Fail safe vs Non fail safe 1

[tulum]

I have a ladder diagram for an MCC which has both fail safe and none fail safe contacts in the schematic. However, there is no differentiation between the two.

i.e...

Fail safe:

1. A contact that is open when no power is applied to the unit
2. It swithces to closed on power up
3. and trips open on UV or GF

This contact is drawn as a normally open contact.

Non-fail safe:

1. A contact that is open when no power is applied to the unit,
2. It STAYS OPEN on power up,
3. and trips CLOSED on GF

This contact is also drawn as a normally open contact.

My question is:

Is there any way to redraw this ladder so it is obvious whether the contact is Fail safe or not? I can just write failsafe beside the contact, but is there any standard symbol?

 

[dpc]

You need to pick a convention for showing contact state and stick with consistently.

Traditionally, contacts were shown in schematics in their **de-energized**, "off-the-shelf" condition. This works well when combined with a legend that describes the conditions under which the contact will be closed **in your circuit**.

The hard part is staying consistent with this presentation when dealing with "fail-safe" devices. But it can be done.

 

[CastleHydro]

Whether a contact is shown normally open or normally closed is purely representational of the physical arrangement of the switch in a state in which all parts of the circuit are de-energised. As DPC states, this is a choice of convention, but to choose any other convention would tend to be very confusing and would need to be very clearly stated on the diagram.

Fail-safe or Not Fail-safe, is purely to do with how the rest of the circuit is wired. If a breaker requires a closure to trip, then the "fail-safe" mode would be normally closed, however if the contact were operating a electrically held contactor, then the normally open contact would be "fail-safe".

If a particular circuit was specifically designed to provide a "fail-safe" operation, then it could be noted as such, but this should not be confused in any way with whether to depict the contact as NO or NC.

JW

 

[jbartos]

Suggestion: Reference:
IEEE Std 100 Dictionary
Definition of Fail-Safe System:
A system whose failures can only be, or are to an acceptable extent, benign failures.
There are other definition listed; however, all tend to be compatible with the above definition.
Therefore, the fail-safe condition is often meaningful system-wise. Various fail safe situations would have to be presented on drawings with corresponding markups of fail-safe equipment including the equipment modes of operation.

 

[GerH]

If you have symbols assocaited with the cpntacts you could use a symbols naming convention that indicates if the contact is Close to Trip or Open to Trip.

e.g. I12.5 Overcurrent Trip (CTT), I12.6 Emergency Stop (OTT)

 

[dbaird]

Are you refering to "Fail-To-Safe" operation?

Fail-To-Safe devices are designed such that a component failure will cause the device to attain rest in a safe condition. This term is generally applied to safety circuit monitoring relays. Such relays are often designed to feature reduncy, self-dianostics, and positive guided contacts.


David Baird
mrbaird@hotmail.com

Sr Controls Engineer

EET degree.

Journeyman Electrician.

 

[jbartos]

Comment: Even though contacts may be marked as fail safe, often, various systems are classified as fail-safe. Then, engineers, designers and technicians/electricians have to deal with such system as with fail safe, including identifying contacts, hardware, operational scenarios, etc. There are many fail-safe systems on nuclear power plant projects.