Page 271 - Electrical Safety of Low Voltage Systems

254 Chapter Fifteen

FIGURE 15.11 Lighting poles in TT systems independently earthed.

protective devices either, as I 2 , limited by the ground resistance R G2 ,
might be too low for their instantaneous pick up.
As a result, the ground fault can permanently energize pole B and
cause stray voltage over it.
Alternatively, earthing the lighting poles collectively would allow
overcurrent devices to trip, in the case of double faults (Fig. 15.12).
The fault current, in fact, corresponds to a short circuit phase-
neutral, and the overcurrent device can promptly disconnect the sup-
ply. Electrical safety is, thus, assured, even if the RCD may still not
intervene due to its desensitization caused by the neutral-to-enclosure
fault.

15.8 Electrical Safety in Medical Locations

15.8.1 Microshock
In medical locations (e.g., hospitals, medical, and dental practices,
etc.), patients are exposed to increased hazard of electric shock due
to their particular conditions. Patients, in fact, may be unconscious,
or anaesthetized and, therefore, unable to let-go of an energized part.
In addition, patients may be connected to medical equipment either
through applied parts to the skin (e.g., sensors, electrodes, etc.), often