Reading and Combining Protective Trip Contacts

[mls1]

I have about a dozen auxilliary trip contacts for a small generator for such things as low hydraulic oil pressure, exciter diode failure, things of that nature. These will all trip a lock-out relay (LOR) but I'd also like to get status via a PLC so there will be immediate indication of what caused the trip. I have only one contact for each. I see two ways to do this:

1) Auctioneer the contacts with diodes so they are segregated into PLC inputs but combined to trip the LOR

2) Read the contacts on the PLC and setup an immediate I/O scheme (essentially an interrupt) that will trigger and output that trips the LOR.

The second scheme relies on the PLC for some of the protection. Any thoughts on a preferred method?

 

[stevenal]

We've had PLCs lock up, and would not rely on them for protection. The PLC would introduce added time. We use the diode method, selecting diodes with specs that greatly exceed the expected duty. And they're still cheap.

 

[controlnovice]

I need some education here:

What do you mean by "Auctioneer the contacts with diodes..."?

______________________________________________________________________________
This is normally the space where people post something insightful.

 

[mls1]

controlnovice - it just means to put them in parallel through diodes on a common bus. If you just put them in parallel without the diodes then there is no way to read the contacts individually (i.e. if one contact closes it would be seen by all discrete inputs). In logic terms, the diodes form an OR gate.

 

[itsmoked]

Here's two common layouts. Hope this helps.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[stevenal]

I have never heard the word "auctioneer" used in this context, but understood by the context of the question.

Our circuits look like the second diagram above, but without the pull down resistor. The LOR coil (at the shutdown position above) has a low enough impedance that the resistor isn't required.