The short answer
Surge Devices
A surge protective device diverts a transient, from lightning or from switching, to ground before it reaches the electronics, and it only works when it is close to what it protects, bonded to ground by a short straight conductor, and rated for the exposure. A control panel needs protection at the supply, where a Type 2 device at the panel entrance clamps what comes in on the power conductors; on every signal, serial, and network conductor that leaves the building or runs to a mast, where a two-stage signal protector clamps and then limits; and on the antenna feed of a radio, where a coaxial arrestor and a grounded mast carry the strike away. The devices are coordinated so that the one nearest the exposure takes most of the energy, and each has an indicator or a monitoring contact, because a surge device that has sacrificed itself protects nothing.
Key points
- Protect three things: the power supply to the panel, every signal or network conductor that leaves the building, and the radio antenna feed.
- A surge device is only as good as its ground lead: short, straight, and bonded to the same ground as the equipment.
- Match the type and rating to the location: Type 1 or 2 at the entrance, signal-rated devices on loops, coaxial arrestors on antennas.
- Coordinate stages so that the upstream device takes the energy and the downstream device clamps the remainder.
- Monitor and replace: every device has an indicator or a contact, and a surge device that has failed is the site with no protection.
Types and locations
| Where | Device | Notes |
|---|---|---|
| Service entrance | Type 1 surge protective device | Installed by the electrical contractor at the service; takes the direct exposure |
| Panel supply | Type 2 surge protective device at the panel entrance | After the disconnect, ahead of the control transformer and power supplies; sized for the voltage and the exposure |
| Control transformer secondary and 24 volt supply | Type 3 or a low-voltage device | Optional third stage close to sensitive loads |
| 4 to 20 milliamp loops and discrete field circuits leaving the building | Two-stage signal protector, DIN rail or terminal type | Rated for the loop voltage; one per pair; located where the cable enters the panel |
| Serial and fieldbus | Data line protector for the bus type | Rated for the signal level and the data rate; both ends of a long bus |
| Ethernet leaving the building | Ethernet surge protector, or fiber instead | Fiber removes the problem; copper needs a protector rated for the data rate and power over Ethernet if used |
| Radio antenna | Coaxial lightning arrestor at the entry | Bonded to the entrance ground; the mast grounded separately |
Ratings
- Voltage rating
- The nominal system voltage and the maximum continuous operating voltage; a device rated too low clamps on normal voltage and fails.
- Voltage protection rating
- The let-through voltage under the test surge; lower is better, and it should be below what the protected equipment survives.
- Surge current rating
- The energy the device can divert; higher for the entrance device, lower for downstream stages.
- Short-circuit current rating
- The device must be rated for the available fault current where it is installed, or protected by a fuse or breaker that is.
- Signal device parameters
- Working voltage, clamping voltage, series resistance, bandwidth, and whether it passes loop power; a signal protector with the wrong working voltage clamps the signal itself.
Grounding is the device
A transient of a few thousand amperes rising in a microsecond develops hundreds of volts across every foot of ground lead, so a surge device with a three foot lead lets most of the transient through. The lead is as short as the layout allows, straight without loops, in a conductor size the manufacturer specifies, and bonded to the same ground reference as the equipment being protected. The ground bar the device lands on is bonded to the enclosure and to the building grounding electrode system by a short path. Signal protectors mount on a grounded rail where the cable enters, with the field cable shield bonded at the same point, so that the surge on the shield and the surge on the conductors go to the same place.
Coordination
Protection is staged: the entrance device takes the bulk of the energy and lets through a clamped remainder; the panel device clamps that remainder lower; a device at the load, if present, handles what is left. The stages need distance between them, which the conductor length provides, so that the upstream device conducts first. Two devices of the same rating mounted side by side do not share; one takes it all. The manufacturer data states the coordination distance, and the panel design uses it.
Indication and replacement
- Every surge device has a visual indicator, and the ones that protect critical equipment have a monitoring contact wired to an alarm.
- Signal protectors on loops are checked during loop checks, since a failed one either opens the loop or leaves it unprotected.
- A site that took a lightning hit gets every surge device inspected, and the ones that clamped hard replaced.
- Spares for the common types on the shelf; the device that fails is the one on the path the next storm uses.
Mistakes that leave a site unprotected
- A power surge device with no signal protection: the strike comes in on the level transmitter cable.
- Signal protectors on the loop but not on the shield or the field ground: the surge arrives on the shield.
- A radio arrestor bonded to a ground rod that is not bonded to the panel ground: the potential difference destroys the radio.
- A protector whose working voltage is below the loop supply, so it clamps the loop itself.
- A device that failed two storms ago and nobody looked at the indicator.
Frequently asked questions
- Do I need surge protection inside the plant on loops that never leave the building?
- Rarely for lightning, since the building bonding limits the exposure; sometimes for switching transients near large drives. The loops that leave the building, cross between buildings, or run to a mast are the ones that need it.
- Fiber or a surge protector on the Ethernet link between buildings?
- Fiber. It removes the conductor entirely and with it the surge, the ground loop, and the noise. A copper link between buildings with protectors at each end is a compromise for short runs where fiber is impractical.
- Will a surge device protect against a direct strike on the antenna?
- A coaxial arrestor with a properly grounded mast survives most strikes and protects the radio from most of them; a direct strike on a poorly grounded mast destroys the radio and often the panel. The mast ground, the arrestor, and the panel ground bonded together are the protection; the arrestor alone is not.
- How do I know a surge device has failed?
- The indicator window or light on the device, the monitoring contact if wired, or a loop that reads open. Devices that fail short trip their fuse; devices that fail open give no indication except an inspection, which is why the schedule includes one.
Related topics
- 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 Select Surge ProtectionSelect the surge protective devices for a control panel and its field circuits: list every conductor that enters, classify its exposure, choose the device type and ratings for the power, the loops, and write it into the drawings and the I/O list.
- 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.
- 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.
- Ground Loops in InstrumentationTwo grounds at different potentials, one signal circuit between them, and a reading that will not sit still. How to find it and how to prevent it.
- Missing Equipment GroundAn enclosure, motor, or device with no effective equipment ground: how it shows up as a tingle, a fault that does not trip, surge protection that does nothing, or instrument noise; how to test the path safely; and why a ground rod is no substitute.
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.