Safety Criteria Window

Overview

The Safety Criteria module calculates permissible touch and step voltage limits in compliance with Standards. These will be used in Display Results to assess safety by comparing the actual touch and step voltages with these safe limits.

Standard

This module can calculate safety criteria limits based on two main safety standards: “IEEE Std. 80 IEEE Guide for Safety in AC Substation Grounding” and “IEC 60479 Effects of current on human beings and animals”.

IEC method is based on more recent science than IEEE. In general, safety criteria limits are higher (less conservative) for IEC than for IEEE when the duration of the fault is < 0.4 seconds.

Notes about the IEC method implementation in SafeGrid:

• Body resistance factors (or Body factor, BF) based on body current path are 0.7425 and 1.015 for touch and step voltages respectively.

• Heart current factor (HF) assumed (conservatively) to be equal to 1.0 for both touch and step voltage calculations.

• Body resistance values used are for large surface area of contact (10,000 mm2 square).
  Important differences between the two standards are as follows:

Safe limits table

You can add multiple voltage limits. You can click on the table to turn the Plots ON of OFF. All the safe limits will be available in Display Results for comparing with the actual voltages.

Foot resistance

Foot resistance is calculated using the method prescribed in IEEE Std. 80 regardless of whether the IEEE or IEC safety standard is selected. The foot resistance is combined with the body resistance in order to calculate the step and touch voltage limits.