Safety

2026.05.26

What is DCavg (Average Diagnostic Coverage)? - A Guide to its Relationship with PL -

When researching machine safety, you will encounter the term “DCavg (Average Diagnostic Coverage).” As you read further, you will come across another indicator “Performance Level (PL),” which might make things seem increasingly complicated.

However, these two concepts are not separate ideas, but are closely connected. Let’s look at them step by step.

DCavg (Average Diagnostic Coverage)

First, in simple terms, DCavg is “the percentage of dangerous failures that a machine can detect on its own out of the total number of dangerous failures occurring in itself.”

Machines will encounter various troubles (failures) as they are used, such as:

sensors may stop detecting
wirings may break
switches may fail

When these failures occur, it is important whether the machine can detect (diagnose) them on its own.

Suppose 10 failures occurred in a machine:

9 were detected → DCavg 90%
Only 5 were detected → DCavg 50%

This is a general idea.
DCavg numerically represents “how many abnormal conditions can be detected without being missed.”

Why does it matter?

The point here is “whether the failure can be detected,” rather than “occurrence of the failure.” This is because continuing to use the machine without realizing the failure can lead to accidents.

For example,

the machine does not stop even though the door is open.
the machine continues to operate even though the sensor is not working.

You may see this is extremely dangerous.

On the other hand, if failures can be detected immediately, it is possible to

stop the machine
activate an alarm

so that accidents can be prevented.

Is a higher DCavg necessarily safer?

Basically, that is correct.

Low DCavg → More likely to miss failures
High DCavg → More likely to find failures

As general benchmarks, DCavg is often classified as follows:

DCavg < 60%: Low
60% ≦ DCavg < 90%: Medium
90% ≦ DCavg: High

What is Performance Level (PL)?

The next is PL, which represents a rank of “how reliable the safety function is.”

The rank is shown as follows. As the rank gets closer to e, the safety is evaluated higher.

PL a (Low)
PL b
PL c
PL d
PL e (High)

What determines PL?

PL is primarily determined by the three factors below:

MTTFd (durability)
Safety Category (structural safety)
DCavg (failure detection ability)

Of these, DCavg is the factor in charge of “the ability to detect abnormal conditions.”

The impact of DCavg on PL

This is the most important point.
The higher the DCavg, the easier it is to achieve a high PL.
This is because:

The proportion of detectable dangerous failures increases.
The proportion of undetectable dangerous failures decreases.

Consequently, it will lead to improved reliability of safety function. For example,

Low DCavg

Faults frequently not detected
Dangerous failures easily overlooked
→ Difficult to achieve high PL

High DCavg

Wider range of fault detection
Dangerous failures less likely overlooked
→ Possible to aim at high PL (d or e)

However, it is not determined by “DCavg alone”

This point tends to be misunderstood. A high DCavg does not necessarily mean a high PL.

In the following cases, for example, the entire safety cannot be considered sufficient even if the detection capability is high:

The component is fragile (Low MTTFd)
The structure is too simple (Low Safety Category)

To put it simply

PL is a “comprehensive evaluation.”

MTTFd: durability
Safety Category: structural strength
DCavg: ability to detect abnormal conditions

It is only when these three factors are well-balanced that a high PL is achieved.

How can DCavg be increased?

To increase DCavg, it is necessary to increase the number of “fault detection (diagnosis) mechanisms.”

The following ingenuities, for example, can decrease the chances of missing a failure (or increase the possibility of detecting abnormal conditions):

Check the presence of faults by sending test signals periodically
Compare input and output (Cross check)
Make two circuits, which perform the same operation, monitor each other

These mechanisms are also used in actual safety devices. For example, although the diagnostic capabilities of safety switches are different between products in the market, the types with internal redundancy and cross-monitoring functions can contribute to enhancing “performance to detect abnormal conditions” of the overall safety functions, thereby helps establish safety functions complying with high PL.

Summary

DCavg (Average Diagnostic Coverage) is the “percentage of dangerous failures that a machine can detect on its own.”
To manufacture a safe machine, it is very important not only that “it is durable” but also that “it is able to properly detect failures when they occur.”
And, Performance Level (PL) represents “how reliable the safety function is.”

The relationship between DCavg and PL is simple.
The higher the DCavg, the easier it is to aim at a higher PL.
However, it is important that PL is determined not only by DCavg, but also by the balance with durability (MTTFd) and structure (Safety Category).

Appendix: Difference between DC and DCavg

DC (Diagnostic Coverage): How well a specific type of failure can be detected
DCavg (Average Diagnostic Coverage): Average detectability across various failures

Suppose there are three kinds of failures in a machine.

Type of Failure	Detectable Ratio (DC)
Wiring breakage	100%
Sensor failure	80%
Contact welding	40%

“Individual detection capabilities” (100%, 80%, 40%) are DC.
In reality, however, it is necessary to evaluate “how well can the entire machine detect?” To achieve this, average of these values is calculated as DCavg.
(NOTE: In practice, it is not a simple average, but the probability of failure is also considered. For illustrative purposes, let us assume it as an average.)

This is a somewhat extreme example, but the relationship can be illustrated as follows: