## Introduction

### Hazardous Voltages During Earth Faults

### Safety Standards

Derivation of minimum safety criteria is based on fundamental knowledge about these effects and numerous scientific investigations have been undertaken to determine safe limits.

Standards have been developed around the science, which provide permissible levels of body current to avoid the deaths of people exposed to electric shocks.

The two main safety Standards are IEEE Std 80-2013 [1] and IEC 60479-2005 [2].

### Threshold of Safety – Fibrillation of the Heart

the human body must be less than the value that can cause ventricular fibrillation.

(within a certain degree of statistical certainty).

### Definitions of Safety Criteria

*Ground potential rise (GPR):*

Maximum potential that an earth grid may attain compared with a distant point assumed to be the potential of remote earth.

*Touch voltage:*

Difference between the GPR and the surface potential at a point where the person is standing while being in contact with an earthed structure.

For touch voltages, unless there are concerns regarding transferred potentials to other remote locations via metallic paths such as overhead lines,

pipes or railway tracks only the area covered by the grounding system needs to be assessed.

*Mesh voltage:*

Maximum touch voltage within a mesh of an earth grid.

*Step voltage:*

Difference in surface potential experienced by a person bridging a distance of 1 m with the feet without being in contact with any earthed structure.

Step voltages must be assessed both within and for a significant distance beyond the extent of the area covered by the grounding system.

Step voltages are not usually a concern within the substation when touch voltages are satisfactory.

## Purpose

- IEEE Std 80-2013 – IEEE Guide for Safety in AC Substation Grounding [1].
- IEC/TS 60479-1 – Technical Specification – Effects of current on human beings and livestock [2].

The software provides an efficient method for determining safety criteria which complies with these Standards.

## Safety criteria – IEEE Std 80-2013

### Tolerable body current limits

### Body resistance

### Touch and step voltage criteria

For 50 kg body weight

Where:

*C*_{s} is the scaling factor due to the presence of the protective surface layer.

*ρ*_{s} is the resistivity of the surface layer in Ω.m.

*R*_{A} is an optional term to account for the effects of an additional series resistance such as from shoes or gloves.

Note if there is no protective surface layer then C_{s} = 1 and ρ_{s} = ρ_{1}.

### Effect of surface layer material

This is because a high resistivity surface layer provides additional series resistance with the body, thereby reducing the body current during a fault situation.

The resistivity of the surface layer should be at least 5 times higher than the top soil layer resistivity to have any great benefit [5].

These effects of the surface layer on allowable touch and step potentials are accounted for with the inclusion of a scaling term (C

_{S}) into the foot resistance (R

_{F}) calculation.

_{F}is computed as follows, assuming the foot to be a conductive plate on the earth’s surface:

Where:

b is the radius of the plate, usually assumed to be 0.08 m.

When a surface layer of resistivity ρs is introduced the formation becomes:

There have been many proposed methods for the calculation of C_{S}. A summary and comparison is given by Hans and Jagdish [8].

The currently accepted as most accurate method for calculating C_{S} is based on the work done by Thaper et al., [9].

This factor generally gives higher values of C_{S} compared with the previous formulation given in IEEE80-1986.

### Decrement factor

Where:

ω is the frequency in radians per second (equal to 2πf)

*t* is the fault clearing time in seconds

## Safety criteria – IEC 60479 [2]

Two main parts to this Standard are useful for deriving substation safety criteria (i.e. allowable touch and step potentials).

- Part 1: General aspects, Technical Specification IEC/TS 60479-1 Edition 4 (published 2005)
- Part 5: Touch voltage threshold values for physiological effects, Technical Report IEC/TR 60479-5 Edition 1 (published 2007)

The body impedance values corresponding to the 5th percentile (representing greater than 95% of the population) are lower and the most conservative from a safety perspective since they would result in higher current through the body. This is different from IEEE80 which uses a fixed value of 1000 Ω. Another difference is that the voltage limits are defined in terms of voltage across the body rather than prospective voltages, as in the American Standards case.

The zones of interest are AC-4 for the boundaries c1, c2 and c3. There is a dramatic reduction shown in tolerable currents (turning point in the graph) at around 400 ms.

This is due to the likely interference of the fault current with the T-phase (occurs at around 400 ms) of the heart pulse which is more likely to cause fibrillation of the heart.

- For a given fault clearing time and assumed probability of ventricular fibrillation determine the value of permissible body current from Figure 20 (IEC 60479 reference).
- For the permissible body current level determine the corresponding body resistance using the Tables.
- Compute the foot resistance in accordance with IEEE Std 80-2013
- Compute permissible touch and step potentials.

*Additional notes about IEC 60479:*

## Comparison of IEEE and IEC safety criteria

_{S}) were calculated using the method from IEEE Std 80-1986.

### Allowable Touch Voltages

Figure 5 shows the values for body resistance used by both Standards.

### Allowable Step Voltages

### Surface Layer Material

_{S}.

Figure 8 shows the affect of varying the depth of surface layer material (from 0.01 to 0.3 m) on allowable touch voltages.

Surface layer resistivity is fixed at 3000 Ω.m.

## Conclusion

*Recommendation:*

It can be concluded that the use of IEC 60479 for determination of permissible body current is preferred over IEEE80. A complete solution for calculating safety criteria limits requires a method combining equations from both Standards.

## References

[1] IEEE Std 80.-2013. IEEE Guide for Safety in AC Substation Grounding, The Institute of Electrical and Electronics Engineers, Inc.

[3] Balda, J. C. (1997). “Measurements of Neutral Currents and Voltages on a Distribution Feeder.” IEEE Transactions on Power Delivery.

[4] Dalziel, C. F. (1946). “Dangerous electric currents.” AIEE Transactions on Power Apparatur and Systems 62: 579-585.

[5] Dawalibi, F. (1982). Transmission Line Grounding. EL-2699, Research Project 1494-1. Montreal, Quebec, Canada, Safe Engineering Services Ltd. 1.

[6] Dawalibi, F. (2003). “Effects of the changes in IEEE Std 80 on the design and analysis of power system grounding.”

[7] Grainger, L. and R. Boulton (2005) A method to apply IEEE Std. 80 safe touch and step potentials to relay coordination. Volume, DOI:

[8] Hans R., S. and A. Jagdish K. (2003). “A Comparative Study of Expressions for Reduction Factor for Ground Resistance of Foot.” IEEE Transactions on Power Delivery 18(No. 3).

[9] Thapar, B., V. Gerez, et al. (1994). “Reduction factor for the ground resistance of the foot in substation yards.” IEEE Transactions on Power Delivery 9(1): 360-368.