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High Level Switch – Initial Troubleshooting

This guide describes a systematic, field-proven approach for troubleshooting High Level Switch alarms and overflow incidents on LiqTech filtration systems. It applies to digital high-level switches installed on process tanks such as permeate, backwash, feed, and similar tanks.

When to use this guide

Use this guide when:

  • A High Level Switch alarm is active on the HMI

  • The system has stopped due to a high tank level safety condition

  • The customer reports an overflow or near-overflow situation

  • The alarm persists even though the tank is expected to be below high level

Preconditions and safety

Before starting troubleshooting:

  • The system should normally be in STOP state

  • Live testing may be required to observe switch behavior

  • Electrical troubleshooting shall only be performed by personnel with relevant electrical competence

  • Always follow site-specific safety procedures

Objective of this troubleshooting

The objective of this guide is to verify that:

  • the High Level Switch can be physically activated,

  • the HMI correctly indicates the activation, and

  • the alarm is configured as a Critical / Stop alarm, not merely a Warning.

In practical terms, it must be possible to:

apply water to the switch → see the HMI indication change → trigger a critical alarm → and stop the system.

If this chain does not work as expected, the system cannot be considered protected against overflow.

Alarm severity can be verified directly in the Alarm Settings on the HMI.

Important principle 

A High Level Switch alarm must always be treated as a real alarm until proven otherwise.

Likewise, a tank overflow without a corresponding High Level Switch stop must always be treated as a critical finding.

The first objective is not to explain the process, but to confirm whether:

  • the switch is physically activated, or

  • the alarm (or missing alarm) is caused by hardware, wiring, or configuration issues.

Step-by-step troubleshooting procedure

1. Confirm switch activation using HMI and local indication

  • Observe the High Level Switch status on the HMI

  • Check the switch for local status indication (LED if present)

  • Inspect the switch area for fouling, buildup, or foam

Outcome

  • Switch appears active → proceed with process-related troubleshooting

  • Switch does not appear active → proceed with hardware-related troubleshooting

2. Hardware-related troubleshooting

(false activation OR missing activation during overflow)

2.1 Manual functional test of the switch

  • Manually activate the switch using water or a wet cloth

  • Verify that:

    • the switch changes state locally

    • the HMI input changes accordingly

2.2 Verify the complete signal path (end-to-end)

The entire signal path must be verified:

  • Switch element

  • Switch hat / connector

  • Cable gland

  • Field cable

  • Junction box (JB)

  • Cabinet terminals

  • Digital input (DI) card

Checks to perform:

  • Measure and/or buzz continuity conductor-by-conductor

  • Verify correct 24 VDC supply and reference

  • Inspect terminals for loose connections, corrosion, or incorrect termination

Common field issues

  • Incorrect pin assignment

  • Wiring landed on wrong terminal

  • Broken conductor inside cable

  • NPN / PNP mismatch

  • Incorrect DI channel

2.3 Verify alarm behavior and severity on HMI

  • Confirm the High Level Switch alarm is configured as Critical / Stop

  • Verify it is not configured as Warning only

If an overflow has occurred, verify whether the input ever changed state on the HMI.

3. Process-related troubleshooting

(correct activation)

3.1 Verify alignment with level transmitter (most common cause)

This is by far the most common root cause.

Verify that:

  • the level transmitter is correctly scaled, see articel "Level Alarms (Transmitter) - Initial Troubleshooting" 

  • transmitter high-level setpoints are not set higher than or too close to the physical switch position

3.2 Identify all possible inflow paths to the tank

Systematically check all ways liquid can enter the tank, including unintended paths:

  • Leakage through permeate FCVs during CIP due to wear or calibration drift

  • Technical or utility water unintentionally entering the tank

  • Internal leakage across valves assumed to be closed

  • Bypass lines or drain returns routed back to the tank

Small continuous inflows can raise the tank level over time.

3.3 Verify downstream conditions

  • Confirm downstream valves are open as intended

  • Check for blocked or closed outlets

  • Verify correct routing and active tank selection on the HMI

3.4 Check flow balance and control behavior (PID)

  • Upstream pump control may be too aggressive

  • Downstream pump may be too slow or stopped

  • Ramp-up behavior may cause temporary overfilling

⚠️ Important
If PID or control behavior is suspected to be the root cause, LiqTech must be contacted.
PID parameters and control philosophy are not intended to be modified during field troubleshooting.

3.5 Marine installations (if applicable)

On vessels, sloshing due to sea state can cause intermittent activation of high-level switches even when average level appears acceptable.

Common root causes

Hardware-related

  • Fouled or contaminated switch

  • Switch mechanically unable to activate

  • Incorrect wiring or termination

  • NPN / PNP mismatch

  • Faulty digital input channel

  • Alarm configured as Warning instead of Critical

Process-related (most common)

  • Incorrect level transmitter scaling

  • High-level setpoint on transmitter misaligned with physical switch

  • Unintended inflow to the tank

  • Incorrect valve or pump state 

  • Aggressive or unstable control behavior

  • Sloshing in marine installations from ship rolling

When to stop and contact LiqTech Service

  • If switch behavior cannot be verified locally

  • If continuity and wiring are verified end-to-end but alarm behavior is incorrect

  • If PID or control behavior is suspected

  • If an overflow occurred without High Level Switch intervention and the root cause cannot be clearly identified

Alarm ID examples (for agent guidance)

The following alarm IDs are non-exhaustive examples of High Level Switch alarms observed in LiqTech systems.
They are provided to help support agents recognize the alarm type when a customer references an alarm number.
Other High Level Switch alarm IDs may exist depending on system type and project.

  • MK6 Generic:
    Alarm ID 51 – Permeate High Level Switch

  • Project-specific systems (e.g. KOC3000):
    Additional High Level Switch alarms may exist for other tanks (e.g. feed, neutralization, process tanks). Alarm IDs vary by project and should be confirmed in the relevant alarm list.