Page 212 - Electrical Safety of Low Voltage Systems

Earth Electrodes, Protective Conductors 195

FIGURE 11.7 The
supplementary
bonding between
ECPs carries the
fault current.

If the supplementary bonding conductor connects an ECP to an
EXCP, as per IEC 60364–5-54, its conductance must be at least half that
of the PE serving the ECP. Also, in this case, a current divider takes
place and the EXCP carries part of the fault current.

11.4.1 Where Should We Use Equipotential
Bonding Conductors?
As we have already substantiated, the only reason to employ an
equipotential bonding conductor is to eliminate or reduce potential
differences in the case of faults. However, an erroneous application
of this concept may induce designers to bond each and every metal
part in the vicinity of equipment. An undue link between ECPs and
metal objects, which are not required to be bonded, causes the safety
to decrease, because the fault potential arising within the ECP may be
transferred to that metal part.
As an example of a “legitimate” bonding conductor, let us consider
a panelboard, whose door contains live conductors (e.g., door with
controls or instrumentation) (Fig. 11.8).
This piece of equipment is composed of a “core” (i.e., the panel
itself) and a “satellite” (i.e., the “active” door). Both the core and satel-
lite are ECPs since, by definition, they are normally “dead,” but likely
to become live upon failure of their basic insulations. Both panelboard
and door must be bonded to the grounding system to allow a prompt
disconnection of supply upon ground faults.
While the enclosure of the “core” is bonded to the grounding bus
within the panel, which is linked to the earthing system, the door
might simply employ an equipotential bonding conductor between
itself and the “core,” as shown in Fig. 11.8. The equipotential bonding
5
conductor, by bypassing the resistance of the hinges, allows a clear
path to the ground current and a prompt intervention of the protective
device, should the door fail.
On the other hand, a “plain” door (i.e., with no live parts on it)
is neither an ECP nor an EXCP and therefore there is no need for
bonding jumpers. The presence of a bonding conductor between the
door and the frame decreases the safety, as a fault in the panelboard
also energizes the door. As a consequence, the probability for persons