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Surge Protective Devices - SPD

The new 18th Edition Regulations are coming in effect on the 1st of January 2019 and here are some key changes to consider in your electrical design projects for including Surge Protective Devices (SPD).

Surge Protective Devices (SPD) are devices for Protection against Overvoltage. SPDs are designed to limit transient overvoltages due to lightning or switching and divert the associated surge currents to earth, so as to limit these overvoltages to levels that are unlikely to damage the electrical installation or equipment.

Regulation 443.4 of BS 7671:2018 states:

Protection against transient overvoltages shall be provided where the consequence caused by overvoltage could:

  • Result in serious injury to, or loss of, human life, or
  • Result in interruption of public services and/or damage to cultural heritage or
  • Result in interruption of commercial or industrial activity, or
  • Affect a large number of co-located individuals

For all other cases, a risk assessment according to Regulation 443.5 shall be performed in order to determine if protection against transient overvoltages is required.

SPDs are NOT only for commercial or industrial installations!

Regulation 443.4 of BS 7671:2018 states: If the risk assessment is not performed, the electrical installation shall be provided with protection against transient overvoltages, except for single dwelling units where the total value of the installation and equipment therein does not justify such protection.

Also, IEC 60364-4-44 states that overvoltage protection is not required for single dwelling units where the total economic value of the electrical installation to be protected is less than five times the economic value of the SPD located at the origin of the installation.

This means that for single dwelling units the electrical installation designer should determine if the total value of the electrical installation and the equipment connected therein justifies the cost of installing the SPDs.


Modeling SPDs in ElectricalOM is straightforward by following the 2 steps below:

Step 1: Insertion of the SPD

From the main design menu (accessed with right click on the schematic area) select Insert -> SPD - Overvoltage Protection.

 

Step 2: Set the SPD properties

To set the SPD properties select the SPD element and within the circuit editor choose the Type / Impulse Current / Maximum Discharge Current /  Voltage Protection Level / Conductor.

The Guides next to each property are derived directly from the standards and will help the designer select the correct value.

Based on the SPD characteristics selected, the software will check for compliance with BS7671 and flag errors where the regulation is violated.



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