Simple Ladder Logic Problem

[Lancie1]

The notion of people running round like headless chickens after a power outage, as suggested earlier, does not happen on systems that have been programmed to correctly recover, or to enter "safe state" from power outages.

Read what happened witht he alarms in the control room during the Browns Ferry Nuclear Plant fire. It was as close to headless chickens as you ever want to see. A manager finally oredered the operators to ignore all the alarms and only work to keep the cooling pumps running. I bet the alarms at Fukushima caused the same confusion, even though they had been tested to the nth degree.

"Control board indicating lights were randomly glowing brightly, dimming, and going out; numerous alarms occurring; and smoke coming from beneath panel 9-3, which is the control panel for the emergency core cooling system (ECCS). The operator shut down equipment that he determined was not needed, only to have them restart again."
The flashing lights, alarms, smoke and continual restarting of the ECCS pumps went on for a full ten minutes before the reactor operators began to wonder whether it might be prudent to shut down the reactors.

http://www.ccnr.org/browns_ferry.html#pr

 

[Geospark]

Programmer: Yes, an OTL here, an OTU there...

Operator: What happens if I do this?...

Entire Plant:


Have we really gone from discussing the use, or misuse of retentive instructions, for retaining an alarm condition, to them supposedly being the root cause of a nuclear meltdown?

I have my views on what I've read so far, but for now I'm off to sleep on them...

G.

p.s. Sorry for all the smilies Phil!

 

[El Matador6667]

Audible alarm is different from a strobe, indicator light, etc.

Even though I said I wouldn't - I'm going to continue to disagree on this...

A horn, or audible alarm, is NEVER provided in any respectable control system to "annoy" operators. It is there for a reason, to "alert" them to a problem that is either affecting production, or more importantly, a potential danger to personnel or equipment.

From a health and safety point of view, it is essential, and probably mandatory in most of the world, that audible alarm devices cannot be silenced by cycling power.

IMHO a control system should power back up in the same state that it went off, although the powering up should instigate placing all controlled equipment into a "safe" state. A horn blaring is not classed as an "unsafe" state, it is simply an indication of the state that existed when the power went off.!! The programmed system is informing personnel that an alarm condition exists which, in all other circumstances, has to be acknowledged, to guarantee that the operators have "seen" that circumstance.

The notion of people running round like headless chickens after a power outage, as suggested earlier, does not happen on systems that have been programmed to correctly recover, or to enter "safe state" from power outages. On many systems I have been involved with, acknowledging or muting alarms is logged, together with "who" and "when". "Power Cycle" is not a logged-on operator, so can't mute the alarms.

I'm currently working on a HTHW- High Temp Hot Water (375 deg) system for a University campus. And on the boiler control panel there is an alarm horn that will be set at 95 db (as per design engineer), this is about 1 ft from the HMI, there are also various strobes connected in parallel to the horn.
I see no problem at all by not latching the horn.
I could just picture a frustrated operator either cutting the wires to the horn or smashing it with a hammer if it is going off in his face.
I installed an alarm silence button to break the seal-in logic to the Horn.
However, the strobes are latching logic.
I see no negative issues with this procedure.
You can't miss the strobes, but they don't annoy the heck out of you as a horn would.
The horn is doing it's job by alerting the operator to an alarm condition while not in front of the panel, and the strobes are doing their jobs of indicating the unsafe state, or previously unsafe state without breaking someone's ear drums.

 

[Lancie1]

there is an alarm horn that will be set at 95 db (as per design engineer), this is about 1 ft from the HMI,

Anything over 95 dB for a total of 4 hours a day is a violation of OSHA 1910.96 occupational noise exposure standards. You should try to get that horn moved farther away from the operator station. Otherwise after a few days you will find it muffled with foam, or worse. Locate the horn where the operator might be when he is away from the HMI, not at the HMI.

Also give the horn a 0.6 second ON, 1 second OFF cycle. A cycling horn can be heard the same distance, but is only about 1/3 as irritating as a continous blast.