Chapter A General rules of electrical installation design

F1
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F - Protection against electric shock
2.5 m
Electrical
apparatus
Fig. F24 : Protection by out-of arm’s reach arrangements and the interposition of non-conducting obstacles
Earth-free equipotential chambers are
associated with particular installations
(laboratories, etc.) and give rise to a number of
practical installation difficulties
> 2 m
(1) Extraneous conductive parts entering (or leaving) the
equipotential space (such as water pipes, etc.) must be
encased in suitable insulating material and excluded from the
equipotential network, since such parts are likely to be bonded
to protective (earthed) conductors elsewhere in the installation.
3 Protection against indirect
contact
b The placing of equipment and obstacles must be such that simultaneous contact
with two exposed-conductive-parts or with an exposed conductive-part and an
extraneous-conductive-part by an individual person is not possible.
b No exposed protective conductor must be introduced into the chamber concerned.
b Entrances to the chamber must be arranged so that persons entering are not at
risk, e.g. a person standing on a conducting floor outside the chamber must not be
able to reach through the doorway to touch an exposed-conductive-part, such as a
lighting switch mounted in an industrial-type cast-iron conduit box, for example.
Insulated floor
Electrical
apparatus
Insulated
obstacles
Earth-free equipotential chambers
In this scheme, all exposed-conductive-parts, including the floor (1) are bonded by
suitably large conductors, such that no significant difference of potential can exist
between any two points. A failure of insulation between a live conductor and the
metal envelope of an appliance will result in the whole “cage” being raised to phaseto-earth
voltage, but no fault current will flow. In such conditions, a person entering
the chamber would be at risk (since he/she would be stepping on to a live floor).
Suitable precautions must be taken to protect personnel from this danger (e.g. nonconducting
floor at entrances, etc.). Special protective devices are also necessary to
detect insulation failure, in the absence of significant fault current.
M
Schneider Electric - Electrical installation guide 2008
< 2 m
Insulating material
Electrical
apparatus
Conductive
floor
Insulated
walls
Fig. F25 : Equipotential bonding of all exposed-conductive-parts simultaneously accessible