| Purpose | Transient overvoltage protection | A three-phase surge protective device diverts short-duration surge current to an appropriate protective conductor and limits the voltage applied to connected equipment. It does not regulate normal voltage or replace overcurrent protection. | Confirm that the SPD is used as part of a coordinated protection system and is not specified as a substitute for circuit breakers, fuses, grounding, or bonding. |
| System voltage | Nominal line-to-line and line-to-neutral voltage | Common three-phase systems include 120/208 V, 230/400 V, and 277/480 V. Select an SPD with a continuous operating voltage rating suitable for the actual voltage between each protected conductor and the connection point. | Check the switchboard nameplate, transformer secondary, phase arrangement, and measured system voltage before ordering the device. |
| System configuration | Grounded, ungrounded, corner-grounded, or high-resistance-grounded system | The internal connection mode must match the system, such as three-phase wye with neutral, three-phase delta, or another configuration approved by the manufacturer. A mismatch can cause premature failure or unsafe operation. | Verify the wiring diagram and terminal identification against the actual system grounding arrangement. |
| Connection mode | Number of protected conductors and neutral connection | A 3P+N device is normally used where all three phases and the neutral require protection. A three-phase, three-wire system may require a 3P configuration or a delta-rated arrangement, depending on the system design. | Confirm whether the neutral is present, solidly grounded, switched, or isolated before selecting the SPD topology. |
| SPD installation location | Service entrance, distribution board, or sensitive equipment | Use a coordinated approach: install a high-energy SPD at the service or main distribution point and add downstream protection near sensitive loads when required by the risk assessment and equipment layout. | Review whether downstream SPDs have suitable voltage ratings and coordination with upstream devices. |
| SPD type or location classification | Type 1, Type 2, or Type 3 under the applicable product standard | In common North American usage, Type 1 devices are connected on the line or load side of the service disconnect, Type 2 devices are installed on the load side of the service equipment, and Type 3 devices are installed close to utilization equipment. These classifications should not be confused with IEC test classifications. | Check the listing, installation instructions, and local electrical requirements for the intended location. |
| Maximum continuous operating voltage (Uc or MCOV) | Voltage the SPD can withstand continuously | The Uc or MCOV rating must be high enough for the normal voltage and expected temporary overvoltages of the system, while remaining appropriate for the required protection level. | Compare the device rating with the actual line-to-neutral or line-to-line voltage for every protected mode. |
| Voltage protection level (Up or VPR) | Residual voltage during a specified surge test | A lower protection level generally reduces stress on equipment, but the value must be considered together with conductor length, connection mode, system voltage, and the equipment impulse withstand rating. | Ensure the selected protection level is compatible with the equipment and the applicable standard test method. |
| Nominal discharge current (In) | Standardized surge-current capability | In is used to compare the device's tested discharge-current capability under the relevant product standard. Higher values may be appropriate in locations with greater exposure, but In alone does not determine total protection performance. | Record the rated In and confirm that the test basis matches the project specification. |
| Maximum discharge current (Imax) | Maximum single surge-current rating used in product testing | Imax indicates the maximum test surge the device can withstand under the specified test conditions. It should be considered with In, Up or VPR, short-circuit rating, and the expected exposure at the installation point. | Do not use Imax as a direct estimate of the current available during a real lightning event. |
| Short-circuit current rating | Available fault current at the installation point | The SPD short-circuit current rating must be equal to or greater than the available fault current where it is installed, subject to the applicable standard and overcurrent protective-device requirements. | Obtain the calculated available fault current from the electrical design or field assessment. |
| Backup overcurrent protection | Fuse or circuit breaker protecting the SPD branch circuit | Follow the SPD instructions for the maximum permitted fuse or breaker size, conductor ampacity, and disconnecting means. Some devices permit connection to an existing upstream protective device; others require dedicated protection. | Verify conductor sizing, overcurrent-device compatibility, interrupting rating, and accessibility of the disconnect. |
| Lead length | Length and routing of phase, neutral, and grounding conductors | Keep all connecting conductors as short and straight as practicable. Avoid sharp bends and unnecessary loops. In many designs, the total connecting path is targeted at approximately 0.5 m or less when permitted by the installation instructions. | Inspect routing after installation and measure the practical conductor path rather than only the visible cabinet distance. |
| Grounding and bonding | Low-impedance path for surge current | Connect the SPD to the correct grounding or bonding point using the conductor size and termination method specified for the device. A high-impedance or discontinuous path can significantly reduce performance. | Check torque, corrosion, continuity, bonding jumpers, and compliance with the local electrical code. |
| Environmental conditions | Indoor, outdoor, humidity, temperature, dust, and corrosive atmosphere | Select an enclosure and device with an environmental rating suitable for the location. Outdoor or wet locations may require a weather-resistant enclosure and properly sealed cable entries. | Inspect the enclosure, seals, cable glands, condensation, contamination, and signs of overheating or corrosion. |
| Status indication | Visual or remote indication of module condition | Many SPDs provide a visual status window, and some provide a dry contact or electronic signal for remote monitoring. A status indicator usually shows the condition of the protective module, not the quality of the building grounding system. | Inspect indicators during routine electrical maintenance and test remote contacts according to the maintenance procedure. |
| Installation sequence | Safe isolation and correct termination | De-energize and lock out the circuit, verify absence of voltage, install the specified protective device and conductors, torque terminals to the required value, and restore power only after inspection. | Installation should be performed and inspected by qualified electrical personnel in accordance with applicable codes and instructions. |
| Coordination with other protection | Power, data, control, and communications circuits | Surges can enter through power, communication, control, and grounding paths. Coordinate protection across interconnected systems to reduce differential voltage between equipment ports. | Review all metallic and wired connections serving sensitive equipment, including network, instrumentation, and control cables. |
| Inspection interval | Routine and event-based inspection | Inspect at intervals established by the facility risk assessment, electrical maintenance program, and device instructions. High-risk sites may require more frequent checks. | Inspect after a nearby or direct lightning event, major switching event, severe overvoltage, or a known equipment fault. |
| Replacement indicators | End-of-life indication or physical damage | Replace the SPD or affected module if the status indicator shows failure, the backup protective device operates, the enclosure is damaged, or there are signs of thermal stress, burning, cracking, or water ingress. | Do not reset or reuse a failed module unless the product instructions specifically permit it and the cause has been investigated. |
| Documentation | Installation records and maintenance history | Record the system voltage and configuration, SPD type, ratings, location, circuit identification, installation date, protective-device details, inspection results, and replacement history. | Keep updated drawings, photographs, test records, torque records, and manufacturer instructions available for future maintenance. |
