In case of a Shutdown Pushbotton connected to a Safety PLC (1oo2, TMR, or whatever), how should be the contact of the pushbotton? Normally open or Normally close? In my opinion a normally close contact can cause an accidental trip due to a broken loop situation, but in case of a normally open contact, what would happen if a broken loop is present, no maintenance is applied an in a short period the operator needs to actuate the pushbotton?

All your clarifications will be very helpfull!
Thanks in advance.

 

Wiring in the normal close loop configuration is referred to as fail safe. If your loop opens then the action is committed. To prevent accidental trips you can do this with redundant loops and compare for differences. But in alot of safety systems the requirement is to be connected 'failsafe'. Yes this does have a chance of a unintended operation if anything goes wrong with loop but you will sure have to react to it. Of course if you cables short out ( which isn't the most likely thing to happen) you will not know it. So if you want failsafe with a proof you need to run to circuits to the pushbutton with 2 sets of contacts. At the PLC level check that if one changes state the other also changes state within say 1/10 sec. If not then give an alarm that this loop is in error.

C. Petersen

 

Responding to C. Peterson's Jul 30, 2:09pm comments...

Fault-diagnosis (opens-shorts-ground, fail-to-danger, fail-to-safe) of inputs is the only true method of improving ESD (now SIS) performance.
My paper "Probabilistic Risk Assessment Equipment Safety Systems" mathematically assesses the N/O vs N/C argument in great detail. Furthermore the difference between "fail-safe" (a misnomer) and
"production-safe" is examined. Finally, the fallacy of TMR is exposed.

For a copy contact me, or:

"Hazard Prevention Magazine, Vol 35, No. 3, 3rd Qtr, 1999" (ISSN-0743-8826)

An official publication of The System Safety Society Email: [email protected]

Publisher: AS Waterman Inc.
Email: [email protected]

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])