1793   Behaviour of output contacts when LN is in blocked mode

Created: 28 Sep 2021

Status: Discussion (red)

Part: Part 7-4 (2020; Edition 2.1)


Page: 397

Clause: Annex A

Paragraph: Table A.1


This request for clarification deals with s/c "boundary" Logical Nodes which are connected to the process.

As soon as the mode of such LN is set to "blocked", "test/blocked" respectively, no (wired) output data (digital by relays or analogue setpoints) will be issued to the process.

It is not defined whether
the concerned outputs shall turn to their idle/default states or
the status of the concerned outputs shall be frozen in the present value.


Consider transient and non-transient controls.

Proposal (for contacts):
-For transient controls the contact to assume its default position (open if normally open and closed if normally closed). Allow to finalize a control in progress at the time of activating "blocked" or "test/blocked" mode to prevent from intermediate equipment positions.
- For non-transient controls the contact to remain in its state.

Discussion Created Status
Agree to add a line in Table A.2 and to complement the wording in Table A.1.

items to consider in the new statement:
- binary outputs (contacts)
- analogue outputs (current loops)
- LPDO LN class in preparation
- external DO connected via bus (intelligent terminals, intelligent breaker units, e.g.)
04 Oct 21 Discussion (red)
Just a correction of wording: we should use the terms like in part 7-3: persistent output and pulse output (OutputSignalKind).

We may consider a formal change in table A.2 of 7-4 or adding a row to distinguish "output to process" from software and relay output (self-holding relay).
01 Oct 21 Discussion (red)
Tissue accepted and open for discussion. 29 Sep 21 Accepted


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