The short answer
Surge Protection for Signal Circuits
Signal surge protection places a clamping device on each conductor of a signal circuit where the circuit enters a panel, so that a surge from lightning or switching is diverted to ground before it reaches the transmitter, the input card, or the radio. Gas discharge tubes handle the energy, transient voltage suppressor diodes clamp fast and low, and hybrid devices combine them in stages. A protector works only through its ground connection, which must be short and bonded to the same ground as the equipment it protects, and it adds series resistance and capacitance the loop design must allow for.
Key points
- Surges enter on every conductor that leaves the building: power, signal, network, antenna. Protect all of them or none of them is protected.
- A protector is a path to ground. A long ground lead makes it useless.
- Two-stage hybrid devices for signal loops: a gas tube for energy, a diode for speed.
- Every protector adds resistance and capacitance. Check the loop and the network for them.
- Protectors fail. Test or replace on a schedule, and use ones that indicate.
Where surges come from
A lightning strike does not have to hit the site. A strike a mile away raises the ground potential unevenly across the area and induces voltage in every long conductor, and a lift station with a transmitter in the wet well, a level cable in a conduit, a power service, a radio antenna on a mast, and a telephone line is a collection of long conductors tied to different points in the ground. The difference in potential between those points appears across whatever connects them: the input card, the radio, the transmitter. Switching surges from utility operations, motor starts, and capacitor banks are smaller and far more frequent, and they wear insulation and electronics down the same way.
The rule that follows is that a surge enters on any conductor that leaves the building or the panel, and that protecting the power service while leaving the signal cables and the antenna unprotected leaves the surge a path through the electronics from the unprotected side to the protected one.
Device types
| Device | How it works | Strength | Limit |
|---|---|---|---|
| Gas discharge tube (GDT) | A sealed gap that arcs over at a few hundred volts and then conducts heavily | Handles very high energy; very low capacitance; long life | Slow to fire, so the voltage reaches hundreds of volts before it clamps; needs a second stage for electronics |
| Metal oxide varistor (MOV) | A resistor whose resistance collapses above a threshold | High energy; inexpensive | Degrades with each surge; high capacitance; used on power circuits more than signal |
| Transient voltage suppressor diode (TVS) | A silicon diode that clamps within nanoseconds at a precise voltage | Fast and precise; clamps low enough to protect a 24 V input | Limited energy; fails short when overwhelmed, which at least takes the circuit down safely |
| Hybrid, multi-stage | A GDT on the line side, a series resistor or inductor, and a TVS on the equipment side | The GDT takes the energy while the resistor lets the TVS clamp first; the standard for signal loops | Adds series resistance, typically a few ohms to tens of ohms per conductor, and some capacitance |
| Isolating transformer or optical isolator | Breaks the galvanic path entirely | Immune to ground potential difference | Not a surge protector as such; used with one, especially on network and serial links |
Where to put them
- At the panel entry
- On each conductor of each signal cable where it enters the panel, on a DIN rail or a terminal block designed for it, before the cable reaches the controller. The protector ground goes to the panel ground bar by the shortest possible lead.
- At the field device
- Transmitters in exposed locations, on tanks, on masts, or in wet wells, get a protector at the device as well, so that a surge induced on the cable is clamped at both ends. Many transmitters offer an integral surge option.
- On the antenna feed
- A coaxial surge protector at the point the feed enters the building, bonded to the site ground with a short strap. The mast is bonded to the same ground. Radios are the most frequent lightning casualty at remote sites and this is the reason.
- On network cables between buildings
- Copper Ethernet between buildings is protected at both ends, or, better, replaced with fiber, which does not conduct a surge at all.
- On serial lines
- RS-485 and RS-232 links that leave a panel, at both ends, with devices rated for the signal voltage and the data rate.
- On power
- A service entrance protector and a panel protector, because signal protection is defeated if the power side floats up.
Grounding that makes it work
Surge protection and grounding are one subject. Every protector, the panel ground bar, the equipment ground, the antenna mast, the service ground, and the transmitter case need to rise together when the ground rises, which means a single low-impedance ground system for the site: a ground ring or grid with rods, bonded to the service ground, to the mast, and to the panel, with the protectors landed on the bar that is bonded to it. Separate isolated grounds for instrumentation, a practice that is sometimes specified for noise reasons, are the opposite of what surge protection needs, and where both matter, the instrument ground bar is bonded to the site ground at one point and the shields are handled as the ground loop guidance describes.
What protectors do to the loop
- Series resistance. A hybrid protector at each end of a 4-20 mA loop adds tens of ohms to the loop. Add it to the loop resistance calculation for the transmitter compliance voltage.
- Capacitance. Protectors on a HART loop or a serial line add capacitance that can attenuate the signal; use devices rated for the protocol.
- Leakage. A protector near its clamping voltage leaks current. On a 24 V loop, choose a clamp voltage with margin above the working voltage, commonly 30 to 36 V, so the protector does not conduct in normal operation.
- Failure modes. A TVS fails short and takes the loop to zero, which is detected. A GDT can fail open and stop protecting silently. Devices with a status indicator or a remote contact are worth the cost on critical loops.
- Bandwidth. Network protectors are rated for a data rate; a device meant for 10 Mb Ethernet degrades a gigabit link.
Maintenance
Protectors are consumed by the surges they absorb. Plug-in devices with a status flag are inspected on the site visit schedule and after any lightning event, and replaced when flagged. Devices without indication are replaced on a schedule, every few years in a lightning-prone area, or tested with a protector tester where the utility has one. A site that has had its radio or its level transmitter replaced twice for lightning damage has a grounding or protection gap, not bad luck, and the investigation starts with the ground lead lengths and the bonding.
Frequently asked questions
- Do I need surge protection on a 4-20 mA loop that stays inside the building?
- Usually not, if the loop never leaves the structure and the building has a proper ground. A loop that goes to a transmitter on a tank outside, down a wet well, or through a buried conduit to another structure does, at both ends.
- Is fiber the answer to network surge problems?
- Between buildings, yes. Fiber conducts no current, so a ground potential difference between two structures does not appear across the switches. Copper between buildings is protected at both ends and still fails more often; fiber is usually the better investment at any site that has lost a switch to lightning.
- Where does the protector for the wet well level transmitter go?
- At the panel entry for the level cable, and at the transmitter where the transmitter offers it or where a junction box at the well top allows a protector. The wet well transmitter is at the bottom of a hole in the ground with a cable running to the panel, which is the classic lightning path.
- Can surge protectors cause a ground loop?
- A protector connects a conductor to ground only when clamping, so in normal operation it does not. A shield grounded at both ends through a protector ground, however, is still a shield grounded at both ends; keep the shield termination rules and use protectors that pass the shield through or ground it only at the panel end.
Related topics
- 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.
- 4-20 mA Signals from the Instrument SideHow a 4-20 mA current loop is powered and wired at the transmitter, why the loop resistance budget matters, and how to check a loop with a meter.
- HARTDigital data riding on the 4-20 mA loop: how HART signaling works, what a handheld or a HART-enabled input can read, multidrop and burst modes, and the wiring conditions that make HART fail while the current loop keeps working.
- How to Diagnose a Ground LoopProve that a noisy or offset analog signal is a ground loop, find the second ground, and fix it the right way: remove the extra ground, isolate the signal, or correct the shield termination. With the measurements that make the diagnosis certain.
- Ethernet Device Drops OfflineAn industrial Ethernet device that disappears and comes back. Duplex mismatch, cabling, spanning tree, and the switch counters that identify it in minutes.
- Hydrostatic Level MeasurementMeasuring level from the pressure of the liquid above a sensor: submersible transducers, bubblers, and base-mounted transmitters, the density assumption behind all of them, venting, installation, and what makes them drift.
Direct contact
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