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P403-3222 Rev. K 4/18
It is the responsibility of the user to establish and maintain a maximum resistance threshold for the protec-
tive ground set to provide a safe working environment.
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Although substantial research has been conducted to determine the reaction of the human body to various
levels of current, no single value can be given as a safe level for all situations. Research has determined that
the body’s reaction is dependent upon the time duration as well as the magnitude of the current flow [1], [2],
[8]. Other variables to consider are: the protective grounding method employed [3], the fault current avail-
able [4], the assumed body resistance of the protected worker [1], [5] along with his weight [3] and the level
of protection being sought by the user. Ultimately the safety department of the using utility must give due
consideration to the variables which affect the degree of worker safety which that utility desires to achieve.
Values of each variable may differ from utility to utility, or even from work site to work site. Once the vari-
ables are defined, the equations discussed below can be used to establish a maximum resistance value for
protective grounds issued to workers for use in a predefined area.
For example, a nearly equipotential zone can be created by placing a protective ground in parallel with the
worker at the work site [4]. The allowable resistance of the protective ground can be higher for low values
of available fault current than for very large values. Also, fast backup circuit protective devices remove the
body current quicker, allowing a somewhat higher body current to flow and still achieve a level of protec-
tion. Many standards and reference literature use 1,000 ohms as the worker’s body resistance [3]. While this
may be not be totally correct, it provides a basis for calculations.
Charles Dalziel, a noted researcher, has published charts which are widely used in the utility industry today
[1], [2], [3], [4]. He determined statistically that the average perception current, the least current detectable
by the body, to be 1.2 milliamperes and the average let-go threshold to be 9 milliamperes [1], [6]. He fur-
ther determined that 99.5% of those receiving shocks will not go into heart fibrillation if the shock current,
for a specified duration, is below the value calculated by Equation 1. [1], [3], [9]:
I = K/(√t)
Eq. 1
Where:
I =
Current flowing through body’s chest cavity, in milliamperes
t =
Duration of current flow, in seconds
K = A constant related to the electric shock energy
116 for a 110 lb. man or,
157 for a 154 lb. man or,
165 for a 165 lb. man
Possible ventricular fibrillation thresholds, with time dependency, may occur above:
0.03 second shock 1,000 milliamperes
3.00 second shock 100 milliamperes
APPENDIX A: THEORY OF RESISTANCE THRESHOLD DETERMINATION
WARNING
!