A 4-20 mA loop or a 0-10 V signal line carries almost no energy, which is exactly why a surge matters there. The transmitter, the PLC analogue card or the drive input is built around a few milliamps and a handful of volts, and a surge that would be a minor event on a power circuit is enough to destroy the input stage. Standard power SPDs are the wrong tool for the job: they clamp at voltages far above what a signal input can survive, and their connection method does not suit a two-wire loop.
Signal surge protection for analogue loops works on a different set of rules. The device has to clamp well below the input’s withstand voltage, pass the signal without measurable distortion, and add so little series resistance that the loop still has enough compliance to drive its load.
Three differences decide it:
| Property | Power SPD | Signal SPD for 4-20 mA / 0-10 V |
|---|---|---|
| Maximum continuous voltage (Uc) | 275 V AC, 1000 V DC and above | Typically 30 V DC to 60 V DC for a 24 V loop |
| Clamping voltage (Up) | Kilovolts | A few tens of volts, below the I/O card withstand |
| Ligação | Parallel across a power circuit | In series with the loop, through a protected feed-through |
A power SPD installed on a 24 V analogue loop will simply never conduct at a level the loop cares about, and its clamping voltage is an order of magnitude above the destruction threshold of the input. The signal SPD sits in series with the pair, passes the loop current through, and diverts the surge to earth through its clamping element.
Uc, maximum continuous operating voltage. Must be above the highest voltage the loop can see in normal operation, with margin. A 24 V DC loop with a 10 percent tolerance needs at least 30 V; a 0-10 V signal inside a drive cabinet may see higher voltages during a fault. Uc below the normal loop voltage means the device conducts continuously and fails.
In, nominal discharge current. Typically 5 kA or 10 kA with an 8/20 µs waveform for instrument loops. Higher values exist but are rarely needed inside a panel; at a field device on a long outdoor run, the higher figure is easier to justify.
Up, voltage protection level. The only number that matters to the electronics, and it has to be compared with the input card’s withstand specification, not with the loop’s normal voltage. If the card datasheet does not state a surge withstand, the safe assumption is that it is low.
Series resistance. Every series-connected SPD adds resistance to the loop. A 4-20 mA transmitter has a maximum loop resistance it can drive — usually stated as a supply voltage versus load resistance graph. The SPD’s resistance, the cable resistance and the receiver’s input resistance all add up, and if the total exceeds the transmitter’s capability the loop reads low or drops out at the top of its range.
A two-wire transmitter regulates its current to represent the measurement, and it can only do that while the total loop resistance keeps the supply voltage within its operating window. Adding a series device consumes part of that budget.
Worked example, with illustrative values: a 24 V supply, a transmitter that needs at least 12 V across itself, and a 250 Ω receiving resistor. The voltage available for the rest of the loop is 24 − 12 = 12 V, and at 20 mA the 250 Ω resistor drops 5 V. That leaves 7 V for cable and any series devices, which corresponds to 350 Ω at 20 mA. An SPD contributing a few ohms is negligible; one contributing tens of ohms per wire is not, especially on a long run.
The point is not the specific numbers, which vary by transmitter, but the method: add the SPD’s resistance to the loop calculation before ordering, and check the transmitter’s load graph rather than assuming headroom.
Protection has to be at the boundary where the surge enters, and there can be two such boundaries.
The two devices must be coordinated. A signal SPD at each end with no consideration of the cable length between them can leave the downstream device doing nothing, because the upstream one has already clamped the surge to a level the downstream one never sees. Where the run is long this is rarely an issue; where the two devices are close together, follow the manufacturer’s coordination guidance.
Most damage to analogue inputs is common-mode: the surge raises the potential of the whole cable relative to the panel earth, and the input sees the difference. That means the SPD’s earth connection is part of the signal path, not an afterthought.
A voltage signal is read by a high-impedance input, so a small series resistance produces almost no error in normal operation. The failure mode is different: the input is sensitive to any leakage path to earth, and a signal SPD with a low insulation resistance or a contaminated terminal block will drag the signal down. For 0-10 V loops, check the device’s insulation resistance and keep the terminals clean and dry rather than worrying about series resistance.
Also check Uc carefully. A 0-10 V signal may sit inside a cabinet where a 24 V control supply is also present, and a device with a 15 V Uc would conduct continuously.
If the cable leaves the building, runs more than a few tens of metres, or connects equipment on different earth references, yes. The input card and the transmitter are both low-energy devices with little tolerance for overvoltage.
Only if the device is designed for two channels. Sharing a single-channel device between two loops defeats the isolation between them and can create a path between signals that should be independent.
Not if the series resistance is small relative to the loop budget. It becomes visible when the transmitter is already near its maximum load resistance, which is why the loop calculation has to include the device.
IEC 61643-21 covers surge protective devices connected to telecommunications and signalling networks. Ask for the test classification rather than relying on a generic “surge protection” description.
Yes, and it is a separate device. A 24 V DC power SPD protects the supply feeding the loop; the signal SPD protects the measurement pair. They are not interchangeable.
O nosso signal surge protector range lists Uc, In and connection format for each model. For the network side of the same problem, RS485 and PLC I/O surge protection covers the digital interfaces, and PoE surge protection covers Ethernet-powered devices.
