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

[leitmotif]

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

That is for sure. Place I worked the cleanup guys wanted a high pressure washer. Maintenance was resistant to that idea.

Dan Bentler

 

[RobImb]

Hi All,

Just to add my 2 pence worth i did work for an American company in the paper industry but we did NOT fuse outputs but now working for a British company we design all our cabinets with fused outputs it adds cost at design time and initial build but yes it does protect the system better, and our maintenance guys are not the brightest spark in the cabinet so i would rather them keep blowing fuses than blowing PLC I/O cards..

 

[tomwhite007_2008]

What about inputs? Again money is no object. I look at it more as a cost savings to fuse an input and have it blow then have the power supply voltage intermitting by a short and have an assembly line down while maintenance tries to fix the problem. Yes / No?? Thanks for your response.

 

[hadji likes plc]

Have not had issues with inputs, have fused a few at upper managements request but no a practice for me. Outputs are a must as far as I care. If the boss says do it I don't care what it cost it will get done, he is the boss.

 

[OkiePC]

What about inputs? Again money is no object. I look at it more as a cost savings to fuse an input and have it blow then have the power supply voltage intermitting by a short and have an assembly line down while maintenance tries to fix the problem. Yes / No?? Thanks for your response.

Fusing the inputs may help with downtime (troubleshooting). If there are hundreds of input devices and they are fused in groups of eight or sixteen, then usually that is sufficient to help divide and conquer during troubleshooting.

It sucks when you have 200+ input devices all fed by the same power source, and the fuse blows dropping them all out. Next, production is breathing down your neck and you have to figure out which one is the culprit.

Even in the nastiest environments, I have never seen any machine with individual inputs fused, but it is quite common to see them split into logical groups. That way, one part of a plant may continue to run, for example. As another example, or all the devices tied into the same junction box could be isolated by removing a fuse.

 

[JOLTRON]

I also use the indicating type fuse holders. Any "Bubba" or even management can find the blown fuse when it's illuminated!

Unless you have one of the rare occurences where the fuse blows but the indicator doesn't show it. I have had it happen 4 or 5 times in the past 5 years. Not quite sure why it happens

 

[OkiePC]

Unless you have one of the rare occurences where the fuse blows but the indicator doesn't show it. I have had it happen 4 or 5 times in the past 5 years. Not quite sure why it happens

Two possible causes:
1. The source of power to the fuse has turned off (very common with PLC outputs...if the output point on the card has turned off, the indicator won't light).

2. The wrong type of indicator was used. If the fuse holder is designed for an AC voltage (like these), then 24VDC will not light it.

I have seen both of these issues crop up recently.

Paul

 

[MasterBlaster]

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

Greg

I strongly suspect that you could have removed the fuse and still had indication, as it sure sounds to me like the fuse holder was conducting, not the remnant of the fuse.

Bill

 

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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

hey folks,
can somebody just confirm weither i'm off me rocker here or not, i remember when i was in tech one of the lecturers told us, if you use an inductive load on an output like a solenoid, contactor, motor etc you have to put an RC network in parallel(he actually drew it in series in his sketch) with it because, the back emf from the inductive load would blow the output relay.. is this true or to i have my wires crossed! excuse the pun
thanks in advance