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Can a contact rated at 10 amps carry 10 amps? Maybe yes, probably not. The reason lies in the test conditions used to rate the contact. If these conditions do not adequately reflect application conditions, actual allowable current levels may be lower than specified levels. For example, many manufacturers test a single contact in air. This gives an accurate measure of the basic current-carrying capacity of the contact. Use the contact alone in air and it can certainly carry 10 amps. Use it in a multiposition connector surrounded by other current-carrying contacts or in high ambient temperatures, and the contact should carry less current.
Similarly, as the contact ages and stress relaxation, environmental cycling, and other degradation factors take their toll, its current-carrying capacity decreases. A prudent design must set current levels for such end-of-design-life (EODL) conditions.
Practical current-carrying capacity is not an absolute, but an application-dependent condition.
New Method Simplifies Ratings
To help the designer set the appropriate current level, AMP has developed a new method of specifying current-carrying capacity. This new method takes into account the various application factors that influence current rating.
The method can be summarized as follows:
- The contact is aged to EODL conditions by durability cycling, thermal cycling, and environmental exposure
- The contact's resistance stability is verified
- The current necessary to produce the specified temperature rise is measured. This t-rise is usually 30 degrees C
- A rating factor is determined to allow derating of multiple contacts in the same housing and for different conductor sizes
Temperature, Temperatures
One other factor influencing current levels is the maximum operating temperature, usually 105 degrees C. If the application has a high ambient temperature (over 75 degrees C), the contact's t-rise is limited by the maximum operating temperature. For example, an application temperature of 90 degrees C limits the contact t-rise to 15 degrees C. Since current produces heat (the I squared R law), the current must be lowered to limit the t-rise.
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This graph clearly shows the relationship between base current, ambient temperature, and contact t-rise. |
A contact's t-rise depends not only on its I squared R Joule heating, but also on its ability to dissipate the heat. Consider a contact in multicontact housing. Joule heating in multiple contacts will raise the local ambient temperature. Since the contact will not be able to dissipate its own heat as well by convection, the maximum t-rise will be realized at a lower current level. Consequently, the allowable current level must be lower to maintain an acceptable t-rise.
For a given connector, the current level will be set by the loading density. A connector containing 50% current-carrying contacts will permit higher currents (per contact) than a connector will at 75% loading. The loading percentage assumes an even distribution of contacts within the housing. If all 10 contacts are grouped together in one section of the 20-position connector, the loading density may approach 100%.
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