Hubbell CHANCE C403-3220, Инструкция по эксплуатации, Страница: 13 / 20

Page 13 of 20 P403-3222 Rev. K 4/18
Figure 1 illustrates a parallel protective grounding scheme is used [4], [10], the current flow through the
worker is:
Fault
Current
Man Protective
Jumper
Current & Current &
Resistance Resistance
Figure 1
I
m
= I
f
· [R
j
/(R
j
+ R
m
)]Eq. 2
Where:
I
m
= Current through the man, in amperes
I
f
= Available fault current at the work site, in amperes
R
j
= Resistance of the parallel jumper, in ohms
R
m
= Resistance of the worker, in ohms
Equation 2 can be rearranged to calculate the maximum jumper resistance, for a given set of conditions.
R
j
= R
m
· [I
m
/ (I
f
- I
m
)]Eq. 3
Example 1:
Assume:R
m
= 1,000 ohmsK = 157
I
f
= 1,000 Amp.t = .25 Sec.
I
max
= 314 milliamperes, fibrillation threshold (from Eq. 1)
With 3:1 safety margin:
R
j
= 100 milliohms for any parallel protective ground length (Eq. 3 divided by 3)
Some utilities, in order to provide a margin of safety in the assumptions used, for their own circumstances,
may desire to limit the maximum voltage drop across the worker to 100 V. If this is the desired approach,
equation 4 provides the limiting body current value.
I
m
= V
m
/ R
m
Eq. 4
For the same worker as defined in Example 1
I
m
= 100 V. / 1,000 ohm = 100 milliamperes (from Eq. 4)
R
j
= 100 milliohm, for any parallel protective ground length.
(from Eq. 3)