
Technical Manual SFH3-800-200-275
Novaris Pty Ltd
72 Browns Road, Kingston, TAS. 7050 AUSTRALIA
Tel: + 613 6229 7233 FAX: + 613 6229 9245
1. Operation
Novaris power line surge filters are designed for the protection of sensitive
equipment from the damaging effects of lightning surges, power transients and RF
interference.
Novaris hardwire filters comprise MOV surge protection plus LC low pass filters for
both single and three phase applications. The prime surge diverting elements are
metal oxide varistors of a rating chosen to suit the application. This is primarily
based upon current rating of the filter but high surge rating options are available for
high exposure sites.
Three stages of protection include metal oxide varistors, inductors and capacitors
forming a low pass filter and a final stage of metal oxide varistors.
The first stage of protection consists of the metal oxide varistors. MOVs are
connected between each phase and neutral to absorb transverse mode surges usually
generated by load switching and other power system disturbances. These MOVs in
conjunction with the gas arrestor between neutral and ground absorb common mode
surges most often caused by lightning induced disturbances or power system earth
faults.
Novaris power filters are designed for general installation at any point in a LV
distribution system. When installed near a main switchboard close to the building
MEN point the neutral-earth components serve no function but are necessary when
the filter is installed at distribution boards some distance from the MEN point to
remove possible induced common mode voltages.
The second stage of the filter consists of the inductors and capacitors. The LC section
low pass filter elements further attenuate surge voltages already clamped by the
MOV stages. In addition the filter stages attenuate noise and power system
harmonics. Filter cutoff frequency is around 800Hz. The filter stage is designed to
attenuate both transverse and common mode noise.
Metal oxide varistors are voltage dependent electrical resistors with symmetrical V/I
characteristics. Their resistance value decreases with increasing voltage, thus "short-
circuiting" a further rise in overvoltage. The metal oxide varistors in Novaris surge
filters are conservatively rated to handle a large number of surge events. However
due to the unpredictable nature of transients a varistor may occasionally be
overloaded. To sense the integrity of each varistor a fuse is connected in series with
each MOV.
Should an overload occur the varistor leakage current will increase causing the fuse
to blow.