Distance problem

[AusLee]

Hi,

I have some manually operated switches that are used to activate a large exterior lighting installation.

The switches are in a building that is 800m away from the switchboard that supplies the lights.

My first approach is to install those switches in a panel where they are and run a 16 core control cable from there to the switchboard and have the PLC that will receive those switches as input and figure out which circuits to activaye in that switchboard as well.

The question is would those 800m be any problem? can you please recommend an approriate AWG or mm sq. cross section of each core?

I think this is better than having the PLC with the switches 800m away from the switchboard. In that case the outputs of the PLC would have to be driven 800m.

I have considered using an ehternet switch at the location of the switches and at the location of the switchboard to benefit from an existing data network, but it seems the cost of the switches and the additional PLC required at the second end to decode the message and take action will be more than the cost of the multi-core control cable...

Any hand drawn sketch much appreciated

 

[Skogsgurra]

It will be a problem if the installation is large. And you say it is. But not haow large. So it is not possible to say anything about AWG.

Install a PLC with relay outputs and control it with 24 V DC. Filter the inputs if necessary. Or use 110 V DC and a voltage divider for each input. That will cover any distance. If every single light circuit is less than around one kW, you can probably use the PLC relay contacts directly. But I would install contactors that takes care of inrush current to the lights and increases power range.

Alternatively, use interposing DC coil relays and control contactors without the PLC.

I do not think that a sketch will be necessary.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[djs]

I would send the outputs 800m rather than the inputs. Most PLC inputs are rather high impedance, and with cable runs over long distances interference and induction pickup are a problem. Most loads feed from a PLC will be much lower impedance and not subject to the same interference.

 

[Skogsgurra]

Backwards thinking. To say the least. "Large lighting load" 800 m?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[AusLee]

Hi thanks for the replies.

The light circuits are more than a KW each so I'll be using contactors.

The PLC is required because there is a rather complicated selection of what lights go ON based on the inputs and the ladder logic diagram is already covered. It's just the physical implementation.

Can you please djs or Skosgurra just sketch the system you're proposing?

Many thanks if that's possible,

 

[djs]

Mount the PLC close to the manually operated switches. Each switch will probably be connected to one discreet input of the PLC. Each remote lighting contactor will be wired to one output (usually of the relay type) of the PLC. In this manner the outputs of the PLC will be run 800 meters from the location where the manual switches are to where the lighting contactors are.

Unless you have some very bad wiring pratices, the induction along the 800 meter cabling will not be enough to cause the lighting contactors to malfunction. You might have to use bigger than normal cable to reduce voltage drop.

 

[RajeevCell]

I have considered using an ehternet switch at the location of the switches and at the location of the switchboard to benefit from an existing data network, but it seems the cost of the switches and the additional PLC required at the second end to decode the message and take action will be more than the cost of the multi-core control cable...

A sixteen-core control cable will off-course be cheaper than an additional PLC. But you said you are designing a "large exterior lighting installation". Other information also indicate a fair-sized project - 800m distance between manual panel and distribution-board, multiple circuits each with kW-level load, etc. Why are you shying away from the two-PLC (or one PLC with distributed I/O) approach?

You said there is already a communication channel there. Assuming it is a fiber-optic link, you can utilize any of the inexpensive PLCs with Ethernet capabilities, like Siemens S7-1200. Ethernet to fiber converters are also very cheap.

 

[Stablefordd]

If you have a fibre optic cable near by why not use a fibre optic multiplexer of the type supplied by Weed instruments. 1 fibre for upto 10 signals- and no programming.
I currently looking at them on a tunnel for distant io.

Regards

Del