PLC Fusing

Bolt

Member
Join Date
Jun 2007
Location
TX
Posts
143
I'm putting together a new PLC setup (perhaps my most complicated one so far) and have some questions about fusing. Lets start at the beginning.


Power supply. 30W. 0.9A input at 100V. 0.6A at 240. Will run it at 120V. So it will draw a bit under 0.9A at full load. 1.3A output at 24VDC. Where am I best off fusing it? I will be running a PLC on this, (+/- 22W, IIRC) and it's inputs. Do I need to fuse the power supply on both sides, in and out? How big do the fuses need to be, and of what type, fast acting, what's the difference between all the letters, AGC, AGX, etc.

Inputs, some are float switches, pushbuttons, and a contact from a motor drive. Fuse them seperate or together? Just off the powersupply and that's it?

Outputs, 120VAC for the most part, most go to relays, and contactors. Is a fuse on the common(s) enough? Max amp on the common is 8A, and each relay output is 2A.

On a somewhat related topic, when am I best off using transistor out/vs a relay output? What does it take to run an external relay off of a transistor output? How about indicator lights, LED andincadescent?
 
Bolt said:
I'm putting together a new PLC setup (perhaps my most complicated one so far) and have some questions about fusing. Lets start at the beginning.


Power supply. 30W. 0.9A input at 100V. 0.6A at 240. Will run it at 120V. So it will draw a bit under 0.9A at full load. 1.3A output at 24VDC. Where am I best off fusing it? I will be running a PLC on this, (+/- 22W, IIRC) and it's inputs. Do I need to fuse the power supply on both sides, in and out? How big do the fuses need to be, and of what type, fast acting, what's the difference between all the letters, AGC, AGX, etc.

Inputs, some are float switches, pushbuttons, and a contact from a motor drive. Fuse them seperate or together? Just off the powersupply and that's it?

Outputs, 120VAC for the most part, most go to relays, and contactors. Is a fuse on the common(s) enough? Max amp on the common is 8A, and each relay output is 2A.

On a somewhat related topic, when am I best off using transistor out/vs a relay output? What does it take to run an external relay off of a transistor output? How about indicator lights, LED andincadescent?

Hello there Bolt,

I fuse the power supply on both sides, one fuse on the input, (the power supply paperwork should suggest an amp rating). One fuse between the output of the power supply and the PLC power in, again refer to the PLC paperwork re amp rating. One fuse for the 24vdc common, 1amp, fast acting, (I've never had a problem). Most PLC's are perfectly happy with a dead short, your fuse is just protecting the WIRES and the power supply in case of a complete foreign ground.

I like a fuse on EACH output, fast acting, match the rating of the relay out, use wire large enough to handle about double the rating of the relay out. (I use 16gua on all pilot duty wiring, more than enough). This fusing setup is to protect the PLC from shorted components because many PLC's don't provide for user replacement of the internal relays.

I ALWAYS use relay out PLCs so I'll let others suggest when you should use transistor outputs.

What's THIS project all about anyway?

Stationmaster
 
I fuse all outputs individually. The main reason for this is that if a fuse blows you know what it is for (rather than 16 possible things). Not a big deal if something goes to ground and stays grounded but if you have an intermittent short it can be a b**** to find.

On the input side most people just fuse each card. I fuse all FIELD inputs individually and just use one fuse for things like PB's. There is no reason electrically to individually fuse all of the field inputs, but again it just makes it easier to troubleshoot when a limit switch gets full of water. It's more wire and fuse blocks but hey wire and fuses are cheap, downtime is expensive.

The main reason people use triac outputs is that these are solid state with no wearing parts and theoretically should last forever. I only use relay outputs when voltage isolation is necessary or if the triac card can't handle the load. There is nothing that I can think of that is special about hooking up loads to triacs. Others may have different opinions.
 
This is for a PLC to control the automation on the milking parlor, most of that is regarding the wash, some of it will be some logic during milking to pump down receivers, vats, turn on cooling, etc.

I'm replacing an Idec FC5A brick style with a FC5A slim with a few input and output cards. The main unit has 6 relay out and 2 transistor outs. I have heard a transistor is never really off, and can be subject to spikes from things like solenoids. It's also better for repeated/pulsed on/off uses, as it won't wear out.

I'm also running some little relays off the relay out's to deal with different voltages on the same common.

At last count I think I have close to 30 inputs and 20 outputs, and these numbers will only go up from there.

The only thing I have against fusing each thing seperate is cost/space/layout consideration, and not knowing if something is down or not. I guess I could put an alarm in the programming that if a certain input hasn't run in X amount of time, it sends a signal. But it still doesn't account for the output's status.

Where and how big should a 4-20mA loop be fused?
 
Bolt said:
This is for a PLC to control the automation on the milking parlor, most of that is regarding the wash, some of it will be some logic during milking to pump down receivers, vats, turn on cooling, etc.

I'm replacing an Idec FC5A brick style with a FC5A slim with a few input and output cards. The main unit has 6 relay out and 2 transistor outs. I have heard a transistor is never really off, and can be subject to spikes from things like solenoids. It's also better for repeated/pulsed on/off uses, as it won't wear out.

I'm also running some little relays off the relay out's to deal with different voltages on the same common.

At last count I think I have close to 30 inputs and 20 outputs, and these numbers will only go up from there.

The only thing I have against fusing each thing seperate is cost/space/layout consideration, and not knowing if something is down or not. I guess I could put an alarm in the programming that if a certain input hasn't run in X amount of time, it sends a signal. But it still doesn't account for the output's status.

That's why I use only one fuse for ALL the inputs, blow the fuse, they ALL stop working.

As for the outputs, you can get fuse holders that light up when the fuse blows. Or a "grasshopper" style fuse that gives you a signal that you can use to set off an alarm when one blows.

Bolt said:
Where and how big should a 4-20mA loop be fused?

Again, the 4-20ma loop(s), at least in MY projects, get ONE fuse between the power supply and the distribution point for power to ALL the sensors. Fuse blows, they ALL stop working. There is a "status" register associated with the analog set-up in the IDEC that you can use to create an alarm if the register is greater than zero. (Zero = normal operation) The fuse should be about 1.5 times the calculated max load (ie: 3 circuits x 20ma = 60ma or .06 amps, .06 x 1.5 = about .1amp), The fuse is just to protect the power supply. If a sensor shorts, it is already blown. In my experience, if you try to fuse the analog ckt low enough to protect the PLC input, you have to fuse each one, and the narrow tolerance results in "nuisance" trips (fuse blows). Believe it or not, the "resistance" created by a dirty/tarnished/corroded fuse holder connection, on such low voltage/current, can cause service problems as well. Instead of individual fuses, I suppose you could use the analog status register to cause a relay to remove 24vdc altogether (and provide an alarm) with a latching relay until reset. hmmmm.........

This project also sounds like it may be a candidate for a touch screen.....

Stationmaster
 
That should get me going. Lots of interesting options and ways to do certain things. This will be a nice system I am able to grow into it. First of all it will replace the current PLC (which will go onto another project....) and then add some functions to it. Then I can slowly piddle with it and add things one by one, all basically during operation. It will slowly grow and take on more "responsibility" and technology as needed/planned. A touch screen is likely in the future as well. I would like to do that too. For now I will use 4 existing pushbuttons and a 3 position selector switch. When I need more, I will replace with a screen. Step by step. Not sure if the little Idec rectangular on I've seen will be enough for me or not. Also, the control side of this is most logically placed in an environment where moisture/washdown/spray is a "problem" so it will need to be capable of handling that.
 
The IDEC touch screens are moisture proof on the front, of course the back will be inside a moisture proof panel. I use the second size up, I'll have to get you the model number, 5.7in diagonal I think. You can wash the cow dukey off of it and it won't even bat an eye. A tomato packing plant I work at (sometimes), has one that I know is at least 10 years old, gets exposed to tomato squeezin's, wash downs, cold in the winter, heat in the summer, and unauthorized finger pokin', and it is still going strong.

I don't think they're too great in direct sunlight though.

When I get time in the "lab" I'll send you some screen shots. Touch screens are a great way to display/control lots of conditions without having to have a lamp/switch for each one. As a result they cut way down on my I/O needs.

Stationmaster
 
Super, I'd really like to see some screen shots or things of that nature. All I really know about em is from sales literature and seen a few here and there, not an indepth examination. This would be mounted inside, so no sunlight issues. Climate is reasonable, fairly humid in this room at times, ambi ent temperature from 55 to 95 at the most I'd guess. 90% of the time it's at a pretty comfortable temperature.

I remembered after my last post that alot of those inputs are just manual toggle switches mounted inside the enclosure, that only I use. Open a water valve, run a chemical pump, turn a pump on, etc. That would be an easy touch screen use, and would cut down on the inputs. I'll go that way in the future. For now, it's manual switches, and in the future I can use these inputs for other things when they are re-assigned to the touchscreen.

I'm slowly going to add monitoring and recording type info to this, where I can see wash temps from the past day, etc.

For now I don't have any analog inputs, just digital. Temperature stuff will be added later.
 
You can get din mount terminal blocks that house fuses with fault indicators. http://web5.automationdirect.com/adc/Shopping/Catalog/Terminal_Blocks_-a-_Wiring_Solutions

I started a basic page for panelbuilding which has numerous links to enclosure and other component manufacturers. With those temp ranges you may want to read about Temp management, there could be condensation issues etc. that you have not considered. At 95 deg F the inside of an enclosure can easily get as high as 140 depending on situation.
http://www.patchn.com/panelbuilding.htm
 
Well 95 may be a bit hotter than it is, I'll do some checking with a thermometer sometime. The room has 2 fans in it, and they pull all the air out in 3 minutes. The room only gets warm when 400 gallons of wash water are diverted to drain at 120+ degrees. But I don't know how much it affects the room temp. Eye glasses and glass doors will fog up rather quickly upon entering but its only for a few minutes 3 times a day. Only the touchscreen will eventually be in this room, not the PLC enclosure.
 
PLC outputs and inputs

Hello


About PLC´s Digital Outputs :



In station master reply, he said we need a fuse for each digital output following the rating of the relay out. But if any external relays uses just some miliamperes to commute the output, why i should use, relay output´s current to specify my fuse?

Supposing a relay 12vdc and needs 8.3mA to commute the output. using formula above, the fuse of each output should be have spec of: 1.5 * 8.3 = 12,45 mA.



Another question , but now about digital inputs:



How the input´s fuse are specified when im using just push buttons to activate PLC binary inputs?



Thanks for your comments people specially for station master



Regards

Bruno
 

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