Specifications

Earth fault disconnection parameters
There are a number of changes to BS 7671 introduced by the 17
th
Edition which affect the way that earth fault disconnection for circuits is
calculated. These include the following:
Max Zs calculation
The formula for calculating maximum Zs is stated in Appendix 3 of BS
7671. Previous versions of the standard require that the open circuit
voltage (Uoc) be divided by the current causing automatic disconnection
of the protective device (Ia) to determine Max Zs. However, in BS
7671:2008 the open circuit voltage has been replaced with the nominal
a.c. rms line voltage to earth (Uo).
This change will result in calculated maximum Zs values which are about
4% lower than those calculated under the 16
th
Edition Wiring
Regulations.
Maximum earth fault disconnection time
BS 7671:2008 section 411.3.2 states a series of different maximum earth
fault disconnection times. The appropriate time limit is dependent on
circuit type, earthing system type and nominal system voltage.
ProDesign v15 will automatically calculate the appropriate maximum
earth fault disconnection time for each circuit.
Socket circuit parameters
BS 7671:2008 section 411.3.3 states the scenarios under which an RCD
is required for a socket circuit. Exceptions are stated for socket circuits
which will be used ‘under the supervision of skilled or instructed persons’
or where the socket is provided for use with a particular item of
equipment (and is labelled as such).
ProDesign v15 allows these exceptions to be declared on the Load
dialog for any socket circuit.
AMTECH Quick Guide 2008 -17
th
Edition • 16