MicroSensor PD140: The Lightning Protection Device That Keeps Your Level Transmitters Alive

If you operate submersible level transmitters in open water — reservoirs, rivers, dams, bore fields, irrigation channels — you already know that lightning is the enemy. A single nearby strike can induce a surge powerful enough to destroy a transmitter’s electronics in milliseconds, leaving you with a failed measurement, a replacement cost, and potentially a critical gap in monitoring data during a weather event when you need it most.

Australia’s tropical north and inland regions experience some of the highest lightning strike densities in the world. In Queensland, the Northern Territory, and northern Western Australia, the wet season brings thousands of lightning strikes per day across the exact type of open, water-facing terrain where level transmitters are typically installed. Even in temperate regions, a single severe thunderstorm is enough to wipe out an unprotected instrument.

The MicroSensor PD140 lightning-proof protection device was developed specifically for this scenario. It sits between your instrument loop and the transmitter, absorbing and diverting surge energy before it reaches the electronics. PCSPL supplies the PD140 as both a standalone protection device and as part of a complete level measurement solution alongside MicroSensor’s MPM4700 intelligent liquid level transmitter.

The Problem

Why Lightning Is the #1 Cause of Level Transmitter Failure

Submersible liquid level transmitters are by nature deployed in exposed, outdoor environments — suspended in water bodies, mounted on structures with long cable runs, and connected via signal cables that can stretch hundreds of metres back to a controller or data logger. That combination of outdoor exposure and extended cable length makes them ideal antennas for lightning-induced electrical surges.

When lightning strikes near an instrument installation — it does not need to strike the cable directly — the electromagnetic pulse induces a transient voltage spike on the signal and power conductors. That spike travels along the cable and arrives at the transmitter’s electronics as a high-energy surge that vastly exceeds the withstand voltage of the components inside. The result is typically immediate and permanent destruction of the transmitter circuit.

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Direct and Indirect Strikes

A direct strike to a cable or structure is catastrophic, but it is far less common than an indirect strike. An indirect strike within several hundred metres of the installation is enough to induce damaging transient voltages on long cable runs via electromagnetic coupling — no physical contact required.

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Power Supply Surges

Lightning can enter the instrument loop through the 24 VDC power supply side as well as the signal side. A surge entering from the power supply travels directly to the transmitter electronics. Without protection on both the power and signal conductors, the transmitter is exposed from multiple directions simultaneously.

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Static and Inductive Discharge

Lightning strikes are not the only threat. Inductive switching transients from solenoids, contactors, and motor start events on the same power distribution system can also generate damaging voltage spikes on the signal loop — particularly where instruments share a common power supply with other field equipment.

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Built-in Protection Is Not Enough

Modern submersible level transmitters include internal surge protection circuits, and MicroSensor’s own instruments are designed with internal protection as standard. However, internal protection alone cannot guarantee survival of a nearby direct strike or repeated indirect surges. External protection is the first line of defence — it absorbs the bulk of the surge energy before it even reaches the transmitter’s input terminals.

⚠️ Don’t rely on internal protection alone: Internal protection circuits in a transmitter reduce surge damage probability — they do not eliminate it. MicroSensor’s own guidance is clear: for any outdoor installation in an area prone to lightning, an external lightning protection device such as the PD140 is required in addition to internal protection. The two work together; neither is a substitute for the other.

The Solution

How the MicroSensor PD140 Works

The PD140 is a passive, self-resetting lightning protection device that installs in series with the instrument signal loop between the field wiring and the transmitter. It contains purpose-built protection components that clamp surge voltages and divert transient currents safely to earth — all within microseconds of a surge event, and without requiring any manual reset or replacement after operation.

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Dual-Channel Protection

The PD140 protects both signal channel types used by MicroSensor transmitters. It covers the 4–20mA current loop used by standard two-wire process transmitters, and the RS485 digital communication interface used by HART-capable and intelligent transmitter variants. A single PD140 unit handles both signal types simultaneously.

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Common Mode and Differential Mode Protection

Surge energy attacks signal circuits in two ways: between each conductor and earth (common mode), and directly between the two signal conductors (differential mode). The PD140 provides both types of protection. Common mode surges are diverted from each wire to earth; differential mode surges are clamped between the two signal wires. Both must be addressed for complete protection.

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High Impulse Current Handling

The PD140 is rated to handle impulse currents of ≥10kA on the 4–20mA channel and ≥5kA on the RS485 channel, measured on the standard 8/20μs surge waveform used in lightning protection testing. These ratings reflect the ability to safely absorb a genuine lightning-induced transient, not just a minor electrostatic discharge event.

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Automatic Self-Reset

After absorbing a surge event, the PD140 resets automatically — no fuse to replace, no manual intervention required. This is particularly important in remote outdoor installations where a maintenance visit following every thunderstorm would be impractical. The transmitter resumes normal operation as soon as the surge has passed.

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IP65 Weatherproof Housing

The PD140’s enclosure is rated IP65 — protected against dust ingress and direct water jets. This makes it suitable for outdoor installation adjacent to the transmitter in exposed field locations, junction boxes, and instrument enclosures subject to rain and washdown.

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Wide Temperature Range

The PD140 operates across a temperature range of −40°C to +80°C, covering the full range of ambient conditions encountered in Australian field installations — from near-freezing highland locations to the extreme summer heat of inland and tropical sites where ambient temperatures regularly exceed 45°C.

Key principle: The PD140 acts as a sacrificial diverter — it absorbs surge energy and redirects it safely to earth before it can reach the transmitter’s input terminals. The lower the impedance of the earth connection, the more effectively this diversion works. Earth resistance below 4 Ω is mandatory for the device to function correctly under a real surge event.

Specifications

PD140 Full Technical Specifications

The table below lists the complete electrical and mechanical specifications for the MicroSensor PD140, taken directly from the manufacturer’s datasheet. Use these figures when specifying the PD140 for your installation or when verifying compatibility with your instrument loop design.

Parameter	4–20mA Channel	RS485 Channel
Max. impulse current (8/20μs waveform)	≥10kA	≥5kA
Clamping voltage — wire to earth	28V	−7V to +12V
Clamping voltage — between wires	28V	6.5V
Max. working current	<40mA	<120mA
Voltage drop at 20mA, 25°C	<2VDC
Voltage drop at 20mA, 80°C	<6VDC
Operating temperature	−40°C to +80°C
Housing protection rating	IP65
Mounting options	3-bolt panel fixing or DIN rail mount
Cable entry diameter	5–10mm (via fixed connector)
Required earthing cable cross-section	≥4mm²
Max. earthing conductor length	≤1m from terminal to earth
Required earth resistance	<4Ω
Reset type	Automatic (self-resetting)

⚠️ Voltage drop at high temperature: At 80°C ambient, the PD140’s voltage drop increases to less than 6VDC at 20mA. In a 24VDC loop, this leaves 18V available for the transmitter and loop resistance. Verify that your loop power budget remains adequate at the maximum expected site temperature — particularly in enclosed instrument enclosures in direct sun in northern Australia.

Installation

Installing the PD140 Correctly — Four Rules That Cannot Be Skipped

A lightning protection device is only as effective as its installation. A PD140 with a poor earth connection, a long earthing conductor, or a cable gland that is too large for the cable will not perform to its rated specification during a real surge event. The four installation requirements below come directly from MicroSensor’s installation guidance and must all be met for the device to provide the protection it is rated for.

⚠️ Install as close to the transmitter as possible: The PD140 should be positioned at the transmitter end of the cable run, not at the control panel end. Installing it at the panel end protects the panel but leaves the full length of the field cable — and the transmitter itself — exposed to the incoming surge. The device must be on the field side, adjacent to the instrument it is protecting.

⚠️ Cable gland sizing is critical: The cable entering the PD140 through the fixed connector must have a diameter between 5mm and 10mm. An oversized cable gland — a common field shortcut — allows moisture ingress and reduces the integrity of the cable entry seal. Use the correct cable size and verify the gland is properly tightened before commissioning.

⚠️ Earth connection quality determines protection quality: The earthing conductor from the PD140’s earth terminal to the site earth must be no longer than 1 metre, must have a cross-sectional area of at least 4mm², and the earth resistance at the earth rod or earth bar must be less than 4Ω. A long, thin, or poorly terminated earth conductor adds impedance that prevents the surge current from being effectively diverted — the surplus energy then passes through to the transmitter instead. Verify earth resistance with a dedicated earth resistance tester at commissioning and after any significant earthing system work.

⚠️ Form an equipotential bond: Lightning protection works best when all metallic components in the instrument installation share the same earth reference. Connect the instrument housing, cable armour, the 24VDC power supply negative terminal, the signal ground, and any data logger or controller I/O ground to the same earth bar or earth rod. Differences in earth potential between components during a surge event create damaging transient currents between them — bonding eliminates those paths.

Maintenance reminder: Add an annual inspection of the PD140 and its earthing system to your instrument maintenance schedule. Check earth resistance, verify cable gland tightness, inspect the earth conductor for corrosion or damage, and confirm all bonding connections are tight. The PD140 itself requires no maintenance after normal surge events due to its self-resetting design, but the earthing infrastructure it depends on must be kept in good condition.

The Complete Solution

PD140 + MPM4700: A Complete Lightning-Protected Level Measurement System

The PD140 is designed to work alongside MicroSensor’s MPM4700 intelligent liquid level transmitter — a two-wire, 4–20mA submersible transmitter with internal lightning protection built in as standard. Together, they provide a layered protection strategy: the MPM4700’s internal circuits handle minor transients from the process side, while the PD140 handles major surge events arriving from the signal cable.

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MPM4700 — Intelligent Level Transmitter

4–20mA DC two-wire output. Internal lightning protection as standard. Available with RS485 interface or HART communication protocol. Suitable for reservoir, river, bore field, and dam level measurement. Custom cable lengths and configurations available to suit your installation depth and site layout.

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PD140 — External Lightning Protection

Protects both the 4–20mA current loop and RS485 signal channel. Rated to ≥10kA impulse on the current loop. Self-resetting — no site visit required after a surge event. IP65 weatherproof housing. Mounts adjacent to the transmitter at the head of the cable run, forming the first barrier against incoming surge energy.

This combination is particularly well suited to unattended or remotely monitored outdoor installations — flood warning networks, water authority level monitoring stations, irrigation dam telemetry, and mining water balance monitoring — where the cost of an emergency transmitter replacement, plus travel time to a remote site, can far exceed the cost of the protection device itself.

Cost perspective: A PD140 is a low-cost, one-time addition to any outdoor level transmitter installation. The cost of replacing an unprotected transmitter destroyed by a lightning surge — including the instrument, cable, site access, and lost monitoring data — typically runs many times higher. In lightning-prone regions of Australia, the PD140 should be considered standard practice, not optional.

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Is It Right for You?

When to Specify the PD140

The PD140 is appropriate for any outdoor level transmitter installation where lightning, inductive switching, or static discharge represents a realistic risk. Use the checklist below to assess whether your application warrants its use — in practice, almost any outdoor installation in Australia will tick at least one of these boxes.

Specify the PD140 if your installation involves any of the following:

Outdoor deployment in open terrain
Installation near water bodies (rivers, reservoirs, dams)
Located in a high-lightning-density region (Qld, NT, WA wet season)
Cable runs exceeding 50 metres in the field
Submersible transmitter suspended in open water
Remote or unattended monitoring station
4–20mA or RS485 signal interface (both covered)
Instrument sharing a power supply with switchgear or motors
History of unexplained transmitter failures at the site
Replacement transmitter cost is significant relative to protection cost
Loss of level data during a storm event has operational consequences
Site is difficult or costly to access for emergency repairs

The PD140 is also an important consideration when:

You’re replacing a previously lightning-damaged transmitter
Your site has inadequate existing surge protection infrastructure
You’re upgrading to a new MicroSensor level transmitter
You’re specifying new instrumentation for a flood warning network

A Note From Our Team

Specifying Lightning Protection for Your Site?

Lightning protection for field instrumentation is a straightforward measure, but getting the installation details right — particularly the earthing — makes the difference between a device that performs to spec and one that provides a false sense of security. The PD140 is a well-engineered, purpose-built solution for the exact scenario it addresses, and it is one of the most cost-effective risk management measures you can add to an outdoor level measurement installation.

PCSPL supplies the MicroSensor PD140 and the full MicroSensor level transmitter range, including the MPM4700 intelligent submersible level transmitter. The PD140 datasheet is available via Birkett Controls, our official online retail partner. If you would like to discuss your specific site conditions — cable run length, local lightning risk, earthing infrastructure, or instrument selection — our applications team is available to help you select and specify the right solution.

When you contact us, it helps to have the following information ready: the transmitter model and signal output type (4–20mA or RS485), the approximate cable run length from transmitter to controller, the site location and whether it has an existing earthing system, and any history of previous lightning or surge damage at the site.

Protect Your Level Transmitters from Lightning Damage

Contact PCSPL to order the MicroSensor PD140 or to discuss a complete lightning-protected level measurement solution for your site.

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