How to size a fuse. Should I fuse an Output?

tomwhite007_2008

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Hi. Can someone please help me? My background is computer programming and not PLC's. I was forced into this job at my work because of the slowdown in the economy so I had to take it or I would be jobless. I am really starting to enjoy it and it is very challenging. So forgive me if I sound like I am just looking for quick answers because I spend hours many hours to try things before I post here.

My question is on how to Size a Fuse properly. I have a 5 Amp DC power supply powering a SLC 500 PLC, 1 (16)Input card with 16 Proxies, 1 (8)Output card powering 6 Valves, 2 lights. There is also Jokab safety relay being powered.

Do I look at the amount of current each device draws and just add them up? What about the proxies. Do I add them up too? What about the Solenoids. There will only be one on at a time for each cylinder is either extended or retracted. So just add the one of them or both. Should I fuse each output to the Solenoid? Am I supposed to fuse the proxies? Really appreciate your response... Thank you. Tom
 
All of your answers are valid. My personal opinion is a 5 amp fuse off the power supply and I fuse for each I/O card.
By going that way you can isolate a shorted component to 1 area. I have seen panels with a fuse for every output and I must say its overkill! I've only seen it in American panels.
 
I would fuse a 5A power supply with a 5A fuse on the 24V.
I'd use a separate fuse for your PLC, safety relay, inputs (1 fuse for all 16) and every output individually. I typically size fuses at 125% rated current, rounded to the next standard size. I also usually use time delay fuses.

You would add up all of your current draws when you size the power supply. Here is where you could only count 1 solenoid per cylinder.
 
If you fuse the power supplying the whole output card, say a transistor DC output card, then your fuse has to be sized large enough for all 16 points to operate. Next, when a single point draws too much current because of a faulty field device, you will very likely destroy the transistor before your fuse blows.

That is why if you fuse every output point of the card, you will save money and time in the long run by being able to fuse the points at a low enough current level to truly protect the card from failure.

Paul
 
OkiePC said:
If you fuse the power supplying the whole output card, say a transistor DC output card, then your fuse has to be sized large enough for all 16 points to operate. Next, when a single point draws too much current because of a faulty field device, you will very likely destroy the transistor before your fuse blows.

That is why if you fuse every output point of the card, you will save money and time in the long run by being able to fuse the points at a low enough current level to truly protect the card from failure.

Paul
Click to expand...

Only in America!o_O
 
I like OkiePc fuse my outputs on the card from past experience working in the food processing industry. Every thing here gets really wet every night and at past places of employment we had issues with water shorting out field devices and blowing the card also. So for a wet shorted coil we lost the $25 coil & $500 card plus labor to find the shorted device then replace it and extra stock needed on the shelf for overhead. Fuse the outputs with a 25cent fuse and we lost the $25 coil and 25cent fuse and labor for about $35 total. So yes I live and work in America but the $5 investment for the fuse & fuse holder make a lot of good sense to me and the bean counters where I work. The blown fuse also tells the tech what device or wire are bad and cut a lot of trouble shooting time out. We would blow 1 to 3 cards a month at one place depending on how rough the cleaning crew was on things. I don’t care if the box is rated for water proof a food processing cleaning crew will get water in it.

Charles
(y)
 
HJTRBO said:
I have seen panels with a fuse for every output and I must say its overkill! I've only seen it in American panels.
Click to expand...

OkiePC said:
That is why if you fuse every output point of the card, you will save money and time in the long run by being able to fuse the points at a low enough current level to truly protect the card from failure.

Paul
Click to expand...

Because I'm an American and I fuse every output on output cards I'll add a little more fuel to the fire.

I also use the indicating type fuse holders. Any "Bubba" or even management can find the blown fuse when it's illuminated!
 
I agree 100% yet I still wouldn't do it (Respectfully MHO). My background is on local and European machinery so you may understand why I think so. I have spent time with Marquip, Ward and Red Bud Industries machines from America and I must say, opening a large double cabinet door to a sea of fuses was a new experience. (And fuses in J-boxes and also on the 0V ARGHHHH!!)
 
If you are using the power supply to also power the PLCs processor I would use several small fuses, perhaps one 2 Amp per group of four or 8 that way if you get a short in the field it can pop the fuse without pulling down the power supply to the processor.
Note: Most 24 VDC power supplies have current limiting so it might sit there all day with a short before it blows a 5 amp fuse.
If the project can afford it I prefer to use circuit breakers instead of fuses.
Indicating fuses are fine if your loads are over 1/4 Amp or so but avoid them if you get to work with 4-20 mA loops as they can blow but still supply enough current to give you a partioal signal. e.g if you power up a level transmitter on a tank it might be 100% full but indicate 70%
Roy
 
Under the Canadian Electrical Code "Under normal conditions, power levels less than 100 V•A do not constitute a fire hazard great enough to require
Rules as restrictive as those for power levels greater than 100 V•A."

Have the maximum fuse size coming out of your 24VDC power supply at 4A. You can still have another branch circuit coming off of it fused at 1A.

Giz.
 
Really?!

Roy Matson said:
Indicating fuses are fine if your loads are over 1/4 Amp or so but avoid them if you get to work with 4-20 mA loops as they can blow but still supply enough current to give you a partioal signal. e.g if you power up a level transmitter on a tank it might be 100% full but indicate 70%
Roy
Click to expand...

I have never seen this and work with a lot of analogs in BAD environments. Has anyone else seen this?

Greg
 
If I'm feeling panicky that my o/p cards may fail, I use some nice weidmuller/klippon interface relays - same width as a regular terminal and come with a nice led! Bet they cost a similar amount to fuse carriers and fuses!

They are good for 5A, so just use 1 fuse and don't fuse the card outputs.
 

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