Page 65 - Uninterruptible Power Supplies

Interconnecting the Standby and Normal Supplies
Interconnecting the Standby and Normal Supplies 63

Within an installation, the Electricity Supply Regulations allow the
high-voltage earth electrode system to be interconnected with the low-
voltage earth electrode system provided that the combined resistance to
earth does not exceed 1 ohm. If the resistance exceeds 1 ohm the
Regulations forbid any interconnection and require the two electrode
systems to be separated sufficiently to ensure that the overlap between
the two resistance areas does not cause danger. In most industrial instal-
lations it is not practicable to ensure that the two earthing systems are
not interconnected, or that the electrode resistance areas do not overlap,
in such cases it follows that a combined resistance not exceeding 1 ohm
is required for high-voltage neutral earthing.

Neutral Conections for Single Sets
Not Intended to Run in Parallel with
the Normal Supply
Where the standby and the normal supplies are not arranged to run in
parallel Engineering Recommendation G59/1 describes this as the alter-
native connection.

Low-Voltage Installations
The neutral of a low-voltage standby supply should be solidly earthed.
In a single set low-voltage installation the generator will usually be
connected, as a four-wire machine, to a distribution board with the star
point connected to the neutral busbar which in turn is connected to the
earth bar and on to the earth electrode (see Fig. 2.9a).
Within the United Kingdom four systems of public supply may be
encountered, they are described in BS 7671 and are:

■ TN-C-S systems in which the neutral and the earthed protective con-
ductors are combined into a single conductor in part of the system.
This is the most common system and uses protective multiple earth-
ing (PME). Where a standby supply is installed in such a system a
triple pole changeover device will be required to select one supply or
the other as illustrated by Fig. 2.7.
■ TN-C systems in which the neutral and the earthed protective con-
ductors are combined throughout the system. Provided that the sys-
tem uses multiple earthing the same considerations apply as to the
preceding TN-C-S systems.
■ TN-S systems in which the neutral and the earthed protective con-
ductors remain separate throughout the system. Where a standby
supply is installed in such a system a four-pole changeover device

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