PLC hook up check

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Hi all,
Just thought I'd ask you if what I have done sounds right. I'm hooking up a Allen Bradley PLC SLC5/03 with 24 vdc cards (dc sinking and dc sourcing)in my simulator I built and want to double check if my wiring sounds right because this PLC belongs to my workplace. I looked up the schematics for the cards and don't want to get super technical as the writing could go on and on, but looking for a rough check. Here goes,
1. I have 120VAC coming from plug to term. strip to fuse to PLC for main power.
2. I have the ground of the 120VAC going to term. strip to PLC marked "chassis ground".
3. I have 120 VAC from term strip to 24 vdc power supply to a d.p.s.t. rocker arm switch that sends 24 vdc to an led with a resistor inline and the other side of the switch sends 24 vdc to the input card.
4. The 24vdc sent to the above has all the leds neg's tied together and goes to a fuse and then to the dc com on the input card mentioned in #3.
5. As for the output card I have all the neg's tied together like mentioned above to a fuse to the dc com screw in the output card.
6. Output card positives go to a inline resistor and then to a output led.

I do have a fan that runs whenever the plug is receiving power (fused) to prevent overheating.

Let me know if you think this should cause no harm to the PLC or cards, backplane, etc... If anyone would care to guesstimate what amp fuse for the PLC main power I'm listening, otherwise I'll run power through a amp meter and go from there.

Thanks for you input (no pun intended, well, I guess so)
Dr Oldz
 
One more question

I was showed to be logged in and when I posted my question (above) it shows me as a guest. What did I do wrong? This happened to me once before. I don't get it
Thanks,
Dr Oldz
 
hmm

this sounds more like an engineering quiz, but, I'll bite.

First of all, I would not fuse the 24VDC return line, and
certainly not if it were grounded.

Second, the AC supply to the PLC should have a fuse, something
small, say 3 amps. You'll find better direction if you
read the installation manual, but, 3 amps should do.

And, you mention a fan... generally, this stuff does not
need 'air-over' ventilation. In extreme cases, you might
want a fan, if the enclosure is small and hot.

As far as simulation is concerned, simply wire neatly, and
keep track of everything. It can get out of hand in a big
hurry, as you've no doubt already seen.

And, by the way, there are 24VDC supply terminals on the 1746-P2
power supply that should be more than enough to run switches and
lights for a simulator... (if you have a 1746-P2, that is) so
you don't even need the extra 24VDC power supply.
 
As I read your post, I immediately came up with the first three items jdbrandt pointed out and I agree with all his statements.

Additionally, you mentioned "dc sinking and dc sourcing" but did not specify whether they were input or output specific or some combination of both. I point this out because if for some reason you are interfacing with open collector inputs and outputs (on servo drives, for example) you will need pay extra attention to your I/O wiring. You may not be able to simply gang all you commons and power wires together as you have described.

My observation here may be purely academic since you are building a simulator, but I thought it worth pointing out anyway.

Steve
 
replies to comments

Hello and thanks to those that have given inputs.

To answer JD
*The 24 vdc is not grounded, the 120vac is connected to the plc itself to the spot labelled chassis ground (hopefully this internally grounded to the chassis because I physically didn't).

*The enclosure is 2' X 1' X 9" but I wanted to be sure I had no failures due to a cost savings of $5.00 for the price of a fan.

* You're right about the amount of wires.

* Do you feel that I can wire the fuse with the neg 24vdc since actually dc electric flows from neg to pos., I think I can since it's still a circuit no matter where you put the fuse.

* Thank you again, it's not the first time you've helped, unfortunately I've only been able to help a few, but hopefully my time will come to help others.

To answer Steve

* The dc sinking is the input and the sourcing is the output card. Luckily I have another unit running a assembly line that I use for reference (voltages, wiring, etc... when uncertain)

* I am glad you mentioned about the servo because while I may not use it on this simulator I do plan on building a servo table with a fellow tech and need to install a PLC and everything since we are building a small satellite station. Thanks for that info, I will research this more in depth.

* Thank you for your time to write and collect your thoughts on the subject. To anyone else who posts I would like to thank you now for your time and knowledge.

Signing off now,
Dr Oldz
 
Droldz wrote:
"* Do you feel that I can wire the fuse with the neg 24vdc since actually dc electric flows from neg to pos., I think I can since it's still a circuit no matter where you put the fuse."
(n)

Mayby it's just that I've been zapped :eek: by poor installations/modifications in the past but, PLEASE get into the habit of placing the fuse on the hot side of a circuit.....
Electrical theory never destroyed equipment and never prevented the 'human grounding rod effect'.

my 2 cents :cool:
 
drold,

Two things to address.

First, you should consider grounding the common side of your 24 VDC power supply. This is primarily a safety issue rather than circuit theory. By not doing so you are creating a "floating" circuit with respect to ground. As such, if someone or something were to make contact with both the low voltage side of the supply and ground, it could be somewhat unpleasant.

Not to say that it isn't done. It is. However it is more widely practiced to ground the common and, furthermore, can make troubleshooting easier (when checking for voltage, one can use ground as a reference rather than having to remember to locate the common wire.)

Second, David Bennett is right. When you only use one fuse, place it on the "high voltage" side rather than the "low voltage" side of your circuit.

While it is true that "electron flow" is in the opposite direction of conventional, or "hole flow", the fuse location is determined with respect to the source of the potential, not the source of ther current.

The reason for this is safety. It really has nothing to do with the current itself. If our circuit were to open on the low voltage side, the system would halt operation (as desired), but all affected equipment would still be at full potential. This is obviously unsafe since it would be very easy for someone or something to inadvertently make contact with the equipment and the low voltage side at the same time. Not good.

Steve
 
Points well taken

Steve and David,
Points well taken with your electrical arguement, now I see the light. I feel stupid to say I didn't think of it, or that I could ground the 24 volts to the chassis. Now should I ground this to the metal project box I'm using and if so, can I and should I ground the 120 line voltage to the project box as well?
So it sounds like I need to use 16 fuses for the outputs correct but only one for the inputs since I have daisy chained the hot going to each switch and only if that switch is thrown it will send juice to the indicator led and the input card as well (both on different poles of the switch). Just a little tidbit from my electrical teacher from back when, it only takes 500mA (1/2 of a Amp) across your heart to stop it. Meaning electricity through a cut finger with connection to your blood to the other hand with blood connection. Looks like I have more work to do. Thanks to all.
Dr Oldz
 
Re: Points well taken

droldz said:
Just a little tidbit from my electrical teacher from back when, it only takes 500mA (1/2 of a Amp) across your heart to stop it. Meaning electricity through a cut finger with connection to your blood to the other hand with blood connection

This won a Darwin Award back in '99 entitled "Resistance is Futile" <---click to read

beerchug

-Eric
 
Let's not start "thinking" that fuses protect people. Fuses protect circuits and equipment, GFCI's protect people. Make sure you fuse any ungrounded feeders, either 24 or 120 volt. Sizing depends upon demand of the equipment and capacity of components. Add the amps requirements of the PLC and the 24 volt supply, this is the size of the 120 volt primary fuse. The amps rating for the output of the 24 volt supply is the fuse size for the 24 volt circuit. Then check the ratings on the individual input/output cards and size those accordingly. Weather you fuse each individual point or just the common, do not exceed the rating for the card.
 

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