By Lonnie Lindell
Short circuit calculations have traditionally been used to select equipment that can withstand or safely interrupt the available fault current. While we attempt to estimate short circuit currents as accurately as possible, we typically make assumptions that produce conservative results for the equipment rating comparisons. These assumptions include ignoring impedance of cable terminations and protection devices, effects of temperature, transformer taps, pre-loading conditions, grouping motor contributions together, ignoring short cables, estimating cable lengths, and using a large or infinite utility contribution estimate. While these simplifying assumptions may be appropriate when selecting withstand and interrupting equipment ratings, they can lead to serious safety problems when used in arc flash hazard calculations.
Before continuing the discussion about how to apply safety tolerances to arc flash calculations, it’s important to review the basis of arc flash hazard calculations and understand why lower fault currents can result in increased risk.
Arcing Faults versus Phase-to-Phase Faults
While a traditional phase to phase or phase to earth fault flows within the power system, an arcing fault releases energy into the environment. This energy takes the form of light, sound, heat and pressure, and can result in fire and burns, hazardous vapors, shrapnel, and pressure-related injuries.
The amount of energy released is the result of the current arcing between the conductors, ionization of surrounding gases and the duration of the arc. The total energy released is typically more sensitive to arc duration than it is to the current magnitude. For example, a 200,000 Amp arcing current for 0.01 seconds results in 2.4 calories per cm2, whereas a 10,000 Amp arcing current for 0.2 seconds results in 4.2 calories per cm2.
Lower arcing fault currents therefore may release more heat energy than higher arcing fault currents because the trip time is often longer for the lower fault current.
The Importance of Safety Tolerance in Arc Flash Calculations
Figure 1 helps explain how a lower short circuit current can result in higher energy. The diagram displays the melting characteristics of a fuse. The current is reported on the horizontal axis and the melt time is read from the vertical axis. Using a maximum estimated short circuit current, the fuse will melt in 0.02 seconds. At the minimum estimated short circuit current, the fuse will not melt for 1 second. At lower currents, it takes longer for the fuse to melt and clear the arcing fault. The longer the arc lasts, the more energy is released into the environment.
Since the exact fault current that will flow during a 3-phase bolted fault isn’t known, and the arcing-fault current and trip times are dependent upon the 3-phase bolted fault, it is prudent to apply a tolerance to the fault calculations. The calculated fault current range can then be used to determine conservative arc flash hazard values.
A similar example using a circuit breaker (Figure 2) indicates that the maximum fault current trips in the instantaneous region (0.03 seconds), whereas the minimum fault current trips in the short time region (0.3 seconds). For this case the instantaneous or the short time delay should be reduced to minimize the incident energy.
Applying Tolerances to Fault Calculations
The fault tolerance can be determined by assigning a tolerance to each component in the short circuit calculation. A sample one-line using tolerance values for the short circuit calculation is shown in Figure 3.
Example
The Utility fault contribution is entered as 15000 Amps Minimum and 25000 Amps Maximum to account for different utility switching configurations. The Transformer impedance is entered as 5.75% with at tolerance of +/- 7.5%. The cable length is entered as 50 feet with a tolerance of +/- 5%. The primary protection device is specified as a 2000 Amp Gould Shawmut A4BT Class L fuse.
The resulting fault current at the MCC equipment bus is 83.6 kA maximum and 61.3 kA minimum. The resulting arcing fault current through the primary protection is 49 kA maximum and 25 kA minimum. The trip time of the primary protection is 0.01 seconds at the maximum fault, and 0.22 seconds for the minimum fault. The maximum energy occurs at the minimum fault current and results in 20 calories per cm^2 (Class 3).
A label reporting the largest incident energy is shown in Figure 4. The energy is 20.6 cal/cm^2 (Class 3).
Figure 4 - Arc Flash from Minimum Fault
If we had used the traditional assumptions that calculate only the largest fault current, the incident energy would have been calculated at 5.6 cal/cm^2 (Class 2). The resulting Class 2 clothing (PPE) would not adequately protect the worker.
Figure 5 - Arc Flash from Maximum Fault
The author, Lonnie Lindell is an electrical industry veteran with over 20 years with SKM Systems Analysis, Inc. In his responsibilities as General Manager he is closely involved in engineering software design, application and training.
