Chapter A General rules of electrical installation design

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J4
J - Protection against voltage surges in LV
Three points must be kept in mind:
b A direct or indirect lightning stroke may
have destructive consequences on electrical
installations several kilometres away from
where it falls
b Industrial or operating voltage surges also
cause considerable damage
b The fact that a site installation is underground
in no way protects it although it does limit the
risk of a direct strike
General
Transient overvoltages at industrial frequency (see Fig. J4)
These overvoltages have the same frequency as the network (50, 60 or 400 Hz); and
can be caused by:
b Phase/frame or phase/earth insulating faults on a network with an insulated or
impedant neutral, or by the breakdown of the neutral conductor. When this happens,
single phase devices will be supplied in 400 V instead of 230 V.
b A cable breakdown. For example, a medium voltage cable which falls on a low
voltage line.
b The arcing of a high or medium voltage protective spark-gap causing a rise in earth
potential during the action of the protection devices. These protection devices follow
automatic switching cycles which will recreate a fault if it persists.
Normal voltage
230/400 V
Fig. J4 : Transient overvoltage at industrial frequency
Voltage surges caused by electrical discharge
In a dry environment, electrical charges accumulate and create a very strong
electrostatic field. For example, a person walking on carpet with insulating soles
will become electrically charged to a voltage of several kilovolts. If the person walks
close to a conductive structure, he will give off an electrical discharge of several
amperes in a very short rise time of a few nanoseconds. If the structure contains
sensitive electronics, a computer for example, its components or circuit boards may
be damaged.
.3 Main characteristics of voltage surges
Figure J5 below sums up the main characteristics of voltage surges.
Type of voltage surge Voltage surge Duration Front gradient
coefficient or frequency
Industrial frequency y 1.7 Long Industrial frequency
(insulation fault) 30 to 1,000 ms (50-60-400 Hz)
Operation 2 to 4 Short Average
1 to 100 ms 1 to 200 kHz
Atmospheric > 4 Very short Very high
1 to 100 μs 1 to 1,000 kV/μs
Fig. J5 : Main characteristics of voltage surges
Schneider Electric - Electrical installation guide 2008
Transient overvoltage
Normal voltage
230/400 V
t