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Guide to commercial installations | Part 2 Panelboard: Standards and Regulations
10
Protection against fault current
You must take into account the prospective fault current
when selecting a panelboard.
If a system is correctly designed and maintained then
electrical faults are very rare. When they do occur
however, the panelboard is subjected to stresses such as:
• High forces acting on components and between
conductors
• High temperatures reached very quickly
• Ionisation of the air. This is due to devices breaking the
fault current which leads to lower insulation values of that
air.
Section 434 of BS 7671 details fault current requirements.
You only need to consider faults belonging to the same
circuit. Where there is a mixture of three phase and single
phase circuits you need to look at these individually.
A line to neutral fault will be about half that of the three-line
fault – or three phase short circuit. This affects the fault
current ratings of the individual devices for the panelboard –
see example image.
L
1
N
L
2
L
3
8kA
16kA
A line to neutral fault will be approximately half that of the
three-line fault - or three phase short circuit.