Created by: Glen Zhu | Updated Date: Sep 21, 2022
As everyone knows, surge protection devices or surge protective devices (SPD) protect electrical equipment against over-voltages caused by lightning. That said, it is not always easy to know which to choose.
Choosing the right surge arrester and protective circuit breakers involves a considering a wide range of parameters related to types of surge protection devices, circuit breaker arrangements and risk assessment.
Let’s try to see things more clearly…
Install in the main switchboard in buildings with lightning protection systems.
Can discharge very powerful lightning currents.
Should be used as part of lightning protection systems
i.e. when lightning rods or meshed cages are installed
Install in the main power distribution switchboard. Discharge currents from indirect lightning strikes, inductive and conductive overvoltages, and switching transients.
Should always be used.
Dedicated to the protection of speciﬁc devices. Very low discharge capacity.
Supplementary surge protective devices:
• Used in Types 1+2+3 combinations in industrial facilities where a lightning protection systems is in place
• Used in Types 2+3 combinations when there is no lightning protection systems
Lightning protection should be approached from an overall viewpoint. Depending on the application (large industrial plants, data c enters, hospitals, etc.), a risk assessment method must be used to guide in choosing optimal protection (lightning protection system, surge protective devices). National regulations, moreover, may make it compulsory to use the EN 62305-2 standard (Risk assessment).
In other cases (housing, offices, buildings not sensitive to industrial risks), it is easier to adopt the following protection principle:
In all cases a Type 2 surge protective device will be installed in the electrical installation’s incoming-end switchboard. Then, the distance between that surge protective device and the equipment to be protected should be assessed. When this distance exceeds 30 meters, an additional surge protective device (Type 2 or Type 3) should be installed near the equipment.
Then, the sizing of Type 2 surge protective devices depends mainly on the exposure zone (moderate, medium, high): there are diﬀerent discharge capacities for each of these categories (Imax = 20, 40, 60 kA (8/20μs)).
For Type 1 surge protective devices, the minimum requirement is a discharge capacity of Iimp = 12.5 kA (10/350μs). Higher values
may be required by the risk assessment when the latter is requested.
Finally, the protection device associated with the surge protective device (circuit breaker or fuse) will be chosen according to the short-circuit current at the place of installation. In other words, for a residential electrical switchboard, a protection device with an Isc < 6 kA will be chosen.
For oﬃce applications, the Isc is generally < 20 kA.
Manufacturers must provide the table for coordination between the surge protective device and the associated protection device. More and more surge protective devices already incorporate this protection device in the same enclosure.
Coordination between the surge protective device and its disconnect circuit breaker
An external disconnecting device must be coordinated with a surge protective device in order to achieve:
Surge protective device in normal operation
Surge protective device in operation during the lightning stroke
Surge protective device at end of life on short circuit
The disconnecting device must be coordinated with the surge protective device. It is designed to meet the following two constraints:
Resistance to lightning current
The resistance to lightning current is an essential characteristic of the surge protective device’s external disconnecting device.
The device must be capable of passing the following standardized tests: not trip upon 15 successive impulse currents at In.
Resistance to short-circuit current
The breaking capacity is determined by the installation rules (IEC 60364 standard):
|External disconnecting device||Fuse protection combined with the SPD||Circuit breaker protection combined with|
|Lightning protection of equipment||=||=|
|All types of disconnecting devices protect the equipment satisfactorily|
|Protection of installation (at end of the surge protective device’s life)||=||+|
|Achieved if compliance with the MCB/SPD coordination table|
|Protection from (impedant) short circuits of low intensity not well ensured||Protection against (impedant) short circuits of low intensity|
|Continuity of service (at end of the surge protective device’s life)||+||+|
|Only the surge protective device circuit is shut down|
|Maintenance (at end of the surge protective device’s life)||=||+|
|Change of fuses||Immediate resetting|
Main reasons why the disconnecting device recommended by the manufacturer should be used:
This table shows: the rating, curve and short circuit current level of the disconnector coordinated with the surge protective device.
When two surge protective devices are installed in an electrical installation, coordination is needed according to IEC 61643-12 to obtain an acceptable stress distribution between the two surge protective devices according to their permissible energy “E”.
L and Zd represent the cable length and impedance respectively between the 2 surge protective devices.
Up2: level of protection of surge protective device SPD2.
Uw: impulse withstand voltage of the equipment to be protected.
Imax: maximum discharge current.
IF: lightning current: Imax of SPD1=I1 + I2
E: permissible energy.
MCB: modular circuit breaker.
SPD: surge protective device.
For coordination between two surge protective devices, a minimum cable length between these 2 surge protective devices is needed to ensure that:
@I2 < Imax SPD2.
@Up2 < Uw.
@E2 < Emax SPD2.
For a cable section of 16mm² and an impulse current equal to the maximum discharge current (Imax) of the upstream surge protective device.
If FLP7 is installed in the incoming panelboard, the second SPD SLP10 must be installed at a cable length of 8 meters from the ﬁrst one.