EN ISO 13849-1 MTTFd

[sdra2]

Hello

In this standard, safety-related parts of control systems, what does the 'd' of MTTFd mean? I know it means "dangerous (failure)", but it also talks about a safety system failing to operate.

If a safety system failing to operate places the machine it is protecting into a default safe state, where does the actual danger come from?

Thanks

 

[controlnovice]

There are two type failures of safety systems:

1: The safety system operates when you don't want it to.

This is normally ok in that it doesn't create an unsafe condition (however, only you can decide that about your system). It causes operations a lot of headaches because it shuts the system or process down. The safety system should be designed to protect the equipment (not damage anything during the shutdown)

2: The safety system doesn't operate when you want it to.

This is the worse of the failures, for what I hope are obvious reasons.

So each of these failure modes should have a MTTF or similar values to indicate the frequency of each.

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