The 24 V supply and the two-wire hydrostatic transmitter both fail on a level loop that runs about 200 m underground to the control room; protecting only the transmitter leaves other surge paths open. The reliable approach is to trace how a transient can reach the loop, power supply, and equipment grounding, then apply compatible protection at the entry points and bond each protector as its manufacturer specifies.
Why do common single-device fixes fail?
Buying one protector because the transmitter costs more may leave the 220 V AC input to the 24 V DC supply unprotected. A surge entering through the AC feed can damage the supply, and a surge coupled onto the long field cable can damage the transmitter, loop wiring, or control-room equipment. A protector installed at only one end of the loop may also leave the other end exposed to the voltage difference that develops along the cable.
Replacing the transmitter or power supply without investigating the path treats the damaged component, not the cause. The cable is underground, but that does not rule out a surge arriving through connected wiring, grounding, or coupling along the route. Likewise, a protector is not simply a fuse: surge protection devices limit or divert transient energy according to their design and connection. They do not guarantee survival after every lightning event.
Do not assume that the damaged equipment identifies the entry point. Repeated damage to both the transmitter and supply is a reason to inspect both the AC supply path and the field loop, along with the bonding and grounding connections they share.
Where can a transient enter this level-measurement circuit?
The measurement chain begins at a hydrostatic transmitter mounted at the tank bottom. The transmitter converts pressure into a 4–20 mA signal and receives its operating power from the two-wire loop. That loop runs roughly 200 m to the control room, where the 24 V DC supply powers it and the instrument reads the current. The supply itself is fed from 220 V AC.
A lightning-related transient can reach this arrangement through the AC power system, the field cable, or grounding and bonding paths. A long cable creates a substantial exposed circuit even when buried. If a transient raises the potential of one connected point relative to another, current can flow through the loop or connected equipment and exceed what the devices can withstand. The fact that the tank is concrete and inside a building does not by itself establish where the surge entered; inspect the electrical routes and connections rather than assuming a strike location.
Keep the protection path in view: a field-end device protects the transmitter-side conductors only when it is connected and bonded as designed; a control-room device addresses the cable entry and loop-side equipment; AC-side protection addresses the supply input. These serve different circuit segments, so one device should not be treated as a universal substitute for the others.
Which circuit segment should each measurement distinguish?
| Signal or segment | Source or destination | Wrong-value symptom or diagnostic clue |
|---|---|---|
| 220 V AC input | Incoming supply to the 24 V DC power supply | Supply damage points to inspection of the AC feed and its protection path; it does not prove that the AC feed was the only entry route. |
| 24 V DC two-wire loop | Power supply through the transmitter and receiving instrument | Transmitter or instrument failure calls for checking loop wiring, polarity, protector connections, and grounding/bonding continuity. |
| 4–20 mA measurement | Transmitter output read by the control-room instrument | An implausible or lost reading after an event may result from damaged loop electronics or an open connection; verify loop current and inspect both endpoints. |
| Protector bonding connection | Protector to its designated bonding or earth point | A long, loose, or interrupted connection can undermine the intended diversion path; inspect the actual connection and installation instructions. |
Record which devices fail, what else is affected, and whether the failure follows a storm or power disturbance. Check the supply output and loop current with suitable instruments, then inspect conductors and terminations before changing the control instrument or transmitter. A 4–20 mA reading is useful for diagnosing loop continuity and transmitter operation, but it does not locate a transient path by itself.
How should protection be applied at the AC and loop boundaries?
Use coordinated protection for the different circuit boundaries: the supply’s AC input and the transmitter loop. For a long field cable, evaluate protection at both the transmitter end and the control-room end. The field-end protector addresses exposure close to the transmitter; the control-room protector addresses the cable as it enters the equipment area. Confirm the device is intended for the actual circuit and install its bonding connection as specified by its manufacturer.
- Map the circuit. Trace the 220 V AC feed into the 24 V supply, the supply’s loop conductors, the two-wire transmitter, the control-room instrument, cable route, and relevant bonding connections. Mark where the cable enters each area.
- Identify the protector’s circuit. Select separate AC-side protection for the supply input and loop protection compatible with the two-wire 4–20 mA circuit. Do not choose a loop device based only on a product name or the fact that it is described as surge protection.
- Evaluate both cable ends. For the approximately 200 m run, determine whether the selected protection arrangement requires devices at both ends. Confirm the manufacturer’s wiring diagram, conductor arrangement, and bonding requirements for the chosen devices.
- Install and bond each device. Keep wiring and bonding consistent with the manufacturer’s installation instructions. A protector with a poor or missing bond cannot provide its designed path for surge current.
- Restore and test the loop. Check the supply output, loop wiring and polarity, and transmitter current before relying on the displayed level. Compare the indicated level with an independent process check where available.
Coordinate AC-side work with a qualified person familiar with the installation. The appropriate AC protector depends on the supply arrangement and device ratings; read those values from the equipment nameplates and manufacturer documentation rather than selecting by the nominal 220 V label alone.
How do you select and service a loop protector?
Read the transmitter and receiver documentation to establish loop operating voltage, current, wiring, and any other circuit limits. Compare those requirements with the protector’s specified operating range and connection diagram. A device must pass the normal loop signal and power while limiting the transient as intended; a protector unsuitable for the loop can disrupt measurement or fail to protect the connected electronics.
Some protection assemblies use a base or support and a replaceable protective plug or element. If a supplier identifies separate base and element components, verify the exact compatible pair and whether both are required for the intended installation. Do not infer compatibility or serviceability from a general product family description.
After an event, inspect the protector’s status indication and follow its manufacturer’s instructions for testing or replacement. Whether an element can be reset, replaced, or must be discarded depends on the specific device. Do not assume that a protector is reusable, or that an intact-looking module remains functional after a surge.
How can you verify the fix after a storm?
Before putting the loop back into service, confirm that the supply produces its specified output, the transmitter is powered, the loop wiring is intact, and the instrument responds across the expected measurement range. Compare the indicated water level with an independent level or process check if available. Record protector status, failed parts, and any damage to wiring or terminations.
After subsequent storms or power disturbances, repeat the inspection rather than replacing equipment automatically. If failures continue, review the complete route and bonding arrangement with the installer or device manufacturer; persistent damage can mean a protector is missing, misapplied, improperly bonded, or that the transient enters by a different connected circuit. Do not respond by adding arbitrary devices in series or changing loop wiring without checking the device diagrams.
What should engineers know about protecting this 4–20 mA loop?
How many surge protectors does a 4–20 mA level loop need?
Evaluate protection for the 220 V AC supply input and the two-wire loop as separate circuit segments. For the roughly 200 m field cable, assess protection at both ends; the device wiring diagram and installation conditions determine the correct arrangement.
How do I protect a two-wire transmitter without interrupting its signal?
Choose a protector specified for the loop’s operating voltage and current, then wire it exactly as its manufacturer shows. Verify the transmitter powers up and the instrument reads loop current correctly after installation.
How do I know whether a surge protector can be reset?
Check the exact device’s status indicator and manufacturer instructions; resettable, replaceable, and non-resettable designs differ. Stop replacing transmitters alone if damage recurs, and have a qualified electrical or controls professional inspect the AC feed, cable route, and bonding; escalate the device selection or repeated failure to the protector and transmitter manufacturers’ official support channels.