Handbook of Electrical Installation Practice - BeKnowledge

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• Fire
• Rodent attack on wiring systems.
The risks associated with agricultural premises are accentuated where livestock are
concerned, owing to their apparently greater sensitivity to electric shock. This has
long been held to be due to their more intimate contact with earth, and the distance
apart of the limbs of a quadruped, which can give it access to a greater substantial
touch voltage in the event of an earth fault. A voltage gradient across the soil in the
vicinity of an earth rod can easily approach levels that are fatal to livestock. An
earth stake or other extraneous-conductive-part will set up concentric lines of
constant potential in the soil under earth fault conditions, with maximum voltage
gradient (and therefore touch voltage) at the earth rod, and reducing to zero many
metres away.
The plastic used in modern cable systems is subject to attack by rodents. Apart
from causing functional failure, this can contribute to risk of shock and fire.
The protective measures
Installations for general use
SELV may be used to provide protection against direct and indirect contact.
However, supplementary protection against direct contact must be provided by
barriers or enclosures to IP2X – or insulation capable of withstanding a test voltage
of 500V for 60 seconds.
Measures for protection against direct contact such as the use of enclosures,
obstacles and placing out of reach, are not excluded, although it is a requirement
that all socket-outlets should be further protected by an appropriate residual
current device.
In respect of protection against indirect contact, maximum disconnection times
are reduced and in these installations, disconnection must occur within 0.2 seconds
for a nominal system voltage of 230V. This should be achieved by designing and
constructing within stated earth loop impedance maxima for a given protective
device. For distribution circuits and circuits supplying fixed equipment, a disconnection
time of 5 seconds is permitted.
Where a residual current device is installed, BS 7671 gives limiting values for
circuit parameters such that the product of earth loop impedance and the rated
residual operating current for the device should not exceed 25V.
The above comments for indirect contact apply to TN systems. Where TT systems
are concerned, there is a further requirement for each circuit that:
RAIa £ 25V
Special Installations or Locations 259
R A is the sum of the resistances of the earth electrode and the protective conductors
connecting it to the exposed conductive parts, IA is the current causing the
automatic operation of the protective device within 5 seconds. In practical terms,
this will frequently be a residual current device and the current should then be taken
as the rated residual operating current.