fail-safe system
fail-safe system
(OP)
suppose you want to design a failsafe system.what position of contact should be used for signalling and interlocking purposes?N.O or N.C position? what is the advantages and disadvantages of each one?
thanx.
thanx.






RE: fail-safe system
It seems a very important consideration is not so much how it works normally, but results of misoperation when part of the system fails.
One scheme uses 5 triacs for a basic 120V AC-control channel, like for, say, a solenoid valve. Four triacs are connected in a series-parallel arrangement, allowing correct operation with a single triac failure—open or shorted. The fifth triac is intended to crowbar a fuse {last-ditch} on one side of the four-triac network.
RE: fail-safe system
As far as switchgear goes, instead of the traditional energise to trip you would need to use an undervoltage release (de-energise to trip, allow close when energised). These are commonly available for LV switchgear and increasingly so these days for MV switchgear as well.
For control relays you can get a SIL 3 "relay" which has 3 coils in parallel and a set of contacts from each coil in series (so that it looks like a single relay). These are used for interfacing to other systems.
Failsafe systems in switchgear can be a pain sometimes.
RE: fail-safe system
The IEEE Std 100 Dictionary defines a "Fail-Safe Circuit" as a circuit in which the occurrence of a failure causes a specified set of outputs of the circuit to assume predetermined values.
Also, a definition of a Fail-Safe System. A system whose failures can only be, or are to an acceptable extent, benign failures.
RE: fail-safe system
RE: fail-safe system
Hybrid systems are sometimes used in power generation facilities, incorporating a two-out-of-three de-energize to trip voting logic for sensing circuits but a one out of two energize to trip scheme for actual tripping.
RE: fail-safe system
RE: fail-safe system
Sometimes on such plants it is necessary for a shutdown system to trip an MV circuit breaker and you have to explain to the instrument engineer that your equipment requires control power to trip and he points an accusing finger at you and says "ooooh but thats not failsafe".
Dont know about North American equipment, but increasingly IEC makes of MV switchgear can be fitted with UV release coils so that it is possible to provide a failsafe tripping mechanism if necessary.
Concur with dpc re substation installations.
RE: fail-safe system
some things should fail on, fail off, fail open, fail close -- depending upon the application; even if you purchase a piece of equipment that has a fail-safe function, you should still evaluate (preferably before purchase) to determine if it fits the fail-safe mode you need... [some applications may be dictated by code, so you should also determine what applies to your application, i.e. fire pump service]
In some respects, this involves a separate study of your systems to make sure proper fail-safe modes are adequately designed into your systems..
to answer your question, both N.O. and N.C. contacts have application -- which to use will depend upon what needs to be done in failure mode...