Q272 : Calculation of Safety Limits in Output Toolbox
Question How are the safety limits calculated in Output Toolbox.
Answer The CDEGS package offers many options to control the computation of the safety limits for touch and step voltages. I can't discuss all of them here, so I will focus instead on the default methods used by the program. The maximum allowable touch voltage is given by the product of the fibrillation current and the total resistance to the current flow. The fibrillation current is given by:
If = 0.116 / sqr(Tf) (Amps)
where Tf is the fault clearing time in seconds. For example, for Tf = 0.25 s, this is 0.232 Amps.
The total resistance to current flow is comprised of two parts: the body resistance and the foot resistance. For the body resistance, the program uses a default value of 1000 Ohms. For the foot resistance, the program uses a series expansion to take into account the presence of the insulating layer. This is the procedure recommended by the IEEE Guide 80-2000. When the insulating layer is absent, the formula for one foot is approximately:
Rfoot = 3 * Rho (Ohms)
where Rho is the resistivity of the top soil layer in Ohms-m. For 2 feet in parallel, this is 1.5 Rho. For a top layer soil resistivity of 100 Ohms-m, this gives about 150 Ohms. The total resistance to current flow is therefore 1150 Ohms, which gives a safety limit for the touch voltage of 1150 * .232 = 267 Volts. The program then applies a derating factor to the fibrillation current to account for the fact that the fault current waveform is asymmetrical.