|
 SHORT
CIRCUIT LIMITATIONS
On power systems with particularly high KVA capacity, the available
short circuit current must be considered in the selection of the
conductor size. The graphs on the following pages show the maximum
amperes Kerite cables and shields can carry for various periods
of time without injury to the insulation system.
BASIC CALCULATION
When calculating the time a conductor can carry a particular
fault current, or determining the fault current which can be carried
for a specific time, it is conservatively assumed that the total
heat generated is stored in the conductor for the brief duration
of the short circuit, without any dissipation of heat to the environment.
The following
basic equation is then used. Either the allowable fault current
(I), the allowable duration of time (t), or the cross sectional
area (A) of metal necessary to sustain a particular fault can be
computed when two of the three variables are known.
A = Total cross-sectional
area concentric neutral, tape shield, lead sheath or phase conductor
(circular mils)
I = Fault Current (amperes)
t = Duration of Fault (seconds)
k = Constant for conductor or shield material
|
Conductor
Copper
Aluminum
|
k
value
5.215 x 10-3
2.341 x 10-3
|
|
Starting
temperature 90°C. Maximum safe temperature 250°C
|
Shield
Copper
Bronze
Zinc
Cupro-Nickel
Lead |
6.258 x 10-3
3.383 x 10-3
1.534 x 10-3
0.560 x 10-3
0.225 x 10-3 |
|
Starting temperature 65°C. Maximum safe temperature 250°C. |
RESISTANCE
AT OTHER TEMPERATURES
Where:
R1 = Resistance at Temperature T1
R2 = Resistance at Temperature T2
|
T1 = |
Reference
Temperature °C |
| T2
= |
Temperature
at Which Resistance R2 is desired °C |
| K
= |
234.5
for copper / 228 for aluminum |
SHORT
CIRCUIT CURVES TIME
VS. FAULT CURRENT FOR SAFE OPERATIONCOPPER CONDUCTORS
(For Aluminum Conductors Multiply Time Scale by 0.45)
SHORT
CIRCUIT CURVES
TIME
VS. FAULT CURRENT FOR SAFE OPERATION
5 mil COPPER TAPE
NOTE: Effective cross-sectional area of tape (A) including lap
conduction equals:
where:
TS = Tape Thickness (mils)
SD =
Shield Diameter (mils)
PL = Tape Lap (Percent)
The above graph
is used to obtain times for safe operation under fault conditions
(tape temperature limited to 250°C). For fusing, the same graph
may be used as follows (tape reaching its melting temperature).
1. To
find the time of fusing for a particular current, enter chart with
current, find safe time and multiply by 4.93 to get time to fusing.
2. To
find the fusing current for a particular time, divide the time by
4.93 and enter the chart with this figure to find the fusing current.
SHORT
CIRCUIT CURVES
TIME VS. FAULT CURRENT FOR SAFE OPERATION
5 mil Cu-Ni TAPE
TIME
- CYCLES (60Hz BASE)
NOTE: Effective cross-sectional area of tape
(A) including lap conduction equals:
where:
TS = Tape Thickness (mils)
SD
= Shield Diameter (mils)
PL = Tape Lap (Percent)
The above graph
is used to obtain times for safe operation under fault conditions
(tape temperature limited to 250°C). For fusing, the same graph
may be used as follows (tape reaching its melting temperature).
1. To
find the time of fusing for a particular current, enter chart with
current, find safe time and multiply by 4.93 to get time to fusing.
2. To
find the fusing current for a particular time, divide the time by
4.93 and enter the chart with this figure to find the fusing current.
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