RS485 and 24 V PLC I/O are the two interfaces that fail most often in industrial installations, and they fail for a reason that has nothing to do with the data rate. Both run on long cables between cabinets that sit on different earth references. When lightning or a switching event raises the potential of one cabinet relative to the other, the difference appears across the signal pair, and the transceiver at one end sees a common-mode voltage far outside its design range.
Protection for these interfaces follows the same principle as analogue loops — clamp below the device’s withstand, keep the series impedance low enough not to disturb the signal — but the details differ, because RS485 is a terminated bus and a PLC input is a single-ended 24 V circuit.
The transceiver is specified for a common-mode range of about −7 V to +12 V relative to its own ground. Anything beyond that, even briefly, forward-biases internal structures that were never meant to carry current. Three mechanisms get you there:
All three are common-mode events. That is why the earth connection of the SPD is not a detail: the device can only clamp what it can reference.
Uc. Set above the transceiver’s common-mode range with margin. The standard range is −7 V to +12 V, so a device with a Uc around 12 V to 15 V is typical for RS485. A device with a much higher Uc leaves the port exposed; one with a lower Uc conducts during normal operation and loads the bus.
In. 5 kA with an 8/20 µs waveform is the common rating for a bus entry device. Higher ratings are available for exposed runs.
Bandwidth and capacitance. The protection element sits across the pair and therefore adds capacitance. At 9,600 baud this is irrelevant; at 12 Mbit/s it is not. Check the device’s stated data rate rather than assuming any signal SPD will do.
Series resistance. Three-stage devices include series resistors between the discharge stage and the clamping stage. That resistance is what makes the fine clamping possible, and it also adds to the bus load. On a bus with 32 nodes and long cable, the accumulated effect is small but should be checked on marginal installations.
| Interface | Typical Uc | Typical In (8/20) | Notas |
|---|---|---|---|
| RS485 / RS422 | 12–15 V | 5 kA | Must pass the data rate without distortion; 3-stage preferred |
| 24 V DC digital input | 30–60 V | 5 kA | Single-ended; the 0 V reference is part of the circuit |
| 24 V DC transistor output | 30–60 V | 5 kA | Check the continuous current rating, not just the surge rating |
| Relay output | Depends on the switched circuit | 5 kA | Contact protection is a separate RC or diode network |
| 0-10 V analogue | 15–30 V | 5 kA | Watch leakage rather than series resistance |
RS485 is designed as a daisy chain: one pair running from node to node, with a 120 Ω termination resistor at each physical end. Two habits break it.
Star wiring. Running a separate pair from a central point to each device produces stubs that reflect the signal and create standing waves. It also means a surge entering on one branch has several parallel paths into the same transceiver. If the installation is already star-wired, an RS485 repeater or a hub is a better fix than more protection.
Termination in the wrong place. The 120 Ω resistors belong at the two physical ends of the bus, not at the master and not at every node. If an SPD is inserted between the terminator and the cable, the termination no longer terminates the line.
Protection goes where the cable enters the enclosure, not at the transceiver pin. A device fitted inside the cabinet on a short tail protects the transceiver from what gets past the entry point, which is not the same thing.
The shield is there to intercept capacitive coupling before it reaches the pair. It only does that if it is bonded to earth, and it only avoids becoming a current path if it is bonded at one point.
A digital input is single-ended: the PLC reads the voltage between the input terminal and its 0 V rail. A surge that lifts the 0 V rail relative to the cabinet earth appears as a signal at every input at once, which is why a single event can take out several channels.
Two details matter for selection. First, the SPD must protect the 0 V conductor as well as the signal conductor — a device that only clamps the signal line has no return path for the common-mode current. Second, the device’s continuous current rating has to cover the load: a 24 V output driving a 0.5 A solenoid needs a device rated for that current, not a 100 mA signal device.
Inductive loads are a separate problem. A solenoid or contactor coil generates a inductive kick when it opens, and no surge SPD will absorb that repeatedly. Fit a freewheeling diode across a DC coil, or an RC network across an AC coil, close to the load.
At every point where the cable enters an enclosure, yes. Nodes inside the same cabinet share one entry point and normally one device. Nodes in separate buildings or separate cabinets each need their own.
It adds a small series resistance and some capacitance, so in principle yes. In practice the effect is negligible at ordinary baud rates; it becomes relevant at high data rates or when the bus is already close to its limits.
Only if the device is specified for both and the Uc suits both circuits. The clamping voltage and the series impedance requirements are different, and a device optimised for a 24 V analogue loop may not pass a high-baud-rate RS485 signal cleanly.
Above the highest normal voltage on the circuit, which for a 24 V system with tolerance is typically 30 V or more. A device with a Uc of 30–60 V is the usual range for 24 V DC industrial circuits.
No. A bus with correct topology, one-point shield bonding and proper termination will survive events that destroy a poorly wired one with the same protection fitted.
O nosso signal surge protector range covers RS485, 24 V DC and analogue variants with their Uc and data-rate ratings. For the analogue side of the same panel, see surge protection for 4-20 mA and 0-10 V loops; for Ethernet and PoE devices, see PoE surge protection.
