Backplane

If a person puts a card designed for 480 volts into a plc slot that is designed for 120 volts will the backplane be damaged why or why not?

To my understanding - No - no damage will occur, any one please correct me if I am wrong.

The backplane of the PLC is having 5V to communicate to the modules. The Voltage rating of the modules is meant for how much it can switch the real world outputs. So if you put a 24 volt module, it is limited to 24 only to switch,

or else if you have a 220V module it switches is 220V

How much real-world PLC experience does your teacher have? There aren't a lot of PLC I/O modules that switch 480 volts, and I'm not aware of any with backplane voltages of either 120 or 480 VAC. As Chavak points out, the field voltage is isolated from the backplane voltage.

Now, if your teacher is talking about field side voltage, then the question becomes, if you connect a device rated for 480 volts to a 120 volt supply, will you do any damage? How about if you connect a device rated for 120 volts to a 480 volt supply?

Tell your teacher that he might reword this question thus: In a system that can be operated on either 115 or 230 VAC, why is the default configuration 230 VAC? Corollary: If, in the above question, the default configuration is NOT for 230 VAC, why should you change vendors?

480v backplane voltage

Hi
I will go out on a limb here and say, THERE IS NO SUCH THING.

I know of no PLC that has slots dedicated for 120v, 220v or 480v, the backplane operating voltage is lower voltage, around 5v.
There must be a miscomunications with the question.

Roger

240vac output

I have seen lots of 240 volt output cards in allen bradley plc 5's

Roger is right!

PLC back planes have only low voltage DC signal and power lines. Most PLCs have only 5 volts on the back plane and DC-DC power converters are used to get other voltage levels. Output cards require external power supplies. Any switching of AC power is done on an isolated part of the the I/O card. The AC is never connected to the back plane.

Cards

eastkodakguy

Yes, so have I but the question referred to a slot being designed for 120 volts and inferred that their were slots for 480 volts.
Have you ever seen an Allen Bradley chassis with slots designed for 120 volts? No you haven't.

Roger

Based on that 'why or why not' phrase at the end of the post, I'm of the opinion that this is a question from a student. I'm hoping that wstone19 has paraphrased the question and not posted it verbatim from his test or homework assignment.

If it is taken verbatim from the test, then wstone19 and everyone else in the class ought to demand to see the instructor's credentials. The instructor ought to be required to demonstrate a better understanding of the subjects he's teaching before being turned loose on an unsuspecting student body.

Steve

Good point Steve, right on.

Roger

some silliness required

Well, if the question is:

If a person puts a card designed for 480 volts into a plc slot that is designed for 120 volts will the backplane be damaged why or why not?

Then, lets suppose that one answer might be that it
OK to do this (since its already been established that this
is not a real-world question)
but,

the control components below the rack need to be guarded
because the extra 360 volts fall out of the card.

jdbrandt,

Is that true for both sourcing and sinking cards?

wstone, physical installation will not damage the card. As long as the 480 VAC isn't wired up, everything will be OK. If you tie into 480 Volt power, though, you will likely become aquainted with the unique smell of burning silicon!

This is an incredibly silly question, for a variety of reasons, and exhibits little or no understanding on the part of your instructor.

1) PLC slots are not designed for specific voltages, or specific functions. The slots in a PLC rack are specifically designed to allow interchange of a variety of modules for a variety of functions and ratings.

2) Backplanes are used to transmit logic level voltage, power for the I/O card (logic side only, usually not the field side power) and data between the CPU and the I/O cards. This is usually 5 VDC, with occasionally 24 VDC or ±15 VDC. I can't imagine the havoc 480 VAC would create on the data communcations if it was run on the backplane parallel to the logic signals!

3) I don't recall seeing higher than 240 VAC rated I/O cards. I suppose you could use relay output cards with contacts rated for 600 VAC, but this would be very bad practice. Normal practice in both the US and Europe is to use low voltage (24 VDC or 120 VAC is most comon) for control, and use separate relays or contacors to switch the higher power loads. Typically, 480 VAC is only used for motor or lighting power, and at amperage high enough you wouldn't want it on a PLC.

4) The heat generated by an electrical device is directly poroportional to the power loss and the current through it. If you exceed the voltage rating on a device, the current and heat dissipation go up as the square of the voltage. If you put 480 V on any load rated for 120 V the heat will be 16 times the heat at rated load. See why the smoke gets out?

5) If you have a higher voltage on a device than rated, you have the chance of shorting through the insulation and arcing to ground or other components. This is a very bad thing!

6) Virtually all PLC I/O cards isolate field voltage from the backplane - if you find one that doesn't don't buy it!

for roger

I could have done without the ****y remark

oops

that is p i s s y
not the other that that has a U in it
lol

Remarks

eastkodakguy-

I was thinking of another 4 letter word that begins with 'S', but I guess that would have been spelled wrong, but, maybe thats why the 'y' didn't get the '*'.


Wow! I must be bored.