DC (Diagnostic Coverage)2018-07-06T09:58:46+00:00

DC (Diagnostic Coverage)

Diagnostic coverage is an important factor in the implementation of the standards and is crucial in determining the performance level of the system.

There are two types of failures. These are Safe failures and unsafe failures. A safe failure is where there are no safety consequences due to the failure. An example of this is a contactor failing with the contacts open. In this case no power is applied to the device it is switching. An example of an unsafe failure is if the contactor welds together so even if the control voltage is disconnected it is possible that the power is still applied to the device.

The diagnostic coverage is applied to individual components in the safety chain and is defined as a function of the effectiveness of the failure detection measures.

So if

ƛDU = Dangerous undetected failures

ƛDD = Dangerous detected failures

ƛD = Total failures = ƛDU + ƛDD

Then the diagnostic coverage DC is

DC          =             ƛDD /  ƛD

And is expressed as a percentage. So if half the dangerous failures are detected the diagnostic coverage is 50%

The diagnostic coverage is split into 4 categories

High:  This is when more than 99% of dangerous failures are detected

Medium:  This is when 90 – 99% of dangerous failures are detected

Low:  This is when 60 – 90% of dangerous failures are detected

None: This is when less than 60% of dangerous failures are detected

Mean Time to Failure (MTTF)

Another factor when determining the performance level is the Mean Time to Failure. This is self explainatory but is split into three

Low is a mean time to failure of 3 – 10 years

Medium is a mean time to failure of 10 – 30 years

High is a mean time to failure of 30 – 100 years

The manufacturer can give a Failure in Time (FIT) figure

Where I FIT = 1 failure in 10hours

For a safety system with several components the individual FIT values are added to gether to form  a total value

FIT Total = FIT + FIT + FIT 3 ………….. = 1/ MTTF

So from the manufacturer’s information a precise figure for the meantime to dangerous failure can be identified. A Diagnostic Coverage figure can also be determined according to the number of dangerous failures detected divided by the total number of dangerous failures in the system. It is now a matter of placing the system in the above chart.

It is clear that to achieve Cat.4 then it is essential to have a  high diagnostic coverage rate. It is also true that it appears that there is some overlap and the exact figure could be crucial in determining the Performance Level

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August 2018

April 2018

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