SPD surge protection device electrical symbol BS EN 60617 — what it represents, where it appears in UK electrical drawings, and how it relates to the rest of the symbol library.
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The SPD symbol represents a Surge Protective Device to BS EN 61643-11 — a component that diverts transient overvoltages from lightning or supply switching safely to earth. On UK drawings it appears at the origin of the installation. BS 7671 defines three types by Lightning Protection Zone: Type 1 (incoming service), Type 2 (consumer unit/distribution board) and Type 3 (sensitive final equipment).
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A Surge Protective Device to BS EN 61643-11. Type 1 (lightning current), Type 2 (transient overvoltage), Type 3 (point of use). BS 7671 443.4 risk assessment usually requires Type 2 at origin.
Used in: New installations + rewires — Type 2 at consumer unit origin; Type 1 where lightning protection is fitted.
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The symbol denotes a Surge Protective Device to BS EN 61643-11 — a component that diverts transient overvoltages (from a lightning strike or supply switching) to earth before they reach equipment. Where it appears on a UK drawing it almost always sits at the origin of the installation, alongside the main switch and distribution.
BS 7671 Reg 443.4.1 — when is SPD protection mandatory? Protection against transient overvoltages shall be provided where the consequence caused by the overvoltage could result in:
Serious injury to, or loss of, human life
Significant financial or data loss
Limb (b) was deleted by the BS 7671:2018+A2:2022 Corrigendum (May 2023).
For all other cases, protection against transient overvoltages shall be provided unless the owner of the installation declares it is not required due to any loss or damage being tolerable and they accept the risk of damage to equipment and any consequential loss. Record the decision on the certificate.
Section 534 (selection and erection of SPDs) was substantially revised in the BS 7671:2018+A4:2026 amendment, including the voltage protection level (Up) selection criteria covered further down this page.
Three SPD types are positioned by the Lightning Protection Zone (LPZ) concept (Figure 534.1). Each protects a different boundary, so a fully protected installation often uses more than one, coordinated together.
Handles partial direct lightning current (10/350 µs waveform). Required at the origin where the structure has an external lightning protection system or otherwise needs protection against direct lightning. Bigger connecting conductors than a Type 2.
Handles transient overvoltage (8/20 µs waveform) at the consumer unit or main distribution board origin. The most common device on a UK domestic or light-commercial drawing.
Fine protection at, or close to, sensitive final equipment. Always installed downstream of, and coordinated with, a Type 1 and/or Type 2 device — never on its own.
Type 2 or Type 3 SPDs may also be placed in sub-distribution boards or close to the equipment to be protected to achieve the required voltage protection level, coordinated with the device(s) at the origin (Reg 534.4.1.5; Figure 534.2).
Once Section 443 has established that SPDs are required, Section 534 decides which type goes where. The trigger is whether the structure has external lightning protection:
No external lightning protection (or no need for direct-lightning protection): install Type 2 SPDs as close as possible to the origin of the installation — the consumer unit or main distribution board.
External lightning protection fitted (or direct-lightning protection required): install Type 1 SPDs as close as possible to the origin, at the LPZ 0/1 boundary on the incoming service.
SPDs with RCDs (Reg 534.4.7): where an SPD sits on the load side of an RCD, that RCD shall have an immunity to surge currents of at least 3 kA 8/20 — a Type S time-delayed RCD satisfies this. Installing a Type 1 SPD downstream of an RCD is not recommended.
For the full set of distribution symbols, see the Distribution Symbols reference page — covering every related symbol with the same level of installation context.
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Start free trialReg 534.4.10 sets minimum cross-sectional areas for SPDs installed at or near the origin of the installation. The protective (earthing) connection and the live-side connection have separate minimums:
| SPD type (at/near origin) | PE / earthing conductor | Live conductor |
|---|---|---|
| Type 2 | ≥ 6 mm² copper or equivalent | ≥ 2.5 mm² copper or equivalent |
| Type 1 | ≥ 16 mm² copper or equivalent | ≥ 6 mm² copper or equivalent |
The live-side conductors connecting the SPD and its overcurrent protective device must also be rated to withstand the prospective short-circuit current expected at that point (Reg 534.4.10, referring to Reg 433.3.1(b)).
Keep the leads short. Reg 534.4.8 requires all conductors and interconnections between the SPD, the line to be protected and any external overcurrent device to be kept as short and straight as possible, with unnecessary loops avoided. Long leads add inductive voltage drop that raises the effective let-through at the equipment.
The voltage protection level (Up) is the let-through voltage an SPD allows to pass. Reg 534.4.4.2 requires Up to be selected against impulse withstand voltage Category II of Table 443.2 and, in no case, to exceed the equipment's required rated impulse voltage. For a standard 230/400 V installation the installed SPD assembly's voltage protection level shall not exceed 2.5 kV.
The 2.5 kV ceiling lines up with the Category II rated impulse voltage for a 230/400 V system in Table 443.2:
| Overvoltage category | Example equipment | Rated impulse voltage (Uw) |
|---|---|---|
| Category IV | Origin of installation, energy meter | 6 kV |
| Category III | Distribution boards, fixed wiring | 4 kV |
| Category II | Appliances, tools (SPD target) | 2.5 kV |
| Category I | Sensitive electronic equipment | 1.5 kV |
Values for a 230/400 V nominal installation, Table 443.2.
Protective distance > 10 m. If the distance between the SPD and the equipment it protects exceeds 10 m, oscillation can raise the voltage at the equipment terminals to as much as twice the SPD's voltage protection level. Add a further coordinated SPD closer to the equipment, or select a device with a lower Up (Reg 534.4.4.2).
Other distribution symbols you may need on the same drawing — and see how to read a wiring diagram for putting them together into a working installation drawing:
A domestic consumer unit (fuseboard) to BS EN 61439-3. Houses the main switch, RCD/RCBO/MCB protective devices, and (since 2018) SPD and AFDD where required.
Used in: Domestic origin of the installation — typically meter cupboard, under-stairs, garage or hallway.
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A three-phase or single-phase distribution board to BS EN 61439-3 (commercial). Holds MCBs, RCBOs and MCCBs serving final circuits in commercial and industrial buildings.
Used in: Commercial buildings, offices, schools, retail; one per floor or department typically.
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A distribution board fed by a sub-main from the main switchboard. Sub-divides the installation into manageable zones; reduces voltage drop on long runs.
Used in: Large buildings, multi-tenant offices, plant rooms, lift motor rooms, sub-zones.
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A Miniature Circuit Breaker to BS EN 60898. Protects against overload and short-circuit fault current. Types B, C and D differentiate by magnetic trip characteristic.
Used in: Every domestic + commercial final circuit; standard protection device on distribution boards.
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A Moulded Case Circuit Breaker to BS EN 60947-2. Higher current ratings (typically 100-1600 A) and higher breaking capacity than MCBs. Used as main incomers or for large feeders.
Used in: Main switchgear, sub-main feeders, large motor circuits, three-phase distribution.
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A Residual Current Device to BS EN 61008. Detects imbalance between live and neutral (earth-fault leakage) and trips. 30 mA RCDs provide additional protection against electric shock.
Used in: Bathrooms, kitchens, outdoor sockets, EV chargers — anywhere BS 7671 requires additional protection.
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A Residual Current Breaker with Overcurrent protection to BS EN 61009. Combines RCD + MCB in one device. Each circuit has individual earth fault and overload protection.
Used in: Modern consumer units — preferred to split-load RCD arrangements; one trip = one circuit affected only.
See all distribution board symbols →A Surge Protective Device to BS EN 61643-11. Type 1 (lightning current), Type 2 (transient overvoltage), Type 3 (point of use). BS 7671 443.4 risk assessment usually requires Type 2 at origin.
Used in: New installations + rewires — Type 2 at consumer unit origin; Type 1 where lightning protection is fitted.
See all distribution board symbols →
An electromagnetically-operated switch for high-current loads. Coil energised = contacts close. Used to control motors, heating, lighting circuits and timed loads remotely.
Used in: Immersion heaters with off-peak timer, motor starters, large lighting banks, HVAC plant.
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A main switch-disconnector at the origin of the installation. Disconnects all live conductors (including neutral on TT/IT) and must be lockable in OFF position.
Used in: Origin of every installation — domestic consumer unit, commercial main switchboard.
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The utility electricity meter — point of supply, owned by the meter operator. Modern smart meters communicate consumption to the supplier automatically.
Used in: Meter cupboard, garage external wall, riser cupboard; the boundary of utility responsibility.
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A switch that transfers a load between two sources (typically mains and generator). Manual or automatic (ATS). Always break-before-make to prevent backfeeding the grid.
Used in: Standby generators, UPS bypass, dual-supply critical loads, farms, remote homes.
See all distribution board symbols →
An Automatic Transfer Switch that detects mains failure and switches to a standby generator without manual intervention. Synchronised return-to-mains when supply restored.
Used in: Hospitals, data centres, telecoms, agricultural sites with standby plant.
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A metal enclosure containing busbars to interconnect multiple sub-main cables — typically at the main switchboard. Allows tap-offs to feed distribution boards without joint boxes.
Used in: Main switchboards, riser shafts, multi-tenant feeder distribution.
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