Page 64 - Uninterruptible Power Supplies

Interconnecting the Standby and Normal Supplies
62 Chapter Two

connections that can be used, the agreement of the network operator
having been obtained. If the normal supply earthing conductor uses
the sheath or armor of a supply cable that has a remote origin it can-
not be relied upon. Such a cable may be disconnected by the network
operator during maintenance or may be severed accidentally by a con-
tractor (on or off site).

Earthing Low-Voltage Supplies
A low-voltage standby supply that runs independently of the normal
supply should have its neutral solidly connected to a local earth elec-
trode. If there are separate earthing systems associated with the nor-
mal supply and the standby supply they should be bonded together.
When devising the interconnections between the two neutrals it must
be kept in mind that the network operator will not allow its neutral to
be connected to earth at a second point unless the system incorporates
protective multiple earthing. If the system does not incorporate protec-
tive multiple earthing a four-pole changeover device will be required as
described in the next section. Within the United Kingdom protective
multiple earthing is normal and in such cases the standby supply earth
electrode should have a resistance to earth not exceeding 20 ohms. This is
the resistance allowed for protective multiple earthing in the Electricity
Supply Regulations.
Where it is intended to run the standby and normal supplies in par-
allel there are additional requirements which are described in the sec-
tion titled “Paralleling the Standby and Normal Supplies.”

Earthing High-Voltage Supplies
A high-voltage standby supply that runs independently of the normal
supply usually has its neutral earthed through an earthing resistor
which limits any earth fault currents to rated current. The earthing
resistor provides two benefits, in the event of an internal fault the
damage to the machine is limited, and in the event of an external
fault the rise of potential of exposed conductive parts is limited. It
should be noted that the earthing conductor between the star point
and the earthing resistor will, under fault conditions, experience a
rise of potential, the magnitude will depend upon the position of the
fault; the earthing conductor should therefore be insulated for the
phase voltage.
Where it is intended to run the standby and normal supplies in par-
allel there are additional requirements which are described in the sec-
tion titled “Paralleling the Standby and Normal Supplies.”

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