Surge protection device or SPD - a device providing additional protection against transient overvoltages (TOV), surges and spikes.

Typical surge protection device (SPD) for a domestic electricity installation

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The technical capabilities and installation of SPDs used in electrical installations are defined in such standards as BS 7671:2018 or IEC 61643-11.

BS 7671 refers to three types of SPDs:

• Type 1 installed at or near the origin of the electrical installation (e.g. main distribution board).
• Type 2 installed in the distribution circuits (e.g. sub-distribution board).
• Type 3 installed close to sensitive equipment.

BS 7671 also specifies two internal connection types:

• Connection Type 1 - all the non-linear components are connected from L1, L2, L3, and N to the common of the PE conductor (4+0).
• Connection Type 2 - the non-linear components are connected from L1, L2, and L3 to the common N, and another component from N to PE (3+1).

The standard Low-voltage surge protective devices IEC EN 61643-11 requires the following marking on the body of an SPD:

• Manufacturer and model number
• Maximum continuous operating voltage Uc in V and type of current (DC, AC or ~)
• Test classification and discharge parameters, for:
  • Test Class I – name of class or [T1] and the impulse current I_imp in kA
  • Test Class II – name of class or [T2] and the nominal discharge current I_n in kA
  • Test Class III – name of class or [T3] and open-circuit voltage Uoc in kV
• Voltage protection level Up in kV (peak voltage across SPD during the impulse test)
• Identification of terminals or leads
• Degree of Ingress Protection
• Rated load current I_L for SPDs with separate input/output terminals

• Surge protection