7.13.8The 20 / 80 assumption#
The ‘informative’ (that is, not mandatory) Appendix B of AS/NZS 3000 under Clause B5.2.1 Determination of Zint and Item (b) states that it may be assumed that there will always be 80% or more of the nominal phase voltage available at the position of the circuit protective device and therefore, Zint should be not greater than 0.8 Zs.
On many Transport and Main Roads installations, it has been found that the assumption of 20% external impedance and 80% internal impedance is not valid and consequently must not be used.
The external network impedance must be determined by measurement, calculation using network data provided by the Electricity Entity, or other method as detailed in the particular requirements for road lighting, traffic signals or ITS installations.
Designers must be able to validate their designs by:
- having an electrical contractor measure and record the external EFLI and prospective short circuit current at the point of supply using a Cat IV 600 V or Cat III 1000 V meter and appropriate protective personal equipment, or
- calculating the external EFLI and prospective short circuit current at the point of supply using network data (transformer type and size, and cable type, size and length between the transformer and point of supply) provided by the Electricity Entity, or
- for traffic signal installations, calculating the external impedance using the Electricity Entity’s fuse.
