 12.8 amps so that the 105 degrees C maximum operating temperature is not exceeded. The current rating then becomes: 12.8 x 0.68 = 8.7 amps
Real-World Values
The new current-rating method gives designers practical values applicable to their applications. While the specified current levels for a contact may be lower for other testing methods, they are more realistic and simplify the system design process. Specmanship is replaced by a realistic assessment of the real-world current-carrying capacity of a contact under varying conditions of temperature, connector loading, and wire size.
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