Page 42 - Electrical Safety of Low Voltage Systems

Fundamentals of Electrical Safety 25

FIGURE 2.15
Double ground
fault in electrically
separated
systems.

This virtual capacitor has a capacitance proportional to the length of
the cables (i.e., the armatures) and its presence can introduce an un-
wanted connection to earth in separated systems. In order to safely
limit the capacitive currents to ground, international standards rec-
ommend that the product of the nominal voltage of the separated
5
circuit (in volts) and its length (in meters) should not exceed 10 V·m,
and the length of the wiring system should not exceed 500 m. These
two conditions actually define the electrically separated systems and,
thereby, the number of transformers necessary to fulfill it. 13
As shown in Fig. 2.15, a hazardous situation can be determined
by a double ground fault involving simultaneously both poles of the
separated system, when a single transformer supplies more than one
piece of equipment.
Persons, then, can be exposed to a dangerous potential differ-
ence while in simultaneous contact with two faulted ECPs. In these
conditions, the person would close the fault-loop, allowing circula-
tion of current through his/her body. This hazardous condition can
be avoided by means of nonearthed equipotential bonding conduc-
tors connecting together ECPs of the same separated circuit. The
equipotential connection, while cancelling, or drastically reducing,
the potential difference between the enclosures, “converts” the dou-
ble ground fault into a short circuit, which can promptly be cleared
by overcurrent devices.
It is important to note that isolating transformers cannot be con-
sidered, per se, an effective protection against direct contact, but must
be coupled with the basic insulation of components (e.g., wires). The
absence of the basic insulation, in fact, would expose persons to the
risk of touching simultaneously bare parts connected to different poles
of the transformer, with lethal consequences.