A surge is a very short spike of voltage, lasting millionths to thousandths of a second, that can damage electronics or slowly wear them out. Surge protective devices (SPDs) limit those spikes by diverting the surge current to ground. They work best as a coordinated system: a device at the service entrance to handle large surges from outside, devices at distribution panels to catch what gets through and what starts inside, and point-of-use protection at sensitive equipment.
Buildings now hold far more electronics than they did a generation ago: LED drivers, building automation controllers, drives on every fan and pump, access control, cameras, network gear and EV chargers. All of them are more sensitive to surges than the motors and incandescent lamps they replaced. Replacing failed control boards after every thunderstorm, or wondering why drives fail early, is often a sign that surge protection is missing or not working.
Where surges come from
From outside. Lightning striking power lines or the ground nearby, and utility switching operations such as capacitor banks switching or faults being cleared, send surges into the building through the service. These are the largest surges, but they are relatively infrequent.
From inside. Most surges start inside the building. Every time a motor, compressor, elevator, transformer or other inductive load switches off, it releases stored energy as a short voltage spike. These internal surges are smaller but happen many times a day, and over years they can degrade electronic components until they fail.
Surges also enter on telephone, cable, data and antenna lines, and through the grounding system when lightning strikes nearby. A plan that protects only the power wiring leaves those paths open.
Protection in layers
No single device handles every surge. The usual approach places devices at several points, each one rated for the surges it is likely to see. The service entrance device takes the brunt of external surges. The panel devices reduce what passes through and catch internal surges generated nearby. Point-of-use devices give sensitive equipment a final, low clamping level.
Types of SPD
| Type | Where it goes | What it handles |
|---|---|---|
| Type 1 | At the service entrance, and can be installed on the supply side of the main disconnect | Large external surges, including from nearby lightning |
| Type 2 | On the load side of the service disconnect, at switchboards and distribution panels | Residual external surges and internal surges |
| Type 3 | At the point of use, some distance downstream of the panel | Final protection for sensitive equipment; includes quality plug-in units |
Choosing and installing SPDs
Ratings that matter
- System voltage and configuration. The device must match the system, such as 120/208 V or 347/600 V wye. A device for the wrong voltage can fail or provide little protection. This is a common error with equipment designed for United States voltages.
- Voltage protection rating. The level to which the device limits a standard test surge. Lower is better, within reason.
- Surge current capacity. How much surge current the device can divert. Service entrance devices need more than point-of-use ones.
- Short-circuit current rating. Like any equipment, the SPD must be rated for the fault current available where it is installed.
- Certification. Use devices certified for use in Canada.
Installation details that make or break performance
An SPD is only as good as its connection. Every bit of lead length between the device and the bus adds voltage during a surge, so leads are kept as short and straight as possible, without loops or sharp bends. Devices are mounted right beside or inside the panel, on the breaker or disconnect the manufacturer specifies. Good grounding and bonding are essential, because the surge current has to go somewhere. Installation is done by a licensed electrical contractor under permit.
Don't forget communication lines
Telephone, data, cable TV, antenna and building automation lines that enter from outside or run between buildings need their own protectors, bonded to the same grounding system as the power SPDs. Otherwise a surge can arrive on a data line and pass through equipment to reach ground.
Maintenance
SPDs wear out. Each surge they absorb uses up a little of their capacity, and a large surge can end their life in one event. Most devices have status lights or indicators.
- Check SPD status indicators during routine electrical room rounds, and after major storms.
- Replace devices that show failure or end of life; a failed SPD gives no protection.
- Record SPD locations and models in your maintenance records and on the single-line diagram.
- Replace worn plug-in surge strips; many have no indicator and give no warning when they stop protecting.
- Include SPDs in the scope when panels are replaced or services upgraded.
A typical case
A low-rise office building replaces control boards on its rooftop units several times a year, usually after summer storms, and the building automation controllers reset at random. There is no surge protection on the service, and the rooftop units are fed from a mechanical panel with long feeders across the roof. A review in a case like this often recommends a Type 1 device at the service, Type 2 devices on the mechanical panel and the panel feeding the controls, protection on the automation network where it leaves the building, and a check of bonding at the rooftop equipment. Failures are then tracked over the following seasons to confirm the improvement.
Surge protection is not lightning protection. SPDs protect electrical systems from surges carried on wiring. They do not protect a building from a direct lightning strike. Lightning protection systems of air terminals, conductors and grounding are a separate design, covered in Canada by CAN/CSA-B72.
How ehZee can help
We design coordinated surge protection for new and existing buildings and include it in service and distribution upgrades. See our electrical design service, and read grounding and bonding explained and power quality and harmonics.
Common questions
Are plug-in surge strips enough?
They help as a last layer for individual equipment, but they cannot handle large surges arriving at the service and they do nothing for hard-wired equipment like rooftop units, drives and lighting. Use them together with panel-level protection.
Which buildings benefit most?
Buildings in areas with frequent thunderstorms, those at the end of long overhead utility lines, sites with rooftop equipment and long outdoor runs, and any building whose operation depends on electronics such as controls, data rooms, medical equipment or security systems. Industrial sites with many motors also benefit, because so many surges start inside.
Where should we start in an existing building?
At the service entrance and at the panels feeding the most critical or most frequently damaged equipment. Records of past failures, especially after storms, are a good guide to where protection will pay off first.
Will a UPS protect against surges?
Many UPS units include surge protection for the equipment plugged into them. They do not protect the rest of the building, and the UPS itself benefits from upstream protection.
Is surge protection required by code?
Requirements vary by edition, occupancy and equipment, and are expanding in some codes. Even where it is not required, it is often worthwhile for buildings with sensitive or expensive electronics. Check the current requirements with your designer.