Pro-tips: Voters

Pro-tips exist to make you a better automation professional. Sometimes, that involves making important decisions, or helping your machines make important decision.

Do not worry, this is not a political post (unless you are a Safety Instrumented System).

Sometimes, a critical decision needs to be made by an automation system. Imagine a high-pressure vessel with hydrocarbons inside. There is a lot of potential for damage, not just to the company's bank accounts or the environment, but people. Such systems are often fitted with multiple, redundant sensors, called Voters. These voters are described as XooY systems. X is the number of devices needed to "win" a vote, while Y is the total number of devices in your system.

There are tradeoffs to this method. Having more instruments that can trigger the failure will lead to "spurior trips," where the system is shut down accidently. The trade

off is in the likelihood of a true disaster that was undetected.

I attached a demonstration calculation for you to try out. This is for educational use only. If you are working on anything like this, you need to verify your calculations yourself. If you unsure about any of this, contact a subject matter expert. While I believe these to be accurate, I put in dummy values for you to test out.

pro tip voters.xlsx

Download XLSX • 13KB

There is a lot to learn about such systems. For an introduction to Safety Instrumented Systems (SIS), I recommend Kenexis Safety Instrumented System Engineering Handbook, which I did a TLDR; on. If you are having trouble visualizing what this looks like, this article might help. If you really want to go more in depth, these articles are better.