CableDataSheet, Cable and Wire Technical Consulting Service

SHORT CIRCUIT RATINGS

The short circuit capacity of a current carrying component of a cable is determined by the following factors:

1. The temperature prior to the short circuit, generally taken to be that corresponding with the maximum conductor operating temperature under normal conditions.
2. The energy produced by the short circuit, a function of both the magnitude and the duration of the current.
3. The limiting final temperature, generally determined by all materials in contact with the conducting component.

The adiabatic (no heat loss) equation for the temperature rise during a short circuit is as follows:

I² * t = k² * S²
Where 
I = Short Circuit Current (r.m.s. over duration) (A) 
t = Duration of Short Circuit (s) 
k = Constant depending on the material and the initial and final temperatures 
S = Cross-sectional Area of Current Carrying Component (mm²)

Rearrangement of the general equation gives the formulae for Isc, the Short Circuit Rating for a particular Conductor Size, and for calculation of Sc, the Minimum Conductor Size to meet a specified short circuit level.

Isc = ( k * S ) / sqrt(t) (A) 
Sc = ( I * sqrt(t) ) / k (mm²) 
(Round up to the nearest standard conductor size.)

Values of k for Cu and Al Conductors with PVC or XLPE Insulation

Insulation Type	Copper Conductor		Aluminium Conductor
	Up to 300 mm²	Over 300 mm²	Up to 300 mm²	Over 300 mm²
PVC*	111	98.7	73.6	65.3
XLPE#	143	143	94.5	94.5

* Insulation material temperature limits for PVC of 75°C to 160°C up to 300 mm² and 75°C to 140°C above 300 mm² apply. 
# Insulation material temperature limits for XLPE of 90°C to 250°C apply.

These values are based on the limits imposed by the insulation material alone. Note that soldered joints impose an upper temperature limit of 160°C, while for mechanical (bolted) joints the manufacturer's recommendations should be observed. The above temperature limits are appropriate for durations of up to 5 seconds only.

One Second Short Circuit Ratings

In practice it is often convenient to work with short circuit ratings converted to a one second basis. Reference may then be made to Table 2.3 which gives one second short circuit ratings for Copper and Aluminium conductors with PVC and XLPE insulation materials respectively.

To convert a one second rating to a t second rating, divide by sqrt(t), e.g., 34 kA for 1s equals 20 kA for 3s. 
To convert a t second rating to a one second rating, multiply by sqrt(t), e.g., 10 kA for 0.04s equals 2 kA for 1s.

Other Considerations

In addition to the temperature rise, consideration should also be given to the thermomechanical (longitudinal) and electromechanical (lateral) forces which can be generated under short circuit conditions.


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