can anyone help?

hi there,
im doing an intro course to plcs at the moment and am after a little help.

what are the implications of programming normally closed, field devices???

what are the hardware changes necessary when converting a relay controlled process to a programmable controlled process???

any help would be greatly appreciated.

thanks

hardy

Finally, the HNC course seems to have added a few new questions, and they've abandoned the question about the differences between unitary, modular, and rack-mounted PLCs.

For your first question, think about the modes of failure of devices wired normally open versus those wired normally closed.

For your second question, identify the hardware devices required to implement a relay-based control system. Then identify the hardware required to implement a PLC-based control system. Don't forget to include things like wire, terminals, enclosures, nuts & bolts, conduit, etc.

hey steve,
thanks for the reply mate. when i said i was doing an intro course i really meant intro. its a 4 week module so your reply is a little over my head. if you could maybe translate for me thatt would be great.

cheers
hardy

A normally closed device is one that closes if power is lost. A normall open on is open when power is lost. For example, on our furnaces with H2 for an atmosphere, we use a normally open N2 solenoid, so if we lose power, it will dump nitrogen into it, and prevent it from going BOOM. Extend this to figure out your question.

Relay control is where you have a series of relays (lots of em possibly) that control logic. Think how that differs from what a PLC is and does.

Matt

Put the device on the bench with no power applied.
The state of the contacts as seen now are the normal states.N/O N/C.

Hardy,

also take into account what can happen if a wire breaks. Suppose you use a normally open contact for a stop function. Try to stop the device if the wire is broken skuller .

Kind regards,