Keeping tanks, sumps, and process vessels at safe levels is a daily priority in water, wastewater, and industrial plants. For many legacy systems, that job still depends on the Endress+Hauser FTW420 level limit transmitter. Even though this classic conductive point level switch has been discontinued and replaced by newer models, thousands of installations still rely on the Endress+Hauser FTW420 level limit transmitter for dependable overfill protection, dry-run protection, and simple pump control.
In this in-depth guide, we’ll walk through what the Endress+Hauser FTW420 level limit transmitter does, how it works, where it shines, and how you can keep getting reliable service from it in your plant. If you’re maintaining existing panels or planning a like-for-like replacement, understanding the strengths of the Endress+Hauser FTW420 level limit transmitter will help you make confident decisions for your level safety loops.
What is the Endress+Hauser FTW420 Level Limit Transmitter?
The Endress+Hauser FTW420 level limit transmitter, sometimes marketed under the Nivotester name, is a compact control unit that turns a simple conductive probe into a reliable point level switch. Installed on a DIN rail in a control cabinet, the Endress+Hauser FTW420 level limit transmitter connects to a stainless-steel rod or cable electrode in the tank and uses the liquid’s conductivity to detect whether the probe is wet or dry.
Unlike continuous level transmitters that output a 4–20 mA signal, the Endress+Hauser FTW420 level limit transmitter is a point level device. It provides a relay output that flips when the liquid reaches a defined level. That relay can directly switch a pump, solenoid valve, or alarm beacon, or it can feed a PLC or DCS input for interlocks. In other words, the Endress+Hauser FTW420 level limit transmitter is designed to give you a simple, robust yes/no level signal that you can trust.
How the Endress+Hauser FTW420 Level Limit Transmitter Works
At its core, the Endress+Hauser FTW420 level limit transmitter uses the principle of conductive level measurement. A metal probe is mounted at the level you want to monitor, and the tank wall or a reference probe acts as the return electrode. When the liquid rises to cover the probe tip, it completes an electrical circuit, and the Endress+Hauser FTW420 level limit transmitter senses this change.
The clever part is that the Endress+Hauser FTW420 level limit transmitter uses a low-voltage alternating current rather than DC. This AC excitation prevents polarization and electrode deterioration, which can otherwise occur with DC-based systems. As soon as the conductive liquid bridges the probe and the reference, a small AC current flows. The internal amplifier of the Endress+Hauser FTW420 level limit transmitter detects this current and responds by energizing or de-energizing the relay, depending on the selected fail-safe mode.
Because the Endress+Hauser FTW420 level limit transmitter relies on conductivity, the medium must have a minimum conductivity—typically easily met by water and water-based solutions. For low-conductivity media, the device includes an adjustable sensitivity range so that you can tune the Endress+Hauser FTW420 level limit transmitter to your specific liquid properties and avoid nuisance trips.
Key Technical Specifications of the Endress+Hauser FTW420 Level Limit Transmitter
When you are maintaining or replacing a legacy device, the hard numbers matter. The Endress+Hauser FTW420 level limit transmitter is a dual-channel unit with an isolated probe input and a robust relay output, giving you flexibility in both wiring and application. It accepts either one probe (with the tank wall as reference) for single-point detection or two probes for high and low level control, which makes the Endress+Hauser FTW420 level limit transmitter ideal for basic pump-up or pump-down control.
On the input side, the Endress+Hauser FTW420 level limit transmitter can be set to detect liquid resistances from roughly 100 Ω up to about 50 kΩ, split across several overlapping sensitivity ranges. That means it covers everything from very conductive wastewater to more resistive process liquids, as long as they’re still aqueous. The recommended maximum cable length from probe to Endress+Hauser FTW420 level limit transmitter is typically up to 300 m, assuming proper shielded instrumentation cable and reasonable capacitance.
On the output side, the relay of the Endress+Hauser FTW420 level limit transmitter is a changeover (SPDT) contact rated up to 4 A at 250 VAC, with a maximum switching power in the 500 VA range for AC loads with a decent power factor. For DC loads, the Endress+Hauser FTW420 level limit transmitter can switch up to around 50 W at 250 VDC, or more at lower voltages, making it suitable for small pumps, solenoid valves, or interface relays.
Power Supply, Housing, and Environmental Limits of the Endress+Hauser FTW420 Level Limit Transmitter
In real-world panels, compatibility with your existing power supply is crucial. Fortunately, the Endress+Hauser FTW420 level limit transmitter was produced in multiple supply voltage versions, such as 24 VAC, 42 VAC, 48 VAC, and common mains ranges like 110–127 VAC and 220–240 VAC. That flexibility made it easy to integrate the Endress+Hauser FTW420 level limit transmitter into different regional standards and cabinet designs without extra transformers or power supplies.
The housing of the Endress+Hauser FTW420 level limit transmitter is a compact “Minipac” module, around 50 mm wide, built for standard 35 mm DIN rail mounting. Plug-in front terminals simplify wiring and replacement because you can remove the terminals from an old Endress+Hauser FTW420 level limit transmitter and insert them directly into a new unit. Ambient operating temperature is usually specified from -20 °C to +60 °C for single mounting, with a slightly lower limit when multiple Endress+Hauser FTW420 level limit transmitter units are tightly banked together.
In terms of protection, the standalone housing of the Endress+Hauser FTW420 level limit transmitter provides IP40 protection, which is suitable for dry indoor cabinets. With an optional field enclosure, you can reach IP55, making it possible to mount the Endress+Hauser FTW420 level limit transmitter closer to the tank or in more demanding plant areas as long as temperature limits are respected.
Fail-Safe Design of the Endress+Hauser FTW420 Level Limit Transmitter
Safety is the main reason you install a level switch in the first place. The Endress+Hauser FTW420 level limit transmitter offers configurable fail-safe modes so that your system defaults to a safe state if something goes wrong. Through an internal jumper, you can configure the Endress+Hauser FTW420 level limit transmitter for “Maximum” fail-safe or “Minimum” fail-safe operation, depending on whether you are protecting against overflow or dry-run.
In high-level fail-safe mode, the relay in the Endress+Hauser FTW420 level limit transmitter remains energized during normal, non-alarm operation. When the level reaches the probe—or if the power supply fails—the relay de-energizes, triggering an alarm or shutting off a feed pump. This ensures that a power loss is treated like a high-level alarm, which is the safer default. For low-level protection, the Endress+Hauser FTW420 level limit transmitter can be set so that the relay drops out when the probe becomes dry, stopping pumps before they run dry and potentially damage seals or bearings.
This simple but flexible fail-safe logic is one of the reasons many engineers still trust the Endress+Hauser FTW420 level limit transmitter in legacy overfill and dry-run protection loops. Combined with a clear front-panel LED that mirrors the relay state, the device makes commissioning and troubleshooting straightforward.
Compliance and Certifications of the Endress+Hauser FTW420 Level Limit Transmitter
When you work in regulated industries, it’s not enough for an instrument to function well—its compliance and certification status also matters. The Endress+Hauser FTW420 level limit transmitter was designed to meet internationally recognized standards for safety and electromagnetic compatibility, including European directives and standards such as EN 61010-1 and EN 61326.
Because of this, the Endress+Hauser FTW420 level limit transmitter carries CE markings and was available with UL, CSA, and VDE approvals for many markets. These certifications mean the device was tested for electrical safety, EMC emissions and immunity, and insulation coordination appropriate for industrial control equipment. When you are maintaining a panel built with the Endress+Hauser FTW420 level limit transmitter, these approvals support continued acceptance by plant inspectors and auditors, as long as the unit remains in good condition.
It’s important to note that the standard Endress+Hauser FTW420 level limit transmitter itself is not intrinsically safe. For hazardous area applications, designers typically added separate Zener barriers or galvanic isolators between the probe and the Endress+Hauser FTW420 level limit transmitter, or they used later Ex-rated versions in the same family. The successor product, the FTW325, includes intrinsically safe sensor circuits, but many existing installations still rely on the original Endress+Hauser FTW420 level limit transmitter with external barriers in classified zones.
Installation Best Practices for the Endress+Hauser FTW420 Level Limit Transmitter
Correct mounting and wiring are essential if you want reliable performance from the Endress+Hauser FTW420 level limit transmitter. In most plants, the device is mounted on a 35 mm DIN rail inside a control panel. Snap the housing of the Endress+Hauser FTW420 level limit transmitter onto the rail, making sure there is enough clearance above and below for cable routing and ventilation. If you are installing multiple units side by side, follow the manufacturer’s guidelines for spacing or derate the maximum ambient temperature for dense mounting.
For power and relay wiring, use the lower set of plug-in terminals on the Endress+Hauser FTW420 level limit transmitter, following the terminal labeling for L, N, protective earth, and the relay’s common, normally open, and normally closed contacts. Always confirm the printed voltage rating on the housing of the Endress+Hauser FTW420 level limit transmitter before energizing. There is an internal fine-wire fuse built in, so no extra external fuse is strictly required, although many designers still add upstream protection for overall panel safety.
On the probe side, the upper terminals of the Endress+Hauser FTW420 level limit transmitter accept shielded instrument cable. For single-point control, you typically connect the probe wire to the designated input terminal, and bond the return terminal to the tank wall or a separate reference electrode. For two-point control, you wire separate probes to the MIN and MAX terminals on the Endress+Hauser FTW420 level limit transmitter, plus a shared reference. Use shielded cable, ground the shield correctly, and keep probe cabling away from high-power cables to minimize noise.
Commissioning and Sensitivity Adjustment on the Endress+Hauser FTW420 Level Limit Transmitter
Once the Endress+Hauser FTW420 level limit transmitter is mounted and wired, commissioning involves verifying the fail-safe logic, checking the relay action, and setting the sensitivity. Start by applying power and noting the LED state; depending on your chosen mode, the LED on the Endress+Hauser FTW420 level limit transmitter will be ON or OFF with a dry probe. Then, simulate a wet probe by briefly bridging the probe terminal to the common terminal with a wire or resistor.
If the LED changes state and the relay in the Endress+Hauser FTW420 level limit transmitter clicks, you have confirmed correct basic operation. Next, check the actual process: allow the tank level to rise or fall through the probe level and confirm that the Endress+Hauser FTW420 level limit transmitter switches at the expected point. If the liquid has very low conductivity, or if probe surfaces are coated, you may need to select a different sensitivity range or fine-tune a potentiometer inside the Endress+Hauser FTW420 level limit transmitter for stable performance.
For turbulent or foaming surfaces, two-point control is often more reliable. By using separate high and low probes wired to one Endress+Hauser FTW420 level limit transmitter, you create automatic hysteresis so that the pump does not chatter on every small level fluctuation. This simple setup is one of the reasons the Endress+Hauser FTW420 level limit transmitter remains popular in older lift stations and sumps.
Typical Applications for the Endress+Hauser FTW420 Level Limit Transmitter
Because it works with virtually any conductive liquid, the Endress+Hauser FTW420 level limit transmitter is widely found in water, wastewater, chemical, and utility applications. One of the most common roles is high-level overfill protection in tanks and sumps. A probe mounted near the top of a tank feeds the Endress+Hauser FTW420 level limit transmitter, which then trips an alarm or shuts off an inlet pump when the level is reached, helping to prevent spills.
Another key use is low-level dry-run protection. If you mount the probe near the minimum safe suction level, the Endress+Hauser FTW420 level limit transmitter can stop the pump before air is drawn in, reducing the risk of cavitation and mechanical damage. This dry-run protection is widely used in cooling water tanks, boiler feed systems, and many process vessels that must not be run empty. In both high-level and low-level roles, the Endress+Hauser FTW420 level limit transmitter is typically wired into safety interlocks or local alarms.
The two-point control capability of the Endress+Hauser FTW420 level limit transmitter also supports simple pump control. In sewage lift stations, rainwater collection sumps, and industrial coolant reservoirs, one low probe and one high probe define the operating band. The Endress+Hauser FTW420 level limit transmitter turns the pump on when the high level is reached and off again when the low level is cleared, providing an easy, stand-alone control solution without a PLC.
Matching Probes and Accessories to the Endress+Hauser FTW420 Level Limit Transmitter
The performance and reliability of any conductive level system depend heavily on the probe design. The Endress+Hauser FTW420 level limit transmitter is designed to work with a wide range of rod and rope probes, so you can match materials and construction to your process. For clean water or mildly aggressive liquids, simple stainless-steel rod probes are usually enough to pair with the Endress+Hauser FTW420 level limit transmitter.
For sticky, coating, or highly corrosive media, you can use partially insulated or fully coated probes. These designs expose only the probe tip to the liquid, reducing unwanted leakage paths and false triggering. When combined with the Endress+Hauser FTW420 level limit transmitter, such probes maintain accurate switching even in challenging tanks. Multi-electrode assemblies, like multi-point probes, can also be used to give several switching levels from one tank penetration, each wired back to the Endress+Hauser FTW420 level limit transmitter or to multiple channels.
In addition to probes, accessories such as IP55 field housings allow you to mount the Endress+Hauser FTW420 level limit transmitter closer to the process where cabinet space is limited. Spare plug-in terminals and front panels simplify maintenance. Finally, shielded instrumentation cable is a critical accessory; it helps the Endress+Hauser FTW420 level limit transmitter resist electrical noise in long cable runs and electrically tough plant environments.
Upgrading from FTW420 to FTW325 While Respecting the Endress+Hauser FTW420 Legacy
Although the Endress+Hauser FTW420 level limit transmitter has been discontinued, its direct successor—the Nivotester FTW325—was designed to fill the same niche. In many cases, the FTW325 can replace an Endress+Hauser FTW420 level limit transmitter with minimal changes to wiring and control logic, while adding modern features like intrinsically safe sensor circuits for hazardous areas.
For plants planning a gradual upgrade, the first step is to document the existing Endress+Hauser FTW420 level limit transmitter: supply voltage, terminal wiring, fail-safe jumper settings, and sensitivity range. With that information, you can map the Endress+Hauser FTW420 level limit transmitter configuration onto a suitable FTW325 order code. During the transition, you might still run several panels with FTW420 units, so knowing how to maintain and support the Endress+Hauser FTW420 level limit transmitter remains important, especially when spare units are still in stock.
If your process media or safety standards now require intrinsic safety, moving from the Endress+Hauser FTW420 level limit transmitter plus external barrier to the FTW325 with built-in Ex-rated input can simplify panel design. However, as long as conductivities are sufficient and barriers are correct, the older Endress+Hauser FTW420 level limit transmitter can continue to deliver reliable service, making it a cost-effective option for legacy system support.
Practical Tips for Getting the Best from the Endress+Hauser FTW420 Level Limit Transmitter
To get long, trouble-free operation out of the Endress+Hauser FTW420 level limit transmitter, it helps to combine good installation practice with regular checks. Keep the unit in a reasonably cool, dry location and observe the ambient temperature limits, especially if several Endress+Hauser FTW420 level limit transmitter units are mounted side by side. Periodically verify that the terminals are tight, and that no moisture or dust has accumulated inside the enclosure.
On the process side, include probe inspection in your maintenance routines. Buildup, corrosion, or mechanical damage to probes can affect how the Endress+Hauser FTW420 level limit transmitter responds. If you encounter nuisance trips or missed trips, check for coating on the electrode, confirm conductivity of the liquid, and then review the sensitivity range settings on the Endress+Hauser FTW420 level limit transmitter. Simple tweaks can often restore reliable switching without major changes to the system.
Finally, make sure your team understands the chosen fail-safe logic. Label the panel to indicate whether the Endress+Hauser FTW420 level limit transmitter relay is energized or de-energized in safe conditions. This clarity helps operators and technicians interpret alarms correctly during abnormal events and can significantly improve response time when something goes wrong.
Related Product Collections for Level and Process Control
If you’re working with the Endress+Hauser FTW420 level limit transmitter, you’ll often need compatible instruments and supporting devices to complete your control loop. These curated collections can help you find the right matching components quickly:
- [Endress+Hauser Collection ➜] – Explore a focused range of Endress+Hauser instrumentation, including level limit transmitters, level switches, and complementary devices for liquid applications.
- [Measuring Device Collection ➜] – Browse precision measuring devices used across industrial plants for level, pressure, temperature, and other critical process variables.
- [Sensors Collection ➜] – Discover a wide variety of industrial sensors that work alongside your level limit transmitter to build robust monitoring and protection systems.
Conclusion: Supporting Your Endress+Hauser FTW420 Level Limit Transmitter for the Long Term
Even in a world of smart transmitters and digital protocols, the Endress+Hauser FTW420 level limit transmitter remains a trusted building block in many plants. Its simple conductive measuring principle, robust relay output, flexible fail-safe modes, and proven compliance history make it ideal for legacy overfill, dry-run, and pump control duties. With proper probes, good wiring practice, and occasional sensitivity checks, the Endress+Hauser FTW420 level limit transmitter can continue to deliver reliable point level detection for years.
If you’re currently operating or maintaining panels that use the Endress+Hauser FTW420 level limit transmitter and want to secure spares or review specific configurations, take a moment to visit the dedicated product page for this model. There you can explore detailed specifications, compatible accessories, and upgrade options so you can keep your level safety loops robust, clear, and dependable.
