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Guide to commercial installations | Part 2 Panelboard: Standards and Regulations
6
Selection and erection
Operational conditions
Distribution boards must be selected for the operational
conditions; which includes voltage, current and frequency.
In the UK the nominal voltage and frequency is typically
400/230V and 50Hz.
Installations and the panelboards used in them will have
particular current requirements. To meet regulation 512.1.2
in the 17
th
Edition you must ensure that the equipment is
suitable for the design current and the current likely to flow
in abnormal conditions. The latter would include short circuit
and earth fault currents.
You will therefore need to assess the current demand, taking
into account diversity, and the prospective fault current at the
panelboard before you select the board and the protective
devices.
The terminology to define the rating of a panelboard in
relation to load/design current used in BS EN 61439 can be
summarised as follows:
• The rated current of a panelboard (
I
nA ), is the maximum
load current it is designed to manage and distribute.
• The rated current of a circuit
I
nc is stated by the
panelboard manufacturer, taking into consideration the
ratings of the devices within the circuit, their disposition
and application.
The current rating(s) of a panelboard circuit may be lower
than the rated current(s) of the device(s) according to their
respective device standard, when installed in the panelboard,
therefore, it is essential that the manufacturer’s ratings and
instructions are followed.
Rated diversity (loading factor) can be stated by the
manufacturer, e.g. for groups of circuits. Diversity recognises
that all outgoing circuits will not normally be fully loaded at
the same time and thereby avoids the need to provide
over-designed panelboards for the actual application. It is
essential that manufacturer’s ratings and instructions are
followed. For example, in the case of a panelboard with a
diversity factor 0.8, any combination of outgoing circuits
within the panelboard can be loaded to 80% of their rated
current, provided the total load on the outgoing circuits does
not exceed the rated current of the panelboard.
The relationship between rated diversity factor and load
current should always be considered i.e. some circuits will be
rated on the basis of inrush currents and intermittent or short
duration loads.