The short answer
How to Select Surge Protection
To select surge protection, list every conductor that enters the panel, power, loops, discrete field circuits, serial and Ethernet lines, and antenna coax, and mark which ones leave the building, run to a mast, or come from another ground, because those are the exposed ones; for the power, choose a Type 2 device at the panel entrance rated for the system voltage with a surge current rating suited to the exposure and a short-circuit current rating for the location, with a Type 1 at the service if none exists; for each exposed loop or discrete circuit, a two-stage signal protector with a working voltage above the circuit voltage and a series resistance the loop can tolerate; for data lines, a protector rated for the signal type and data rate, or fiber instead; for the antenna, a coaxial arrestor for the frequency and power; then plan one entrance with a ground bar bonded by a short conductor, check the coordination distances between stages, specify indicators or contacts for monitoring, and put every device on the drawings and the I/O list beside the point it protects.
Key points
- List every entering conductor and classify its exposure; leaving the building, a mast, or another ground means protect it.
- Power: Type 2 at the panel entrance with the right voltage, surge current, and short-circuit current rating; Type 1 at the service.
- Signals: a two-stage protector per exposed circuit with a working voltage above the circuit voltage; fiber instead of copper for data between buildings.
- One entrance, one ground bar, a short bond; coordination distance between stages.
- Monitoring contacts or indicators, and the devices on the drawings and the I/O list.
What you need
- The panel schematic and the I/O list
- The site plan showing what leaves the building and where the mast and other buildings are
- The available fault current at the panel and the system voltage
- Manufacturer data for the candidate devices: ratings, let-through, series resistance, dimensions
- The grounding design or the electrode test results for the site
Procedure
- 1
List the conductors
Every wire and cable that enters the enclosure, from the schematic and the I/O list: supply, each loop, each discrete field circuit, each serial and Ethernet cable, each coax.
- 2
Classify exposure
High: leaves the building, runs to a mast or a pole, or runs underground between structures. Medium: within the building but on another ground or near large switching loads. Low: within the panel room. High and medium get protection.
- 3
Select the power device
Type 2 at the panel entrance: nominal voltage and configuration matching the supply, a maximum continuous operating voltage above the highest expected supply voltage, a surge current rating for the exposure, a voltage protection rating low enough for the electronics, and a short-circuit current rating at or above the available fault current or a fuse that provides it. A Type 1 at the service entrance if the building lacks one. A third stage at the 24 volt supplies for severe exposure.
- 4
Select the signal devices
For each exposed loop: a two-stage protector with a working voltage above the loop supply, usually 30 volts for a 24 volt loop, a series resistance the loop budget tolerates, and a rated surge current for the exposure; DIN rail or terminal type. For discrete circuits: a protector for the circuit voltage per pair or group. For serial: a protector for the bus type. For Ethernet: a protector rated for the data rate and power over Ethernet if used, or, better, fiber.
- 5
Select the coaxial arrestor
For the antenna: rated for the frequency band, the transmitter power, the connector type, and the coax, with a gas discharge or a quarter-wave design as the radio vendor recommends.
- 6
Plan the entrance
One location where every conductor enters, a protector rail with a ground bar behind it, and a bonding conductor to the panel ground and the site electrode system as short and straight as the layout allows, sized per the device manufacturer.
- 7
Check coordination
The conductor length between the service device and the panel device, and between the panel device and any third stage, against the manufacturer coordination distance, so the upstream stage conducts first.
- 8
Specify monitoring
Indicators on every device and monitoring contacts wired to the controller for the power devices and the critical loops; the alarm on the list.
- 9
Document
Each device on the schematic, on the panel layout at the entrance, and on the I/O list beside the point; the bonding conductor on the grounding drawing; the spares on the parts list.
| Circuit | Device | Key ratings |
|---|---|---|
| 480 or 240 volt supply | Type 2 at the panel; Type 1 at the service | Voltage and configuration, surge current, voltage protection rating, short-circuit current rating |
| 120 volt control | Type 2 or 3 on the control transformer secondary | Voltage, voltage protection rating |
| 24 volt supply output | Low-voltage device at the supply | Working voltage above 28 volts |
| 4 to 20 mA loop leaving the building | Two-stage loop protector | Working voltage 30 volts, series resistance, surge current |
| 24 volt discrete field circuit | Discrete circuit protector | Working voltage, surge current |
| RS-485 leaving the building | Data line protector | Working voltage, bandwidth, both ends |
| Ethernet between buildings | Fiber; else Ethernet protector | Data rate, power over Ethernet |
| Antenna coax | Coaxial arrestor | Frequency, power, connectors |
Frequently asked questions
- Do I need protection on loops that stay inside the plant?
- Usually not for lightning if the building is bonded; consider it for loops near large drives or between buildings on separate grounds. Prioritize what leaves the building.
- How do I pick the surge current rating?
- From the exposure: higher for the service entrance and for remote sites on hills or with masts, lower for downstream stages. The manufacturer application guides give ranges by location; when in doubt, choose the higher rating at the entrance.
- What series resistance can a loop tolerate?
- The loop voltage budget: supply voltage minus the transmitter minimum minus the receiver drop minus the cable drop, divided by 20 milliamps, gives the ohms available; the protector resistance must fit within it with margin.
- Can one device protect several loops?
- Multi-channel protectors exist for groups of circuits at the same voltage, and they save space; each channel is still a device with its own ratings, and a failure in one channel is found only by testing that channel.
Related topics
- Surge DevicesSurge protective devices in a control panel: the types and where each belongs, the ratings that matter, supply protection, signal and network protection on conductors that leave the building, stage coordination, grounding, indication, and replacement.
- Panel Surge ProtectionSurge protection of a controller panel as a system: a single entrance and the ground bar it lands on, staged protection on the power, protectors on every loop and data line that leaves the building, the radio feed, layout, and the checklist after a strike.
- Surge Protection for Signal CircuitsProtecting 4-20 mA loops, discrete inputs, network cables, and radio feeds from lightning and switching surges: where surges enter, the device types and how they clamp, the grounding that makes them work, the resistance and capacitance they add, and where to put them at a remote site.
- How to Design Panel GroundingDesign the grounding and bonding of a control panel and its site so that faults clear, signals are clean, and surges have a path: bond the enclosure and everything in it, and verify with measurements before the panel is energized.
- Short-Circuit Current Rating (SCCR)What a control panel’s short-circuit current rating means, how it is determined under UL 508A, why the NEC requires it on the label, and why the number is usually smaller than people expect.
- Remote Site Stops CommunicatingOne radio site silent while its neighbors answer: site power, the radio, antenna and coax after a storm, a reset configuration, the controller behind it, or a changed path. What the master diagnostics say before anyone drives, and the order of checks on site.
Direct contact
Have a controls question?
Reach Eric Sullivan directly about anything on this site, a controls or automation topic, or one of his personal projects.