Electric switching surge protection

An electric switching surge protector comprising a non-linear, voltage and frequency sensitive RC network connectable between earth and a phase of a switch controlled, AC electric load circuit. The RE network includes a linear resistance component; a non-linear resistance component connected in parallel with the linear resistance component and having a pre-determined knee-point voltage value; and a capacitance component connected in series with the parallel connected linear and non-linear resistance components. The capacitance component is operative to decouple the resistance components from the circuit at power supply frequency but to couple the combined resistive impedance of the parallel connected linear and non-linear resistance components effectively into the circuit under high frequency conditions. The capacitance component is further operative to increase the rise time of that portion of a steep fronted surge which exceeds the knee-point voltage value of the non-linear resistance component.

This invention relates to electric switching surge protection. 
It is known that during the switching of a medium or high voltage 
[typically from 1 kV to 11 kV], switch controlled, cable connected, 
electric motor circuit the following types of transient voltages may be 
encountered. 
[a] A steep wavefront is injected at the instant of a prestrike or a 
restrike into the load cable which is connected to the motor terminals. 
This wavefront may increase in magnitude by up to two times upon its 
refraction at the motor terminals. Because of its very short rise time 
[typically 0.2 to 1 micro-seconds], it is known to stress the interturn 
and/or intercoil insulation of the line-end coils of the motor windings. 
It is also known that this type of electrical transient generally 
constitutes the most severe form of insulation stressing encountered in 
the switching of high voltage motors. 
[b] In the case of motors controlled by air, oil, SF6 or similar switchgear 
which do not interrupt at high frequency current zeros, a switch-on 
operation is usually accompanied by a single pre-strike on each phase. The 
magnitude of the associated steep wavefront impressed on the motor input 
terminals could be as high as four times the nominal peak phase-to-earth 
voltage of the system. 
[c] In the case of motors controlled by vacuum or similar switchgear 
capable of interrupting at high frequency current zones, the switch-on 
operation is usually accompanied by sequential multiple pre-striking [i.e. 
re-ignitioning]. The magnitudes of the associated steep wavefronts imposed 
on the motor input terminals could exceed four times the nominal peak 
phase-to-earth voltage of the system. 
[d] Overvoltages which are generated when an LC circuit is de-energized. 
The transient surge voltage generated within the load subsequent to 
de-energisation is made up of two components which are of the same 
frequency, but which are 90 electrical degrees out of phase. The one 
component comprises a load recovery transient which occurs when the energy 
stored within the load circuit capacitance is re-distributed and/or is 
dissipated within a three phase RLC load circuit upon de-energisation. 
This change in capacitive charge generally occurs in the form of a lightly 
damped oscillation. The other component is normally referred to as a 
current chopping surge and results from energy trapped within the 
inductive load winding when the inductive current is interrupted at a 
finite value, and this energy is dissipated within the three phase RLC 
load circuit in an oscillation of a similar frequency to the first 
component, but 90 electrical degrees out of phase. The overvoltages thus 
generated are generally of a medium frequency [typically 1 to 10 kHz] and 
result in stressing of the motor winding to earth insulation. 
[e] Re-striking [i.e. re-ignitioning] may occur when the motor is 
de-energised during starting. This usually applies to all types of 
switchgear. The associated steep wavefronts imposed on the motor terminals 
may have magnitudes in excess of five times the nominal peak 
phase-to-earth voltage of the system. 
[f] Sequential multiple re-striking [i.e. re-ignitioning] transients which 
may occur in circuits controlled by vacuum switchgear or other types of 
switchgear capable of interrupting high frequency currents, and which are 
generally associated with escalation of the inductive load current and 
therefore of the peak values of successive de-energisation surge voltages. 
This phenomenon is particularly associated with multiple re-striking 
during stalled tripping [i.e. when a motor's starting current is 
interrupted] and may increase significantly the magnitudes of the steep 
wavefronts associated with such re-strikes. 
[g] An increase of re-strike voltages in the manner described in [f] above 
may often result in forced current interruption [also referred to as 
"virtual current chopping"] of one or both of the adjacent phases. The 
phenomenon causes surges which may have very severe consequences and 
should be avoided at all costs. 
Surge arrestors are often employed to limit overvoltage surges to a level 
below the overvoltage surge withstand level with respect to earth 
potential of the load. However, steep wavefronts which represent abrupt 
changes in voltage without reference to earth potential can result in 
severe stressing of the inter-turn insulation of motor windings without 
exceeding the surge arrestor protection level which is defined with 
respect to earth potential. As such, surge arrestors or similar voltage 
limiting devices do not normally offer adequate protection against steep 
fronted surges. 
It is also known that undamped surge capacitors may be included at either 
end of the load cable to slope steep wavefronts [i.e. to increase the 
wavefront risetime] to acceptable values and also to decrease the overall 
load surge impedance and thus the magnitude of the current chopping surge 
component. However, the use of an undamped surge capacitor suffers from 
the following disadvantages: 
[i] It does not eliminate multiple re-striking. 
[ii] It provides a low impedance capacitive coupling at high frequencies 
between phases and may therefore increase the probability of forced 
current interruption [i.e. virtual current chopping] of the adjacent 
phases in circuits controlled by vacuum or similar switchgear. It is well 
known that the latter is a particularly severe phenomenon which can 
generally not be tolerated in motor circuits. 
[iii] If more than one load circuit in a particular system includes an 
undamped surge capacitor, the high frequency inrush current associated 
with a pre- or re-strike in the circuit being switched may be excessively 
high and may eventually result in failure of the surge capacitor. 
A parallel connection combination of a surge arrester and an undamped surge 
capacitor may be able to avoid or at least minimize the disadvantage 
resulting from virtual current chopping if the combination is located 
close to the load terminals, but it retains the disadvantage described in 
sub-paragraph [iii] above and also has the additional disadvantages of 
relatively high cost and relatively large size. 
It is also known to use damped surge capacitors for surge suppresion. A 
conventional damped surge capacitor comprises a capacitor which has a 
typical capacitance value of 0.2 to 0.5 mfd and which is connected in 
series with a damping resistor having a typical resistance value of a 
hundred ohms or more. 
If correctly applied, a conventional damped surge capacitor offers 
effective protection against the effects of medium frequency switching 
surges in a wide variety of motor circuits. The role of such a device may 
be summarised as follows: 
[a] It increases the capacitance-to-earth of the load and therefore 
decreases the overall load surge impedance [Z.sub.o =.sqroot.L/C, where L 
is the load inductance and C is the total load capacitance-to-earth]. The 
peak medium frequency current chopping surge is therefore accordingly 
reduced. [V.sub.c =I.sub.o Z.sub.o, where I.sub.o is the inductive load 
current interrupted by the switch]. 
[b] A critically damped surge capacitor, inserted at any point in the load 
circuit, serves to dampen the combined medium frequency de-energisation 
surges [i.e. both the load recovery and current chopping surges]. For this 
purpose the value of the series damping resistance should be of the order 
of two times .sqroot.L/C.sub.s, where L is the load inductance and C.sub.s 
is the value of inserted surge capacitance. Typical resistance values for 
this application lie in the range from about 100 to 1000 ohms. A 
critically damped surge capacitor may drastically reduce the magnitude of 
a de-energisation surge whilst at the same time decreasing the frequency 
of such a surge [i.e. increasing its time-to-peak] by as much as five 
times. This lowers the probability of a re-strike occurring. 
However, a conventional damped surge capacitor suffers from the 
disadvantages that it is relatively large in size and relatively 
expensive. Furthermore, it does not reduce effectively the magnitudes of 
high frequency wavefronts which are imposed on a motor in the course of 
pre- and re-striking. It also does not eliminate multiple pre- and 
re-striking. 
It is also known to provide an RC surge suppressor comprising a surge 
capacitor which has a typical capacitance value of 0.1 to 0.3 mfd and 
which is connected in series with a resistor having a typical resistance 
value lying in the range from about 50-100 ohms. 
A conventional RC surge suppressor should preferably be located close to 
the motor terminals and its function may be summarised as follows: 
[a] The capacitive component serves firstly to lower the overall load surge 
impedance and thus reduces the peak magnitude of the current chopping 
surge component. 
[b] Secondly, the RC surge suppressor serves to provide a sufficiently long 
charging time constant R.sub.s C.sub.s [where R.sub.s and C.sub.s are the 
respective values of inserted damping resistance and surge capacitance] in 
order to prolong the restrike current and render it aperiodic. 
[c] With the RC surge suppressor connected close to the motor terminals, 
the series resistor is intended firstly to reduce the magnitude of the 
refracted voltage wavefront, and secondly to extend and render the high 
frequency restrike current aperiodic in order to suppress multiple 
restriking and voltage escalation in circuits controlled by vacuum 
contactors and similar switching devices. 
The following disadvantages are often associated with the use of 
conventional RC surge suppressors: 
[i] They are relatively large in size and serious practical difficulties 
are often experienced in the installation of these large devices 
sufficiently close to the motor terminals. Even if connected to the motor 
by cable, problems are often encountered in suitably enclosing these 
devices outdoors or in hazardous or polluted areas. 
[ii] An RC suppressor reduces the ratio between the magnitude of a steep 
wavefront [as seen by the motor] and the pre- or re-strike voltage, to a 
fixed value, typically 1 to 1.2. However, an RC suppressor does not impose 
a definite limit on the magnitude of a steep fronted surge that can 
impinge on the motor terminals. 
[lll] An RC suppressor does not increase the rise time of a steep fronted 
voltage surge. 
It is an object of the present invention to provide improved surge 
protection for switch controlled load circuits. 
According to the invention a surge protector comprises an RC network 
adapted to be connected between earth and a phase of a switch controlled, 
AC electric load circuit, characterized in that the RC network includes a 
linear resistance component having a substantially linear V-I 
characteristic; a non-linear resistance component connected in parallel 
with the linear resistance component and having a pre-determined 
knee-point voltage value; and a capacitance component connected in series 
with the parallel connected linear and non-linear resistance components, 
the capacitance component being operative to decouple the resistance 
components at least partially from the circuit at power supply frequency 
but to couple the combined resistive impedance of the parallel connected 
linear and non-linear resistance components effectively into the circuit 
at frequencies associated with wavefront rise times of up to 2.0 
microseconds and the capacitance component further being operative to 
increase the rise time of that portion of a steep fronted surge which 
exceeds the knee-point voltage value of the non-linear resistance 
component. 
For the purposes of this specification, the term "non-linear resistance" is 
used to signify a resistance having a non-linear V-I characteristic with 
relatively high resistance values at voltages up to a predetermined 
voltage value which is referred to herein as the "knee-point voltage 
value", and with decreased resistance values at voltages in excess of the 
knee-point voltage value. 
The non-linear resistance component may have any suitable V-I 
characteristic in regard to the resistance values at voltages below the 
knee-point voltage value, the actual knee-point voltage value and the 
resistance values at voltages in excess of the knee-point value. The 
knee-point voltage value may be selected to suit the relevant insulation 
characteristics of the load to be protected. 
A surge protector according to the invention may be used in a load circuit 
in which the load terminals are directly connected to a switch, but is 
particularly applicable to load circuits in which the load terminals are 
connected to a switch by means of a load cable. 
In a surge protector according to the invention which is adapted to protect 
a load circuit including a load and a cable connecting the load to a 
switch, the linear resistance component of the RC network may have a 
resistance of up to four times the surge impedance of the load cable. 
Further according to the invention there is also provided a surge protected 
3-phase AC electric load circuit including a load; a switch connected to 
the load; and a surge protector according to the invention for each phase 
of the load circuit which is connected between the phase and earth. 
The switch may be connected directly to the load or by means of one or more 
load cables. 
Where the switch is cable connected to the load, each RC network according 
to the invention is preferably connected to an associated load input 
terminal in a position at or near the load. 
Each RC network may be connected to the associated load input terminal by a 
separate cable having a length of not more than 5 meters and a surge 
impedance of not more than twice the surge impedance of the cable 
connecting the switch to the load terminals. 
The invention is applicable to load circuits operable at line voltages of 
about 1 kV and higher. The invention is particularly suitable for motor 
and other load circuits operable at line voltages in the range from 1 kV 
to 11 kV, but may also be used at higher line voltages, such as in arc 
furnace transformer circuits which may operate at line voltages of 33 kV 
or even higher. 
In a switch controlled, cable connected load circuit adapted to operate at 
a line voltage of at least 1 kV, the role of the linear resistance is to 
terminate the load cable at the high frequencies associated with 
pre-strikes and re-strikes and thus to prevent voltage doubling of 
travelling waves and to minimize the reflected current component. 
The linear cable terminating resistance component may have a resistance 
lying in the range from about one to three times the surge impedance of 
the cable. 
The principal role of the series connected capacitance component is to 
decouple the resistance components during normal operation at the normal 
power supply frequency, thereby to minimize the steady state voltage 
across the resistance components. However, at the high frequencies 
associated with travelling waves, the impedance of the capacitance 
component is sufficiently low to couple the combined resistive impedance 
of the parallel connected linear and non-linear resistance components 
effectively into the load circuit. 
The capacitance component may have a value to couple the combined resistive 
impedance effectively into the circuit at frequencies associated with 
wavefront rise times in the range from about 0.2 to 1.0 microsecond. 
For a nominal power supply frequency of 50 Hz or 60 Hz the value of the 
capacitance component may be in the range from 0.02 to 0.3 microfarad and 
preferably in the range from 0.05 to 0.2 microfarad. 
With a power supply frequency of 50 Hz or 60 Hz and a capacitance value in 
the range from 0.02 to 0.3 microfarad, the value of the linear resistance 
component may lie in the range from 10 to 75 ohms. 
The non-linear resistance component which is connected in parallel with the 
linear resistance component has a pre-determined knee-point value which 
refers to that voltage value above which its effective resistance reduces 
rapidly with increasing current. Should a high frequency [i.e. steep 
fronted] surge voltage exceed the knee-point voltage value of the 
non-linear resistance component, the resistance of the latter decreases 
sufficiently for the resultant resistance of the parallel combination of 
the non-linear resistance component and the linear resistance component 
which is connected in series with the capacitance component, to become 
sufficiently low for the capacitance component to act effectively as a 
wave sloping capacitor and to increase the rise time of that portion of a 
steep fronted surge which exceeds the knee-point voltage value of the 
non-linear resistance component. 
The non-linear resistance component may comprise a zinc oxide element or 
any other suitable element, such as a silicon carbide element or a spark 
gap, which has a non-linear V-I characteristic. 
The knee-point voltage value of the non-linear resistance component may 
typically lie in the range from about 0.5 to 2 times the nominal peak 
phase-to-earth voltage of the system. 
Preferably, the linear resistance, the non-linear resistance and the 
capacitance are substantially non-inductive. 
By connecting the non-linear resistance component in parallel with the 
linear resistance component, a non-linear, voltage and frequency sensitive 
RC surge protector network may be obtained. 
At the normal power supply frequency of the system the capacitance 
component acts to decouple the parallel combination of the linear 
resistance and the non-linear resistance at least partially from the 
circuit. 
At the medium frequencies associated with oscillatory load de-energisation 
transients, the peak transient current through and the peak voltage across 
the linear resistance is sufficiently low so that the knee-point voltage 
value of the non-liner resistor is not exceeded. Under medium frequency 
conditions the characteristics of the combined non-linear RC surge 
protector network according to the invention, are therefore similar to a 
conventional RC surge suppressor with similar linear resistance and series 
capacitance values. 
Under high frequency conditions and for steep fronted surge voltages not 
exceeding the knee-point voltage value of the non-linear resistance 
component, the characteristics of the non-linear RC surge protector 
network according to the invention are similar to that of a conventional 
RC surge suppressor with similar linear resistance and capacitance values. 
Under high frequency conditions and for steep fronted surge voltages 
exceeding the knee-point voltage value of the non-linear resistance 
component, the rise time of that portion of the steep fronted surge which 
exceeds the knee-point voltage value of the non-linear resistance 
component is increased. Thus, a non-linear RC surge protector network 
according to the invention acts effectively as a wave sloping device for 
that portion of a steep fronted surge voltage which exceeds the knee point 
voltage value of the non-linear resistance component. 
The high frequency surge current in the capacitance component of the 
invention is essentially of an aperiodic nature with a relatively short 
duration. Under severe conditions where a high frequency [i.e. steep 
fronted] surge voltage exceeds the knee-point voltage value of the 
non-linear resistance component so that the capacitance component acts 
effectively as a wave sloping capacitor, both the peak value and duration 
of the surge current in the capacitance component of the invention is less 
than that in a conventional undamped surge capacitor. In addition, unlike 
a conventional undamped surge capacitor, an RC network according to the 
invention does not substantially increase the probability of high 
frequency capacitive coupling between phases. 
A non-linear RC surge protector network according to the invention is 
capable not only of minimising the voltage doubling effects associated 
with the refraction at the load terminals of steep fronted travelling 
waves in cable connected motor circuits, but also of modifying the 
reflection and refraction of such steep fronted travelling waves in order: 
[i] to suppress multiple pre- and re-striking in high voltage motor 
circuits switched by vacuum switchgear or other switchgear capable of 
interrupting high frequency pre- and re-strike currents; and 
[ii] to limit the magnitude of a steep fronted voltage surge travelling 
along a load cable and impinging on the load terminals, to a predetermined 
voltage value and also to increase to an acceptable value the rise time of 
that portion of a steep fronted voltage surge impinging on the load 
terminals which exceeds the predetermined knee-point voltage value of the 
non-linear resistance component. 
It will be appreciated that the capacitance component of the invention 
effectively decouples both the linear resistance and the non-linear 
resistance from the circuit at normal power supply frequency so that the 
power rating and size of these components may be reduced. The non-linear 
resistance only becomes active under high frequency conditions when its 
knee-point voltage value is exceeded and its resistance decreases, so that 
its required energy handling capability is minimized. 
Preferably, the value of the capacitance component of the invention is as 
small as possible commensurate with effective coupling of the combined 
resistive impedance into the circuit at the high frequencies associated 
with steep fronted travelling waves, in order to minimize current flow 
through and power losses in the linear resistance and in the non-linear 
resistance during normal operation at the normal power supply frequency of 
the system, thereby to minimize the respective power ratings of the linear 
resistance and the non-linear resistance. 
With a power supply frequency of 50 Hz or 60 Hz and a decoupling 
capacitance value in the range from 0.05 to 0.2 microfarad, the value of 
the linear resistance may lie in the range from 10 to 75 ohms with a 
typical continuous power dissipation of about 1 mW to 12 W in a system 
operating at a line voltage in the range from 1 kV to 11 kV. With such an 
arrangement, the maximum energy dissipated in the non-linear resistance 
during a pre- or re-strike when the knee-point voltage value is exceeded, 
is typically less than 200 Joules. 
Preferably, a surge protector according to the invention is locatable in 
the terminal box of the load for connection to the load input terminals. 
In the case of a 3-phase system, a surge protector according to the 
invention may be provided for each phase at or near the load end of a load 
cable. 
The invention is particularly, but by no means exclusively, applicable to 
vacuum switching devices. The invention is particularly suitable for 
electric motor protection, but may be applied to protect any suitable 
switch controlled load, particularly an inductive load. Thus, a surge 
protector according to the invention may be used in a transformer load 
circuit where a need exists for the reduction of high frequency voltage 
surges. 
With the arrangement according to the invention there may be provided a 
non-linear RC surge protector network which is capable of reducing and 
limiting the magnitude of steep voltage wave fronts in cable connected 
motor circuits and which may be sufficiently small in physical size to be 
located inside a motor terminal box and be connected to the motor input 
terminals. 
In the majority of high voltage motor circuits, non-linear surge protectors 
according to the invention may be used to replace conventional surge 
capacitors and surge suppressors. 
The connection of a non-linear surge protector according to the invention 
to the load input terminal of each phase of a motor serves to minimize and 
limit the magnitudes of steep wave fronts associated with pre- and 
re-strikes and also, where applicable, eliminating multiple pre- and 
re-striking, thereby to provide adequate protection against switch-on and 
switch-off surges. 
Where applicable, the capacitance component of a non-linear RC surge 
protector according to the invention, may be increased up to a value of 
about 0.3 microfarad. This modified arrangement may, for example, be 
applied where a need exists for additional protection against severe 
stalled tripping surges, such as where a motor is required to perform 
inching duties. However, in this case it is possible that the non-linear 
RC surge protector may not be of sufficiently small dimensions to be 
fitted inside the motor terminal box and it may have to be positioned 
close to the motor. It does, however, still have the additional advantage 
over a conventional RC surge suppressor in that the magnitudes of steep 
wavefronts are limited to a predetermined level, typically of the order of 
twice the nominal peak phase-to-earth voltage of the system. 
A surge protector according to the invention may also be used in 
conjunction with a conventional damped surge capacitor. With such an 
arrangement the damped surge capacitor serves to dampen medium frequency 
surges while the surge protector according to the invention serves to 
reduce the magnitudes of any remaining high frequency pre- or restrike 
voltage wavefronts. 
A surge protector according to the invention may also be used in 
conjunction with conventional gapped or gapless surge arrestors where 
additional overvoltage protection is required. 
The invention is applicable not only to cable connected load circuits but 
also to load circuits in which switch means is connected directly to the 
load without a load cable. 
A surge protector according to the invention may be connected to the input 
terminals of the load cable in cases where it is not practically possible 
to connect the surge protector to the load terminals in a position at or 
near the load terminals. If a surge protector according to the invention 
is connected to the input terminals of the load cable it will reduce 
effectively the magnitude and increase effectively the rise time of a 
steep fronted voltage surge injected into the load cable.

Referring first to FIGS. 1 and 2 of the accompanying drawings, the load 
circuit comprises a 3-phase inductive load 1 with an input terminal 2 for 
each phase. Load 1 is adapted to operate at a line voltage lying in the 
range from 1 kV to 11 kV or higher. The input terminal 2 of each phase of 
load 1 is connected by means of a load cable 6 to a load-side terminal 3 
of a 3-phase switching device 4 which is located remotely from load 1 and 
which provides a switch for each phase. Load cables 6 may comprise single 
core cables or may comprise 3-core cables. 
A non-linear RC surge protector 5 according to the invention is provided 
for each phase of the load circuit. Each surge protector 5 is connected at 
the load end of the associated cable 6 to the associated input terminal 2 
of load 1 and also to earth. It will be appreciated that each non-linear 
surge protector 5 is connected between a phase of the load circuit and 
earth. 
Each surge protector 5 comprises a non-linear, voltage and frequency 
dependent RC network comprising a substantially non-inductive linear 
resistance 7 having a substantially linear V-I characteristic; a 
substantially non-inductive zinc-oxide or other suitable non-linear 
resistance element 8 connected in parallel with the linear resistance 7, 
and a substantially non-inductive capacitor 9 which is connected in series 
with the parallel combination of the linear resistance 7 and the 
non-linear resistance 8. 
As can be seen from FIG. 2, the non-linear resistance 8 has a non-linear 
V-I characteristic with relatively high resistance values at voltages up 
to the knee-point voltage value Vkp and with substantially decreased 
resistance values at voltages in excess of the knee-point voltage value 
Vkp. This characteristic of voltage dependent, non-linear resistance 
elements is well known. 
Each resistance 7 has a resistance R.sub.s lying in the range from one to 
four times Z.sub.c where Z.sub.c is the surge impedance of the associated 
load supply cable 6. Each non-linear resistance 8 has a knee-point voltage 
value Vkp in the range from 0.5 to 2 times the peak nominal phase-to-earth 
voltage of the system. The value of each capacitor 9 may lie in the range 
from 0.02 to 0.3 microfarad and preferably in the range from 0.05 to 0.2 
microfarad for a power supply frequency of 50 Hz or 60 Hz and the value of 
each linear resistance 7 may lie in the range from 10 to 75 ohms. 
It will be appreciated that many variations in detail are possible without 
departing from the spirit of the invention. For example, the inductive 
load 1 may comprise an electric motor or a transformer with star (wye) or 
delta connected windings. 
Where load 1 comprises an electric motor requiring additional protection 
against severe stalled tripping surges, such as where the motor is 
required to perform inching duties, each capacitor 9 may have a value of 
up to 0.3 microfarad for a power supply frequency of 50 Hz or 60 Hz and 
the value of each linear resistance 7 may lie in the range from 10 to 75 
ohms, each non-linear resistance 8 having a knee-point voltage value lying 
in the range from 0.5 to 2 times the peak nominal phase-to-earth voltage 
of the system. 
Instead of switching device 4 being connected to load terminals 2 by means 
of load cables 6, the load cables 6 may be dispensed with in certain 
circumstances and the load-side terminals 3 of switching device 4 may be 
connected directly to load terminals 2. Each surge protector 5 may be 
connected directly to its associated load terminal 2 as before. 
The arrangement of FIG. 3 is similar to that of each of the phases of the 
arrangement of FIG. 1, with the exception that the non-linear RC surge 
protector 5 of FIG. 3 is connected to the load input terminal 2 by means 
of a separate cable 10. The surge impedance Zc2 of the separate cable 10 
should not exceed about 2Zc where Zc is the surge impedance of the load 
supply cable 6 and the length of the separate cable 10 should not exceed 
about 5 meters. The resistance value of the linear resistance 7, the 
knee-point voltage value of the non-linear resistance 8 and the 
capacitance value of capacitor 9 may lie in the ranges specified above in 
relation to the arrangement of FIG. 1. 
Instead of each surge protector 5 being connected to the associated load 
input terminal 2 at the load end of the load cable 6 as shown in FIGS. 1 
and 3, the surge protector 5 of each phase may be connected to the 
associated load-side switch terminal 3 in a position at or near the switch 
end of the associated load cable 6. Each surge protector 5 may be 
connected to the associated load-side terminal 3 of switching device 4 and 
to earth so that the surge protector 5 is connected between a phase of the 
circuit and earth. 
In a 3-phase load circuit as shown in FIG. 1, the surge protector 5 of each 
phase may be located in its own enclosure as a self-contained unit. 
Alternatively, the three surge protectors 5 of the three phases may be 
located in a common enclosure.