Page 273 - Electrical Safety of Low Voltage Systems

256 Chapter Fifteen

FIGURE 15.13 Patient leakage current caused by the interruption of the PE.

fault condition, is very dangerous because the same low amount of
leakage current flowing through the catatherized patient, defined as
patient leakage current I P , can cause the microshock.
In Fig. 15.13, it is shown how in a single fault condition, the patient
leakage current I P can directly circulate through his/her heart due to
contacts with any Class I equipment (I en ) and/or due to applied parts
to the body (I ap ).
In the above situation, the person is at great risk of microshock, as
the patient current may exceed the fibrillation threshold.

15.8.3 Local Equipotential Earthing Connection
In addition to the “chronic” problem of leakage currents from reg-
ularly operating Class I equipment emphasized by the interruption
of their protective conductors, the safety of the patient can also be
endangered by actual ground faults.
Figure 15.14 exemplifies a ground fault in TT systems because of
the failure of an electrical component in the patient vicinity. The patient
vicinity is defined as the space with enclosures likely to be touched
by the patient, which extends 1.83 m beyond the perimeter of the bed
and 2.29 m above the floor. 13
In the above situation, a potential difference V A , caused by the
voltage drop on the protective conductor of resistance R PEA , appears
between the two pieces of equipment the patient may be in simulta-
neous contact with by touch and via catheters.