Help on fail safe s7 plc

Hi every body

How the fail safe signals bypassed (for maintenance or start-up)without affecting the program safety? Could any body help me for that

Thank you in advance for your help

Best regards

Elkawafi

Originally posted by elkawafi
How the fail safe signals

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Sorry, I'm out, what means fail safe signals ?

elkawafi, I'm afraid your question isn't very clear. Are you asking how to bypass hardwired failsafe fault circuits without affecting safety? The answer is, you can't. Please be a little more specific in your questions, and give a little more background on the machine and your problem.

Seppoalanen, here in the states we use the term "failsafe" to indicate a signal that uses a noramlly closed contact, and the contact opens on a fault. It is considered more reliable for fault condition indications because if there is a broken wire or loose connection it will show up as a fault and the machine will stop. With normally open contacts that clse on a fault a broken wire means the fault will never register and unpleasant things could result.

Normally open contacts can be used in fail-safe circuits in certain cases. For example, a vacuum switch in a system that normally runs under a vacuum and an alarm is a loss of vacuum or increasing pressure. The shelf state of the switch is no pressure positive or negative. Therefore in order to be fail-safe the switch would be wired using the N.O. contacts. If you were interested in verifying the wiring prior to start up then the N.C. contacts would be used. But how do you verify that you do not have a broken wire while running? Then there’s the case of a switch that is being powered by a separate supply. What happens if the supply is lost? If it is a level switch that is detecting high level then the N.C. contacts are used. If the supply is lost then the switch will not change state. You could run the circuit energized in which case the N.O. contacts would be used. So what is normal anyway?

(guest),

You make some excellent points. I hadn't really thought of most of your examples for failsafe applications.

To address your last question "So what is normal anyway?" - normal typically refers to the "shelf state". Obviously any "normal" condition for a given application can be different from the next application, so we have to have a common reference point. I think most will agree that "shelf state" or the state when no external energy is applied has to be that common reference.

BTW - you are obviously a seasoned veteran to the electrical wars, so how about becoming a member of the site?

Steve

Thanks Tom and Steve !
I understand 'normally closed/open' contacts as follows:
- Pressure switch, in 0-pressure
- Thermostat, in 0-kelvin
- Level sw., as empty tank
- relay, as on the shop-shelf, without wiring
etc..