Surge Protective Device Coordination

How to choose the right surge protective device?

Created by: Glen Zhu | Updated Date: Sep 21, 2022

How to choose surge protection devices?

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…

First of all, current standards define three categories of surge protective devices for low-voltage electrical installations:

Type 1 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
Type 2 Install in the main power distribution switchboard. Discharge currents from indirect lightning strikes, inductive and conductive overvoltages, and switching transients. Should always be used.
Type 3 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

What surge protective devices should be chosen and where should they be installed?

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.

SPD location in the building without Lightning rod

SPD location in the building without Lightning rod

SPD location in the building with Lightning rod

SPD location in the building with Lightning rod

And the sizing of surge protective devices?

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.

How to choose the protection devices associated with the surge protective devices?

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.

Simplified selection principle (excluding full risk assessment)
Coordination between the surge protective device and its disconnect circuit breaker
Disconnector coordinated with surge protective device

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

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 the SPD
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:

Coordination between the surge protective device and its disconnect circuit breaker in the event of a short circuit

This table shows: the rating, curve and short circuit current level of the disconnector coordinated with the surge protective device.

Coordination between the surge protective device and its disconnect circuit breaker in the event of a short circuit

Coordination between the surge protective device and its disconnec fuse in the event of a short circuit

Coordination between the surge protective device and its disconnec fuse in the event of a short circuit

Coordination between two surge protective devices, upstream/downstream

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”.

Coordination between two surge protective devices, upstream and downstream

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.

Minimum distance between two surge protective devices, upstream/downstream

For a cable section of 16mm² and an impulse current equal to the maximum discharge current (Imax) of the upstream surge protective device.

Example

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.

Minimum distance between two surge protective devices, upstream and downstream

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Surge Protective Device Coordination

Coordination between the surge protective device and its disconnect circuit breaker (or fuse).

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