SPD Wiring Diagram
This is a free printable spd wiring diagram: download the diagram as SVG or open it and print to paper or PDF.
Learn how to correctly wire a surge protective device (SPD) across L-N, L-E, and N-E modes in Type 1, Type 2, and Type 3 configurations per IEC 61643-11 and installation codes.
A Surge Protective Device (SPD) — sometimes called a transient voltage surge suppressor (TVSS) or lightning arrester in older terminology — protects electrical installations and connected equipment from transient overvoltages arising from lightning, switching events, or utility faults. The relevant international standard is IEC 61643-11 (low-voltage SPDs); regional implementations include IEC 60364-5-53 Section 534, BS 7671 Chapter 53, and NEC Article 285.
SPDs are classified into three types based on where in the installation they are fitted and what class of surge they are designed to handle:
Type 1 SPDs (formerly Class B / Category I) are installed at the service entrance or main distribution board, typically between the utility supply and the main busbar. They are designed to handle direct lightning strike currents (impulse current I_imp, 10/350 µs waveform). Spark gap (SG) technology is commonly used for Type 1 because of its high energy handling capability.
Type 2 SPDs (formerly Class C / Category II) are installed in sub-distribution boards and are the most common type in commercial and residential installations. They handle the surge current remaining after the Type 1 has conducted (8/20 µs waveform, I_max / I_n ratings). Metal Oxide Varistors (MOVs) are the dominant technology at this level.
Type 3 SPDs (formerly Class D / Category III) are installed at the point of use — at socket outlets, appliance panels, or directly at equipment terminals. They supplement Type 1 and Type 2 devices; they must always be used in conjunction with an upstream Type 2 SPD and cannot be used in isolation.
Connection modes define which conductors the SPD protects between: L-N (line to neutral), L-E (line to earth), and N-E (neutral to earth). A fully protective installation should cover all modes. A 'Common Mode' SPD connects between each phase and earth; a 'Differential Mode' SPD connects between phases and neutral.
MOV degradation is a key maintenance consideration. MOVs absorb energy during surges and have a finite lifetime energy absorption capacity. A thermal fuse (overcurrent disconnector) is built into quality SPDs to disconnect the degraded MOV before it fails short-circuit. The SPD status indicator (LED or window) must be visible and inspected regularly.
How to wire spd wiring diagram
- Determine SPD type required for each installation point Classify each installation point: main service entrance (Type 1), sub-distribution boards (Type 2), or point-of-use equipment (Type 3). Lightning risk assessment for the structure (IEC 62305 risk assessment) informs whether Type 1 is required at the main board or only Type 2 is sufficient.
- Verify SPD ratings against system parameters Confirm the SPD's maximum continuous operating voltage (Uc) is appropriate for the system voltage: for a 230/400 V TN-S system, a Type 2 L-N SPD requires Uc ≥ 275 V; an L-E SPD requires Uc ≥ 255 V. Verify the voltage protection level (Up) is below the equipment's impulse withstand voltage (Uw).
- Select and install upstream short-circuit protective device Install a fuse or circuit breaker ahead of the SPD, rated at or below the SPD's maximum upstream SCPD rating. A dedicated MCB or fuse for the SPD avoids interference with the main distribution board protection coordination. Use the SPD datasheet's maximum backup fuse rating as the upper limit.
- Install SPD with minimum lead lengths Connect SPD terminals to the busbar and earth terminal using the shortest possible conductors. Aim for ≤ 0.5 m total lead length (SPD-to-busbar plus SPD-to-earth). Use a V-connection layout: SPD connects from the busbar with one lead, and to the earth bar with a second lead, rather than wiring in series with the circuit conductors.
- Connect all required modes For complete transient protection, install SPD elements or connections covering L-N, L-E, and N-E modes. Many modern multi-mode SPD units provide all three in a single device. In split configurations, verify that all three mode protection paths are present at the installation point.
- Verify status indicators are visible After installation, confirm the SPD status window or LED is unobstructed and visible for routine inspection. The indicator shows green (healthy) or red/faulted (degraded or failed MOV). Document the SPD installation location, type, and installation date for maintenance records.
- Label and document Label the SPD and its upstream SCPD clearly in the distribution board. Record the SPD type, Uc, Up, and I_max/I_imp ratings in the installation's circuit schedule. Note the manufacturer's recommended inspection interval and replacement schedule.
Specifications
| SPD Type classification | Type 1 (10/350 µs, I_imp); Type 2 (8/20 µs, I_n / I_max); Type 3 (point of use, supplementary) |
|---|---|
| Maximum continuous operating voltage (Uc), L-N, 230 V TN-S system | ≥ 275 V (IEC 60364-5-53 recommendation) |
| Voltage protection level (Up) target | ≤ equipment impulse withstand voltage (Uw); ≤ 1.5 kV for standard 230/400 V equipment |
| Recommended maximum SPD lead length | ≤ 0.5 m total (SPD to busbar + SPD to earth) |
| Primary technology | Metal Oxide Varistor (MOV) for Type 2/3; Spark Gap or combined for Type 1 |
| Connection modes | L-N (differential mode), L-E and N-E (common mode) |
| Applicable product standard | IEC 61643-11 (Low-voltage surge protective devices) |
| Installation standard reference | IEC 60364-5-53 Section 534; BS 7671 Chapter 53; NEC Article 285 |
Safety warnings
- SPD installation in a distribution board involves working within a live LV switchboard. This work must be performed by a licensed electrician with the supply isolated and verified dead, or under appropriate live-working procedures where isolation is not practicable. Accidental contact with live busbars is lethal.
- A failed SPD that has not disconnected internally can present a short-circuit path to earth, causing sustained overcurrent and fire. The upstream SCPD must be correctly rated per the SPD datasheet. Never bypass, oversized, or omit the upstream SCPD.
- MOV-based SPDs are consumable protection devices that degrade with each surge event. A degraded SPD showing a fault indicator provides no effective protection. Inspect status indicators at least annually and after any known lightning event. Replace immediately if fault is indicated.
- An SPD with N-E protection (neutral-to-earth connection) must only be installed in TN-S or TT systems with a separate neutral and PE. Installing N-E SPD elements in TN-C systems (combined neutral/PE, PEN conductor) is incorrect and may cause dangerous earth currents or touch voltages.
- Follow all applicable standards: IEC 61643-11 (SPD product standard), IEC 60364-5-53 Section 534 (installation requirements), BS 7671 Chapter 53 (UK), NEC Article 285 (USA), or relevant local codes. SPD selection and installation must be coordinated with the installation's lightning protection system (IEC 62305) where applicable.
Tools needed
- Digital multimeter (voltage verification, SPD Uc measurement)
- Non-contact voltage tester (verify supply is dead before panel access)
- Insulated screwdrivers and terminal tools
- Wire stripper and cable cutter
- Torque screwdriver (terminal torque verification per SPD datasheet)
- Cable labels and marker
- Phase tester (identify L, N, PE before SPD connection)
Common mistakes
- Exceeding 0.5 m total lead length between SPD terminals and connection points: inductance in long leads defeats the SPD's clamping performance, allowing higher let-through voltage to reach equipment.
- Using an oversized upstream SCPD beyond the SPD's rated maximum: if the MOV fails short-circuit, the SPD's internal disconnector and the backup fuse must jointly clear the fault; an oversized fuse may not clear, leading to sustained arcing or fire.
- Installing a Type 3 SPD without an upstream Type 2: Type 3 devices are designed only as final-stage supplementary protection and cannot handle the surge energy that a Type 2 absorbs; without coordination, the Type 3 will be destroyed by a significant surge.
- Selecting an SPD with Uc below the system's maximum continuous voltage: in TT systems or systems with neutral-earth voltage, the SPD can overheat and fail if Uc is set too low.
- Failing to account for N-E protection in TT systems: in TT systems, surge current flows via the N-E path and can cause dangerous voltages if N-E protection is absent.
- Not labelling or recording SPD installation date: without records, there is no way to know when scheduled replacement is due or whether an SPD has been replaced after a surge event.
Troubleshooting
- SPD status indicator shows fault/red
- Cause: MOV has reached end of life through accumulated surge energy absorption, or a significant single surge event has caused internal failure Fix: Immediately replace the SPD module. Do not continue to rely on a faulted SPD for protection. Check upstream SCPD is intact. Investigate whether the surge event that caused failure also caused other equipment damage or indicates a recurring surge source that should be addressed.
- Upstream MCB or fuse trips when SPD is connected
- Cause: SPD MOV has failed short-circuit and is drawing fault current; or SPD is being connected while supply is live (surge on connection) in some cases Fix: Disconnect and isolate the supply. Test the SPD with a multimeter: a very low resistance between L and PE terminals at room temperature indicates a failed short-circuit MOV. Replace the SPD. Verify the upstream SCPD rating is correct and not overdue for replacement.
- Equipment continues to suffer damage from transients despite SPD installation
- Cause: SPD lead length too long, wrong SPD type for the application, missing protection modes, or surge source is downstream of the SPD Fix: Measure and minimise SPD lead lengths. Verify all L-N, L-E, and N-E modes are covered. Consider adding a Type 3 SPD at the affected equipment. Investigate internal surge sources (VFD switching, contactors) that may require separate mitigation.
Frequently asked questions
What is the difference between Type 1, Type 2, and Type 3 SPDs?
Type 1 handles direct lightning currents at the service entrance (10/350 µs waveshape, I_imp rated). Type 2 handles the attenuated surge at sub-distribution boards (8/20 µs, I_max rated) and is the most common residential and commercial type. Type 3 provides final protection at equipment terminals and must be used with an upstream Type 2; it cannot be used alone.
What does L-N, L-E, and N-E protection mode mean?
These describe the conduction path of the SPD during a surge. L-N (line to neutral) clamps differential-mode transients between line and neutral. L-E (line to earth) clamps common-mode transients between line and earth. N-E (neutral to earth) prevents dangerous neutral-earth voltage differentials during a surge. Full protection requires all three modes to be covered.
How close to the equipment should an SPD be installed?
The lead length between the SPD and the busbar (and between the SPD and earth) adds inductance that increases the effective let-through voltage. IEC 60364-5-53 recommends total lead length (SPD to connection point) of ≤ 0.5 m. Longer leads reduce the SPD's effectiveness. Use a V-connection (short leads to both busbar and earth) rather than a series in-line connection.
Do MOV-based SPDs need replacement after a lightning strike?
MOVs degrade with each surge event, progressively losing clamping voltage accuracy and energy absorption capacity. After a significant direct or nearby lightning event, all SPDs in the system should be inspected — check the status indicator, LED, or inspection window. An SPD showing a fault indication must be replaced, as it may no longer provide protection. Scheduled replacement at the manufacturer's recommended interval is also advisable.
What backup fuse or circuit breaker does an SPD need?
An SPD must be protected by a short-circuit protective device (SCPD) — typically a fuse or circuit breaker — upstream of it. The SCPD rating must be coordinated with the SPD's short-circuit current rating (I_SCPD) from its datasheet; using an oversized SCPD beyond this rating means the SPD's internal disconnector cannot safely interrupt a fault. The SCPD also protects the SPD wiring.