Page 276 - Electrical Safety of Low Voltage Systems

Applications of Electrical Safety 259

FIGURE 15.16 Isolating transformer supplying the circuits in the patient
vicinity.

patient vicinity, in fact, there may be small equipment, not supplied
by the isolating transformer, requiring the ground connection avail-
able at the local earthing bus. The above arrangement is a violation
of the general rule, which prohibits the ECPs of separated systems
to share the earth with nonseparated systems (prohibition also ap-
plicable to Class II equipment). As seen, this rule intends to prevent
enclosures of separated systems from becoming dangerously “live”
due to potentials transferred by means of earthing connections.
In the presence of the supplementary equipotential bonding in
medical locations, this risk is, indeed, very low and deemed accept-
able. In fact, even if the earthing bus attains a certain potential under
fault conditions, all the ECPs in the patient vicinity will simultane-
ously reach this same value, as Fig. 15.16 shows; ergo, no potential
differences can appear among them and the patient is safe. As a con-
sequence, the grounding connection in separated systems adopted in
medical locations is deemed safe in the presence of the local equipo-
tential bonding.

15.8.4.1 Interruption of the Protective Conductor
in Separated Systems
The interruption of the PE is dangerous even in separated systems be-
cause the resulting capacitive current through the patient may exceed
the fibrillation limits (Fig. 15.17).