Need for terminal strips? 2

[controlnovice]

We are designing the installation of a DCS in a very tight spot.

The existing DCS cabinets, and every one I've seen, has terminals inside the same DCS cabinet to which the field wires are terminated. Then from the terminal strip, wires continue to the DCS cards.

What is the reasoning for the terminal strips? Can we take the field wiring directly to the DCS cards to save cabinet space (no terminal strips)?

We have a marshalling panel located in another room where the field wires already terminate. We were going to come off the marshalling panel terminals and run to the new DCS cabinet (straight to the DCS cards).

Am I missing something?

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This is normally the space where people post something insightful.

 

[itsmoked]

Often the terminal blocks serve as strain relief for the cards.

If you transfer any strain to those cards...pfffft!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[davidbeach]

If you can close the door when you are done and walk away forever, sure go ahead. If you think you'll ever have to open the door and do any troubleshooting, maintenance, or modifications you will regret not having terminal blocks.

 

[Skogsgurra]

It depends a lot if you are doing this for a customer - then keep to good practice. Use terminal strips.
If you are doing it for your employer. Same thing.
If you do it for your own use only. Then do what you find convenient. But do not forget code.
Strap the cables to internal structure of cabinet for strain relief. Do not use self adhesive anchors. They do not stay more than months.

Gunnar Englund

http://www.gke.org

 

[ScottyUK]

Field cabling into our Foxboro I/A Series DCS was terminated directly to the I/O under the direction of prime contractor NEI to save a little bit of space, plus time and money. It is a maintenance nightmare and both engineering and maintenance curse the idiots responsible for this installation on a daily basis. Field cabling in our case is almost exclusively armoured multicores, some with diameters approaching 2", and removing or adding anything in the cramped termination area of the DCS enclosure is difficult and on occasions impossible. We have had to abandon precious spare I/O because it is simply not possible to connect to it.

Think very carefully before doing what you are currently considering. Your customer will rightly blame you for the resultant mess, plus knowingly designing something which is unfit for purpose is hardly what a professional engineer should be aspiring to.

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Sometimes I only open my mouth to swap feet...

 

[dogleg43]

Here are my two main reasons FOR having the terminal strip:

1.) Let's say an input module has 16 points but you are only using 12 of them and the rest are spare. Having the terminal strips you are able to (neatly) wire all 16 points to the terminals now. In the future when you need to use up some of the spares you are able to complete the job quicker. Trying to add a wire into the the bundle going up to the wiring arm of the input module can never be done as neatly later on as when orignally installed.

2) When your company wants to upgrade to a newer version of the I/O modules you will not need to disturb and field wiring. All that need's to be done is change out the I/O chassis.

In short, pay me now or pay me later.

 

[Mike2468]

And don't forget, if the "big guy" on third shift strips a terminal block screw when moving a wire, it is a $5 mistake. If he strips a terminal block that is part of a PLC output card, it is a $500 mistake and it takes much more time to get everything repaired and running again.

 

[dpc]

If you'll allow me to pile on - interposing terminal blocks are a really good idea, for all of the reasons stated above. Field wiring conductors are generally larger than panel wiring and the PLC/DCS I/O modules terminals are generally not really large enough or adequately spaced. It's pretty easy to break an output card while you are trying to train in a bunch of #12 wire.

Also, you can use sliding link or disconnecting-type terminal blocks to allow isolation of the I/O cards for testing/troubleshooting.