Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

EN ISO 13849-1 MTTFd

EN ISO 13849-1 MTTFd

EN ISO 13849-1 MTTFd


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?


RE: EN ISO 13849-1 MTTFd

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.

This is normally the space where people post something insightful.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close