AS 4070-1992 (R2015) pdf free.Recommended practices for protection of low-voltage electrical installations and equipment in MEN systems from transient overvoltages.
This Standard recommends a simple system of insulation coordination for nominal
240/415 V. 50 Hz electrical installations in multiple earthed neutral (MEN) systems to avoid damage to wiring and equipment due to transient voltages exceeding 2500 V caused by lightning discharges and switching.
Circuits in some installations may be protected for lower overvoltages, but Lhis is not dealt with in this Standard. (For guidance see AS 1768.)
This Standard does not apply to the protection of installations from temporary overvoltages which could result from faults in the electricity supply system.
It does not apply to the protection of equipment connected to both electricity supply and information technology services (e.g. telephone cables and TV antennas). Consideration needs to he given to a common earth ing system for the protection of all electrical services.
NOTE: Reference should be made toAUSTEL mandatory Technical Standard 001 which specifies requirements for overvoltage protective devices in equipment lobe connected to telecommunication networks. For most applications of this Standard, the telecommunication networks need not be taken into consideration. However, if connections are to be made between systems. or if both systems are enclosed in common ducts, the requirements of this Standard and AUSTEL mandatory Technical Standard 001 should be met. Reference should also be made to AS 3260.
3 INSTALLATION CATEGORY Ills recommended that 240/415 V. 50 Hz electrical installations be protected to Installation Category II in accordance with IEC 664, i.e. 2500 V maximum transient overvoltage in any circuit.
NOTE: Frequent reference is made to IEC installation categories in this Standard, and attention is drawn to a statement in fEC 664 that, in order to apply the concept of insulalion coordination, distinction should he made between two types of transient overvoltages—
(a) transient overvolt ages originating in the system to which the equipment is connected through its terminals: and
(b) transient overvoltages generated within the equipment itself.
Clearly, 240/415 V. 50 Hz equipment suitable for Installation Category II should be capable of withstanding impulse voltages of 2500 V peak, and the equipment itself should not generate overvoltages exceeding 2500 V.
4 PROTECTION FROM OVER VOLTAGES DUE TO LIGHTNING For(MEN)electricity supply systems. it is recommended that a 5000 A surge arrester, having a rated voltage suitable for the system and a maximum residual voltage of 1300 V. complying with AS 1307.2, be connected between each phase and neutral. The total length of the active and earth connections for each arrester should not exceed 1 m and such connections should be 6 mm2 copper conductors and be as straight as practicable. For optimal protection the total length of connections should not exceed 3(X) mm. Connections between surge arresters and phase conductors on the line side of the main switch should be through active links to facilitate replacement of the arresters if required.
The connection between neutral and earth should be as short as practicable. and for optimal protection should not be longer than 150 mm.