2025-10-22

What is fault masking? & how to prevent it

What is fault masking?

In its simplest terms, fault masking happens when one fault in a system hides or prevents detection of another fault.

Fault masking often occurs where several safety devices—such as gate interlocks, emergency stops, and non-contact switches—are linked in series to a single safety relay or safety controller input.

How it happens:

A fault occurs – for example, a short circuit in safety device
A Secondary fault appears– A subsequent fault occurs, potentially in a separate area
The first fault is masked – The safety relay only detects the second fault, leaving the first one unnoticed
Safety is compromised – While the original fault remains undetected safety is compromised, and if another fault occurs the entire safety system could fail.

Why Fault Masking Matters

Fault masking can lead to hidden safety failures, which are among the most dangerous kinds of faults. A system may appear to be operating normally, when in reality, its ability to protect people has been compromised. Preventing fault masking is essential for maintaining the integrity and reliability of any safety system.

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Real World Example

Imagine a robotic manufacturing cell protected by several safety devices, all connected in series to one input of a safety relay/controller.

These include:

Solenoid interlocks on one entrance
Non-contact safety switches on another
Emergency stop buttons
Safety interlocks

Each device is designed to stop the machinery when someone enters the area.

If one of the less-used entrances to the cell—guarded by a non-contact switch—develops a fault, it may go unnoticed because it is rarely operated. Later, a second fault occurs at another entrance, triggering a fault warning on the relay. The technician fixes that fault and restarts the system, unaware that the first switch is still not functioning.

If someone later enters through the faulty gate, the system might not stop as expected—creating a serious risk of injury.

HOW THE IDEM Z-SERIES SOLVES FAULT MASKING

The IDEM Z-Series is designed to overcome fault-masking by combining rigorous safety design, component redundancy, and flexible wiring.

With series wiring for up to 30 safety devices while maintaining full safety performance, diagnostic outputs and standard compliance, the Z-Series eliminates fault masking. A simple solution to ensure safety in your application where no fault will go unnoticed.

See an example of how the Z-Series can be wired in a series to a single safety relay or controller, as well as wiring and connection diagrams. Click to enlarge images.

Series Wiring for Up to 30 Devices

You can connect up to 30 Z-Range (Z-Series) safety devices in series (i.e. daisy-chain) on a single safety relay or safety controller, while maintaining full safety performance (Category 4 / Performance Level e, PLe / SIL 3).

This allows large numbers of safety switches, interlocks, emergency stops etc., to be wired together without each needing a separate input. It simplifies wiring and reduces cost, especially in systems with many guards or access points.

Crucially, the Z-Series devices include internal self-monitoring (via the OSSD outputs), which ensures that even though devices are in series, faults in upstream devices will not be masked by downstream faults. Each device monitors itself and reports via the dual OSSD channels.

Diagnostic Outputs & Visibility

Besides the dual OSSD safety outputs, the Z-Series devices provide auxiliary outputs and LED indicators to help quickly diagnose which device in the chain is faulty or has changed state. This reduces downtime.

Housing types include rugged and hygienic designs (e.g. 316 stainless steel, IP69K) so that devices can survive harsh environments (washdowns, chemical exposure, etc.). This ensures diagnostic features remain reliable over time.

Maintaining Safety Category & Standard Compliance

Even in series with 30 devices, the Z-Series maintains Category 4 / PLe according to ISO 13849-1 and SIL 3 according to IEC 62061.

The OSSD outputs are pulsed and self-testing, which aids in detecting faults immediately rather than relying only on what the safety relay sees. This mitigates hidden or masked faults.

In Summary

Fault masking is a hidden danger that can undermine even the most carefully designed safety systems. IDEM’s Z-Series products provide a simple and robust solution, ensuring every safety device is independently monitored and no fault goes unnoticed—keeping your people, processes, and production truly safe.

Illustration of 2 robotic arms working on a conveyor belt with several IDEM safety devices

Idem Z-Series - 4 switches daisy chained to one safety controller

Choosing trusted Safety equipment

OEM Automatic offers the full IDEM Z-Series range of safety products, tailored for diverse industrial applications.

Our product specialists can help you identify the right safety equipment needed for your project, ensuring maximum safety, optimum performance and reliability across your operations and most importantly eliminating fault masking in your application.

Need Help with a Safety Application?

Our Sensors & Safety Team is here to help! Whether you need expert advice, product recommendations, or a proof-of-concept demonstration:

Phone support – Call us at 0116 284 9900

Email support – Contact us at [email protected]

Live chat – Available on our website

On-site visits – Our expert sales engineers can provide in-person demos and application support. Email [email protected] to arrange a visit or call us.