Page 155 - Electrical Safety of Low Voltage Systems
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138 Chapter Seven
These values, or their interpolation, allow the application of the
following basic safety criterion:
R G I F = V (7.13)
G ≤ V TP
where V TP is chosen based on the communicated clearing time and R G
is the measured value of the earth resistance of the electrode. 9
It is evident that Eq. (7.13), if solved for R G , provides the criterion
to determine the maximum acceptable value of the electrode earth
resistance that makes the installation safe.
If Eq. (7.13) is true, the risk of indirect contact is acceptable, as
|V T |≤ |V G |; if it is false, it might still be possible that |V T |≤ |V TP |,
and it is necessary to perform the actual measurement of the touch
voltage, thereby assessing the effectiveness of the grounding system.
The above procedure is shown as the flowchart in Fig. 7.23.
FIGURE 7.23 Procedure to deem adequate the resistance R G of the grounding
electrode.
