Page 272 - Electrical Safety of Low Voltage Systems

Applications of Electrical Safety 255

FIGURE 15.12 Lighting poles in TT systems collectively earthed.

locally treated to lower its resistance, and/or through the introduction
11
of catheters directly into their body’s organs (e.g., the heart).
The use of conductive intracardiac probes, electrically connecting
the heart to medical equipment, makes the patient extremely vulner-
able to electric currents, because it lowers the threshold of danger. In
fact, in a catheterized patient subject to touch voltages, leakage, or
fault, currents will entirely flow through his/her heart, 12 and leave
the body via the catheter. In these conditions, the current is no longer
limited by the body resistance-to-ground, because this resistance does
not form part with the fault-loop. Patients become particularly sus-
ceptible to the adverse effects of electricity, and currents of magnitude
of a few tens of microamperes can trigger ventricular fibrillation. This
phenomenon is defined as microshock.

15.8.2 Leakage Currents
Ordinary Class I equipment, medical or not, during its use may leak
current through the insulation, and into the protective conductor. As
discussed in Sec. 15.3, in the case of interruption of the protective
conductor, the leakage current may circulate through the persons in
contact with the enclosure. For ordinary equipment (i.e., equipment
having low protective conductor currents), the magnitude of such
current is so low that it does not constitute a hazard for persons. In
medical locations, though, the interruption of the PE, defined as single