Equipment Damage Curves Cables

  
     
  This technical guide contains information about equipment damage curves. The information is presented for review, approval, interpretation and application by a registered professional engineer only. SKM disclaims any responsibility and liability resulting from the use and interpretation of this technical guide.

Reproduction of this material is permitted provided proper acknowledgement is given to SKM Systems Analysis Inc.

Purpose

The purpose of this guide is to provide basic information about insulated power cable damage curves.

Ampacity (> 1-6 hours)

The ampacity of a conductor is the rated continuous current carrying capacity at a referenced ambient temperature and allowable temperature rise. If a conductor is loaded continuously above its rated ampacity the conductor and insulation temperature design limits will be exceeded. This will lead to loss of life not instantaneous failure.

For insulated power conductors the industry standard ambient reference temperature level is 40°C, for underground insulated power cables (>2000V) 20°C, for buried for insulated power cables (0-2000V) all installations 30°C. Temperature correction factors are then given to adjust the base ampacity for other ambient temperature levels.
The ampacity landmark is located in the top decade of a time-current curve (TCC) at 1000 seconds.

Intermediate Overload Limit Curve (10 seconds to 1-6 hours)

Conductor overcurrent operating limit that if exceeded will damage the insulation of an insulated power conductor. This will lead to loss of insulation life, not instantaneous failure. Limit curves are based on the thermal inertia of the conductor, insulation and surrounding material. As a result, it can take from 1 to 6 hours for the temperature of a cable to stabilize after a change in load current. Therefore, under these emergency operating conditions, currents much greater than the rated ampacity of the cable can be supported. See IEEE 242-2001 pages 312 to 318 for more information.

Short Circuit Damage Curve (0.01 to 10 seconds)

Conductor short circuit current operating limit, which if exceeded, will damage the insulation of an insulated power conductor. Damage curves are plotted in the lower 3 decades of a TCC.		  
   
  Empirical formulas that consider all heat absorbed by the conductor metal and no heat transmitted from the conductor to the insulation are listed in equations 1 and 2. Both equations relate conductor temperature rise to conductor size, fault current magnitude and fault duration.

Copper conductors

t = 0.0297 log10[(T2+234)/(T1+234)] (A/I)2 (1)

Aluminum conductors

t = 0.0125 log10[(T2+228)/(T1+228)] (A/I)2 (2)

where:

A = conductor area – cmils
I = short circuit current – RMS amps
t = time of short circuit – 0.01 to 10 seconds
T1 = operating temperature
T2 = maximum short circuit temperature

Example

Plot the conductor landmarks for 3-1/C, 500MCM, THWN copper conductors installed in 2-1/2" conduit on a 480V distribution system.

FLA from NEC table 310.16

Ampacity = 380A

Intermediate points calculated from Tables 2 and 3		  
   
  Damage points calculated from (1) using:

A = 500,000 cmils
t = time of short circuit – 0.01 to 10 seconds
T1 = 75°C (Table 1)
T2 = 150°C (Table 1)		  
   
  References

Other SKM Technical Guides offered at www.skm.com.

The latest revision of IEEE Std 242, IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems (IEEE Buff Book).		  
     
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