Electrical Calculations Software

Comparison of substation safety criteria given by the American (IEEE) and European (IEC) Standards

Calculate touch and step voltage limits to different Standards and understand the differences

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Introduction

Hazardous Voltages During Earth Faults

Substation earthing grids are built to ensure that no electrical safety hazards exist within or outside of a substation perimeter both during normal operation and/or fault conditions.  This report will focus on safety hazards caused during fault conditions.
 
During an earth fault the flow of current into the earth produces potential gradients in and around the substation.  Figure 1 shows a computer simulation the surface potential rise for a 30×30 m grid consisting of four (4) meshes.  Design procedures must ensure that the maximum potential gradients and the voltages developed between earthed structures do not pose any dangers.
 
Figure 1. Surface potential (z-axis) for 30x30 m earth grid. Burial depth = 0.5 m. Top soil layer resistivity (ρ1) = 1000 Ω.m; Bottom soil layer resistivity (ρ2) = 100 Ω.m. Fault current = 1000 A.
Figure 1. Surface potential (z-axis) for 30×30 m earth grid. Burial depth = 0.5 m. Top soil layer resistivity (ρ1) = 1000 Ω.m; Bottom soil layer resistivity (ρ2) = 100 Ω.m. Fault current = 1000 A.

Safety Standards

 
Effect of electric current on the human body depends on a number of factors including the magnitude and duration of the fault and the frequency of the current [1]. 
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 most important physiological threshold is that which causes fibrillation of the heart.  The magnitude and duration of the current flowing through
the human body must be less than the value that can cause ventricular fibrillation. 
 
Safety criteria have been developed in the Standards which for a maximum tolerable body current define the tolerable total effective voltage
(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

 
To compare the safety criteria as prescribed by the Standards:
  1. IEEE Std 80-2013 – IEEE Guide for Safety in AC Substation Grounding [1].
  2. IEC/TS 60479-1 – Technical Specification – Effects of current on human beings and livestock [2].
The methods for determining safety criteria described by the respective Standards are implemented in software (see Figure 2). 
The software provides an efficient method for determining safety criteria which complies with these Standards.
 
Figure 2 Safety Criteria as implemented in SafeGrid earthing software for both IEEE and IEC Standards
Figure 2 Safety Criteria as implemented in SafeGrid earthing software for both IEEE and IEC Standards
 

Safety criteria – IEEE Std 80-2013

 

Tolerable body current limits

 
Fibrillation current is assumed to be a function of a person’s body weight.  This idea comes from studies undertaken by Dalziel [4]. 
Thus the formulae for allowable body current which can be survived by 99.5% of persons are given for two weights (50 kg and 70 kg):
 
50 kg body weight formula
70 kg Body weight formula
 

Body resistance

 
A constant value equal to 1000 Ω is used for body resistance.  This represents resistances from hand-to-feet and from hand-to-hand and from foot to foot.
 
Hand and foot contact resistances are neglected as well as glove and shoe resistances assumed equal to zero (the latter is easily incorporated through simple modifications to the forthcoming equations).

Touch and step voltage criteria

 
For a person to remain alive (to avoid ventricular fibrillation) the energy which is passed through the human body must be limited to a safe level.  This safe level is defined in terms of a set of maximum permissible voltages as follows:
 
For touch voltage the limit is:
 
eTouch Formula
For 50kg body weight
 
e Touch (4)
For 70kg body weight
 
E Touch 70 Formula
 
For step voltage the limit is:

e step Formula

For 50 kg body weight

e Step (7)
For 70 kg body weight
 
e step 70 (8)
 

Where:

          Cs       is the scaling factor due to the presence of the protective surface layer.

          ρs       is the resistivity of the surface layer in Ω.m.

          RA      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 Cs = 1 and ρs = ρ1

 

Effect of surface layer material

 
Spreading a thin (usually 8-15 cm thick crushed rock) layer of high resistivity material over the surface within a substations increases allowable touch and step potentials. 
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 (CS) into the foot resistance (RF) calculation.
 
For a uniform soil the foot resistance RF is computed as follows, assuming the foot to be a conductive plate on the earth’s surface:
 
Effects of surface layer material formula

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:

Formula

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

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

This factor generally gives higher values of CS compared with the previous formulation given in IEEE80-1986.

Decrement factor

 
An adjustment factor used to determine the root-mean-squared equivalent of the asymmetrical current wave for a given fault duration, accounting for the effect of initial DC offset and its attenuation during the fault. 
The decrement factor is defined as follows [7]:
 Decrement factor Formula

 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]

 
The Standard IEC 60479 is the technical specification which describes the effects of current on the human body. 
Two main parts to this Standard are useful for deriving substation safety criteria (i.e. allowable touch and step potentials).
  1. Part 1: General aspects, Technical Specification IEC/TS 60479-1 Edition 4 (published 2005)
  2. Part 5: Touch voltage threshold values for physiological effects, Technical Report IEC/TR 60479-5 Edition 1 (published 2007)
A body impedance model is given with touch voltage thresholds related to the touch current thresholds by the body impedance according to Ohm’s law.
 
Body impedances are given for the 5th, 50th and 95th percentiles of the population for both wet and dry conditions. 
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 current threshold for ventricular fibrillation for a current path hand to feet is given in Figure 3. 
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.
 
Figure 3. Permissible body current versus duration curve (Figure 20 from IEC 60479-1)
Figure 3. Permissible body current versus duration curve (Figure 20 from IEC 60479-1)
While IEEE Std 80 provides a direct method for establishing safety criteria (allowable touch and step potentials) for substations, IEC 60479 does not.  The necessary method is explained in the following steps:
  1. 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).
  2. For the permissible body current level determine the corresponding body resistance using the Tables.
  3. Compute the foot resistance in accordance with IEEE Std 80-2013
  4. Compute permissible touch and step potentials.
Additional notes about IEC 60479:
IEEE80 defines safety criteria for a given body weight.  IEC 60479 states it has been shown that body impedance is not greatly influenced by body mass. 
Physically large people have lower internal body resistance due to their large cross-sectional area.  Physically small people generally have higher body impedance.
Impedances external to the body are not considered in IEC 60479-1, hence the approach taken in IEEE80 needs to be taken.
 

Comparison of IEEE and IEC safety criteria

 
The following sections provide a comparison between the Standards in terms of allowable touch and step voltages for practical ranges of critical design parameters.  An nsight into the differences with the body resistance models used is given.  Note the curves were produced for decrement factor = 1 and scaling factors (CS) were calculated using the method from IEEE Std 80-1986.
 

Allowable Touch Voltages

 
Figure 4 shows the allowable touch voltages for varying fault clearing times (synonymous with electric shock duration) from 0.01 – 10 seconds.
 
Figure 4Allowable touch voltages for varying fault clearing time (surface layer resistivity, ρs = 100 Ω.m and top-layer soil resist
Figure 4 Allowable touch voltages for varying fault clearing time (surface layer resistivity, ρs = 100 Ω.m and top-layer soil resistivity, ρs = 100 Ω.m and top-layer soil resistivity ρ1 = 100 Ω.m)
Table 1 Observation(s) about allowable touch voltages
Table 1 Observation(s) about allowable touch voltages

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

Figure 5 Body resistance values (touch voltages) for varying fault clearing time (surface layer resistivity, ρs = 100 Ω.m and top-l
Figure 5. Body resistance values (touch voltages) for varying fault clearing time (surface layer resistivity, ρs = 100 Ω.m and top-layer soil resistivity ρ1 = 100 Ω.m)

 

Table 2 Observation(s) about body resistances
Table 2 Observation(s) about body resistances

Allowable Step Voltages

Figure 6 shows the allowable step voltages for varying fault clearing times (synonymous with electric shock duration) from 0.01 – 10 seconds.
 
Figure 6. Allowable step voltages for varying fault clearing time (surface layer resistivity, ρs = 100 Ω.m; top-layer soil resistiv
Figure 6. Allowable step voltages for varying fault clearing time (surface layer resistivity, ρs = 100 Ω.m; top-layer soil resistivity ρ1 = 100 Ω.m)
Table 3 Observation(s) about varying surface layer depth
Table 3 Observation(s) about varying surface layer depth

Surface Layer Material

 
Spreading a thin layer of surface material in a substation (when the surface material resistivity is greater than the top soil layer resistivity) increases the contact resistance between a persons feet and the earth which can reduce the current through the body considerably.  The reduction depends on the relative resistivites of the surface and the top soil layer resistivities and on the thickness of the surface material, HS.
 
Figure 7 shows allowable touch voltages for varying surface layer (i.e. crushed rock or ashvalt) resistivity from 100 Ω.m (same as top soil layer, ρ1) to 10,000 Ω.m.
 
Figure 7 Figure 7. Allowable touch voltages for varying surface layer resistivity, ρs (top-layer soil resistivity ρ1 = 100 Ω.m; sur
Figure 7. Allowable touch voltages for varying surface layer resistivity, ρs (top-layer soil resistivity ρ1 = 100 Ω.m; surface layer depth, Hs = 0.15 m; fault clearing time, t = 0.3 s)

 

Table 4 Observation(s) about varying surface layer resisitivity
Table 4 Observation(s) about varying surface layer resisitivity

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.

Figure 8 Figure 8. Allowable touch voltages for varying surface layer depth, Hs (surface layer resistivity, ρs = 3000 Ω.m; top-laye
Figure 8. Allowable touch voltages for varying surface layer depth, Hs (surface layer resistivity, ρs = 3000 Ω.m; top-layer soil resistivity ρ1 = 100 Ω.m; fault clearing time, t = 0.3 s)
Table 5 Observation(s) about varying surface layer depth
Table 5 Observation(s) about varying surface layer depth

Conclusion

 
The safety criteria for IEEE Std 80-2013 and IEC 60479 have been compared and there differences have been quantified.  There are cases when IEEE80 is more conservative than IEC 60479, and vice versa. 
 
The calculation of safety criteria prescribed in IEC 60479 is not straight-forward whereas for IEEE80 it is.  IEC 60479 does not provide any method for calculating foot resistance.
 
IEE80 assumes a fixed body resistance value of 1000 Ω.  This simplification may compromise safety in earth grid design.
 
IEEE80 defines safety criteria for a given body weight.  IEC 60479 states it has been shown that body impedance is not greatly influenced by body mass.
 
IEC 60479 safety limits based on recent knowledge surrounding 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.
 
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.
 
SafeGrid earthing software can calculate safety criteria based on both IEC and IEEE Standards.
 

References

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

[2]  IEC 60479-1:2005. Effects of current on human beings and livestock – Part 1: General aspects.
 

[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.