Re: [SystemSafety] Units for Functional Safety variables ? - A Friday afternoon question on a Wednesday.

From: Robert Schaefer at 300 < >
Date: Wed, 2 Oct 2013 16:47:03 +0000

In this case the label or number or what-have-you corresponds to (1 - probability of something unsafe occurring) wrt some amount of time, for example millions of hours.

Sent: Wednesday, October 2, 2013 12:09 PM To: Jon Hind
Cc: systemsafety_at_xxxxxx Subject: Re: [SystemSafety] Units for Functional Safety variables ? - A Friday afternoon question on a Wednesday.

Not everything has a dimension. (Although I suppose even dimensionless constants could be considered dimensionless ratios). Hardware Fault Tolerance (assuming you mean in the 61508 sense or equivalent) is of the same "type" as a Safety Integrity Level - they are assigned labels. This is indicated in the fact that assignment and manipulation is according to arbitrary rules, not normal arithmetic.

The other way of looking at it, if you reject the idea that HFT is a "label", is that it is a count. The dimension of a count is whatever you are counting, in this case "levels of redundancy". This lets you perform dimension maths - you can weigh your system, and come up with "Levels of redundancy per kilogram", or cost your system and determine "$$ per level of redundancy".

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On 2 October 2013 15:49, Jon Hind <jon_at_xxxxxx I posted this (only partially) frivolous question on out internal system, without any reply.

So trying here for some erudition...

What dimension and or units does Hardware Fault Tolerance (HFT) have ? - A Friday Poser on Functional Safety Terminology (FST).

I remember being taught at university in the last century that everything has a dimension in terms of Length (L) , Mass (M) and or Time (T), or is a ratio (factor). A good check of the validity of an equation is that it must balance dimensionally.

Availability Common Cause Factor (?, CCF) and Safe Failure Fractions (SFF) are dimensionless ratios, failures rates (?) are T^-1, Proof Test intervals are T etc.

And HFT ? HFT is an integer - In our (oil & gas process) world, 0 1 or 2 ; is it part of a case statement, or heaven forbid a pointer like in the C language ?

I don't want to call it a factor as we already have that in CCF and SFF.

Answers on a postcard please....

Jon Hind

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systemsafety_at_xxxxxx Received on Wed Oct 02 2013 - 18:48:30 CEST

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