7.16.3 - Functional
extra-low voltage (FELV)
If a separated extra-low voltage (SELV)
system is earthed, or if the insulation of the supply which
feeds it does not meet the necessary requirements, it ceases
to be a SELV system and becomes a functional extra-low voltage
(FELV) system. This difference is illustrated by {Fig 7.25}.
If a functional system only fails to be
classified as a safety system because it is earthed, it
must then be protected by enclosures that prevent it being
touched or by insulation capable of withstanding a test
voltage of 500 V r.m.s. for one minute. In such a case there
is no need for the earth-fault loop impedance of the extra-low
voltage circuit to be low enough to prevent danger; compliance
of the safety source supply is sufficient. There is therefore
no need to bond and earth all associated non-current-carrying
metalwork.
Such metalwork must, however, be bonded
and earthed if the insulation between the low-voltage supply
circuit and the functional extra-low voltage circuit does
not meet the requirements stated above. In the event of
the supply circuit being of the earth-free bonded type {5.8.3}
all non-current-carrying metalwork of the FELV system must
he connected to the non-earthed protective conductor of
the supply circuit. Plugs and sockets in FELV systems must
not be interchangeable with those of other Supply systems
in use in the same premises.
becomes a functional extra-low
voltage system when
Fig 7.25 - Earthing relationship,
SELV and FELV systems
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