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6.7 SRFS ON SUB-CIRCUITS
Where SRFs are installed to protect equipment on a particular
sub-circuit, it is strongly recommended that additional
protection be installed at the power point of entry. Primary shunt
protection at the point of entry should be used to divert the peak
surge currents away from the sub-circuit. This will reduce the
risk of cross-coupling of transients onto adjacent circuits and
will reduce the risk of flashover between the locally grounded
chassis and the earth circuit.
Figure 13 details the role of point of entry protection in these
instances. The cables supplying the input to the SRF and the
connection to the earthing system will carry a proportion of the
surge energy which has been let through the primary
point-of-entry protector.
Care should be taken therefore, in the routing of these cables to
ensure that this energy will not couple onto adjacent circuits.
In some instances it will be necessary to provide a separate
earth electrode for the SRF (subject to compliance with
relevant wiring regulations), particularly where the filter is to be
installed on a sub-circuit some distance from the existing earth
electrode. In this instance, the new electrode should be located
as near as possible to the SRF. This secondary electrode must
be electrically bonded to the existing earthing system via the
most direct route possible, using flat copper tape, and should
be buried to an appropriate depth.
This earthing arrangement is depicted in Figure 13. If a
secondary earth cannot be installed, or the earth impedance
through the sub-circuit to the earth is significantly above 10Ω
special care must be taken. The risk of flashover between the
locally grounded chassis and the earth circuit may exist.
Careful attention must be paid to equipotential bonding of
the protected equipment.
Figure14. Earthing of an SRF remote from MEN point. Subject to compliance with relevant national stan-
dards). Care is needed ro avoid earth loops within protected environment.
Figure13. Operation of primary shunt protection and SRFs on sub-circuit
Installation and Operating Instructions
