Page 61 - Uninterruptible Power Supplies

Interconnecting the Standby and Normal Supplies
Interconnecting the Standby and Normal Supplies 59

ious consumers’ installations, in order to ensure the safety of the sup-
plier, the consumers, and any other personnel who may be involved.
The following text is not intended as a summary of the Regulations; the
text draws attention to those parts likely to be of particular interest to
installers of standby generating plant.
Within the Regulations high voltage is defined as being above 1000 V ac.

Earthing
It is required that both high-voltage and low-voltage supplies shall be
connected with earth at, or as near as is reasonably practicable to, the
source of voltage. When high- and low-voltage systems are to be con-
nected with earth in the same location there are two requirements:
1. The two earth systems shall not be interconnected unless the com-
bined earth resistance does not exceed 1 ohm.
2. The systems shall not be connected to separate earth electrodes unless
the overlap of the areas of resistance is not sufficient to cause danger.

Multiple Earthing
The supplier may connect the supply neutral with earth at places in
addition to those described under the heading “Earthing” above, pro-
vided that the supply neutral conductor for three-phase circuits has a
cross-sectional area not less than one-half of the phase conductors. For
single-phase circuits the neutral shall have a cross-sectional area of not
less than the phase conductor.
(Note: The minimum cross-sectional area requirement leads to a
limitation of voltage difference between the earthing electrodes.)
There are special requirements for protective multiple earthing.

Protection against Overload, Earth Leakage,
and Excess Voltage
“The public electricity supplier is to include protective devices in every
system to prevent, so far as is reasonably practicable, danger arising
from excess current or from current leakage to earth. Similarly, the
supplier is to make arrangements which ensure, so far as is reasonably
practicable, that danger will not arise from accidental contact between
low- and high-voltage conductors.

The Consumer’s Installation
The supplier is to be satisfied that the consumer’s installation is prop-
erly installed, safe to use, and does not present danger. If an alternative

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