Intermediate Zone Characteristics of Cables- Error in IEEE Std. 242 2001?

[recs]

IEEE Std. 242 2001 in page 314 describe how to obtain the overload curves (i.e. intermediate zone characteristics) for cables. this is the zone between the continues load capability and the short-circuit capability. It says:

"Intermediate zone characteristics of medium-voltage cables and 75 °C and 90 ˚C
thermoplastic cables are tabulated in Table 9-6 with the characteristics of medium-voltage
cable illustrated graphically in Figure 9-17a. These factors all apply to preloading at rated
ampacity at 40 ˚C ambient temperature. For lower ambient temperatures and when cable
ampacities have been increased to take this into account, the intermediate overload current
percent should be reduced by the following factors for each degree decrease in ambient
temperature below 40˚C"

I believe this might be wrong. The overload curve should be increase by that factor if the cable is operated at a ambient temperature below 40˚C. By increasing this the overload capability by that factor, the overload curve moves to the right (instead to the left) therefore you may overload the cable a little bit more at lower ambient temperatures.

Is my reasoning wrong? If it is, why?

Thanks.

 

[davidbeach]

You're omitting the and when cable ampacities have been increased to take this into account part. The extra capacity has already been used.

 

[7anoter4]

As davidbeach already said the sentence written in IEEE-242 CH.9.5.2.4 Development of intermediate characteristics: "These factors all apply to preloading at rated ampacity at 40 ˚C ambient temperature. For lower ambient temperatures and when cable ampacities have been increased to take this into account, the intermediate overload current percent should be reduced by the following factors for each degree decrease in ambient temperature below 40 ˚C:" is correct.
Let's say your cable works at the rated temperature at the ambient of 40dgr.C and the current will be 100 A. Let's say the k=1 [small cable underground].According to Figure 9-17a—Emergency overload current percent of continuous rating we get 500% for 100 sec overload-that means 500 A.
If the ambient would be 30 the maximum permissible current could be 110 –indefinite time- so for 100 sec could be 5*110=550 A. But you have to reduce this current by 0.004 for each dgr.C-that means 40-30=10 10*0.004=0.04 1-0.04=0.96 0.96*550=528 A.

 

[recs]

7anoter4

Thanks so much for your help. I kind of understood the first kindly response of Mr. davidbeach, but an example is the equivalent to a thousand words.

Thanks again.

RECS