Segment loading-Critical Loop

[prachides]

Hi,

While going through AG-181, i came acroos terms like Level-1,level-2 critical loops.
i just wanted to know, if one of the loop is assigned as level-1 crtical loop, then i shall be assigning that loop on single H1 segment.
Does it mean that i can not connect any more monitoring devices to that H1-segment which are independent of that critical loop.
If yes, then why?


Ms.Prachi

[rezabejd]

The short answer is, the guidelines allow additional monitor-only devices on the Level-1 segment.

Bear in mind, the reference is a guideline, and your circumstances may drive you to be more or less conservative. In my mind, the guidelines are a little dated and overly conservative for most cases. What drove early adopters to restrict the number of devices, and specifically control valves, on critical loop segments?

1) No spur short-circuit detection (became available c. 2001) - one fault (maybe a maintenance person or installer working on a new device) would cause the whole loop to go down. This is not a huge issue any more.

2) Uncertainty about H1 card availability - redundant H1 cards have since become very common, and simplex cards have also become very reliable. By distributing critical valves (no 2 on the same segment or the same interface cards) you were distributing your risk. This is still true today, but in my mind the risk (of H1 interface card causing issues) is very low.

3) Uncertainty about the impact of configuration changes / downloads on devices. That is, if you have other loops or even a multitude of indicate-only devices on a critical segment, you would at some point want to make a change, and might shy away from that change if it could impact a critical loop. This was a valid concern, but I think today, after multiple software upgrades, the risk is much smaller. Consider whether the operating culture of your new installation would be nervous about such changes.

4) General concern about how robust H1 was likey to be. Similar to (2), the object was to distribute the risk. In practice, most of us have found H1 to be extremely robust.

[Mike ONeill]

The subject of criticality with regard to FF segments should be seriously reviewed for the next update of AG-181, if only because it is now possible to make every item on a critical service redundant - H1 card, power conditioner, field cable and device connection. This doesn't eliminate the possible software interaction issues that <rezabejd> mentions (which may prevent the mixing of other loops on that segment) but it certainly improves the physical layer hardware availability. Fear of hardware failure & subsequent loss of data/control/production was a key factor in many users adopting resticted practices for their initial FF deployments.