Page 37 - Electrical Safety of Low Voltage Systems

20 Chapter Tw o

As discussed later, it is important that protective devices are co-
ordinated with the value of the earthing system resistance so as to
prevent the persistence of fault potentials on accessible parts for a
dangerous amount of time.

2.3.1.1 Nuisance Trippings of RCDs
Nuisance trippings of RCDs disconnect the supply in the absence
of any actual danger for persons, thereby causing an unnecessary
loss of service. Typical reasons of nuisance tripping of RCDs are the
overvoltages resulting from both switching transients and lightning. 9
Surge voltages momentarily overstress the capacitance-to-ground of
cables and equipment, forcing the circulation of leakage current to
earth for a few microseconds. RCDs may, then, react and initiate the
parting of the breaker’s contacts, which can take few milliseconds.
As a result, the supply will be disconnected when the overvoltage is
already expired.
RCDs may also trip upon starting of three-phase motors. High
inrush currents, in fact, may not be perfectly balanced among the
phases and therefore cause the tripping of the RCD.
Another cause of nuisance trippings may be the leakage currents
inevitably flowing through the insulation of equipment during its
normal operations. The issue of high leakage currents in equipment
will be discussed in Chap. 15.

2.3.2 Protection Without Automatic Disconnection
of Supply
Passive means of protection, that is, not involving disconnection of
supply, may be used to prevent the occurrence of hazardous situations
in case of failure of the basic insulation. The continuity of the service
is, then, preserved, which is particularly important in installations
where the loss of energy can be detrimental to safety. Such protective
measure is typically used when skilled and instructed persons strictly
supervise the installation.

2.3.2.1 Protection by Use of Class II Equipment or
Equivalent Insulation
If the basic insulation fails, in order to prevent the appearance of po-
tentials on the exposed parts of electrical items, a supplementary and
independent layer of insulation material may be added to safeguard
persons against indirect contact (Fig. 2.10).
Basic insulation plus supplementary insulation form a double in-
sulated, or Class II, piece of equipment, which is identified by the
symbol in Fig. 2.11.
To reduce the probability of simultaneous failure of the two in-
sulations, manufacturers must install (and test) them in a way that