PLC output fused for Isolating relay

Question... for those of you that use isolating relays on the outputs of your PLC cards; do you still fuse the outputs of the card? See pdf for example with outputs fused. It seems overkill to me to fuse an output just to fire a little relay, but part of me just can't leave those outputs un-fused. I need to use the isolating relays to drive loads that are higher than the PLC outputs will handle.

Thanks,
Patrick

If I have outputs turning on an isolating relay, I rarely fuse the outputs, and instead have a fuse on the supply voltage to the relay output.

Or if it's a small enough power supply, I just put a fuse on the output of the power supply and let that take care of the rest of the machine. I have lots of machines that only have a 50W 24VDC power supply. They have a 2 amp fuse on the power supply, and that's it.

sportster said:

Question... for those of you that use isolating relays on the outputs of your PLC cards; do you still fuse the outputs of the card? See pdf for example with outputs fused. It seems overkill to me to fuse an output just to fire a little relay, but part of me just can't leave those outputs un-fused. I need to use the isolating relays to drive loads that are higher than the PLC outputs will handle.

Thanks,
Patrick

Click to expand...

Yes I do. I don't want any one output taking out all outputs.

Tharon said:

If I have outputs turning on an isolating relay, I rarely fuse the outputs, and instead have a fuse on the supply voltage to the relay output.

Click to expand...

I do the same thing. Just one fuse for every relay output card. If it is a regular output card with interface relays mounted in the same cabinet I do the same. If the outputs are connected directly to the equipment or to another cabinet, I usually use a single fuse for every output.

I agree with Mickey. I fuse every output. Just because one of them goes doesn't mean I want to lose the others. Plus, sometimes a shorted solenoid can burn up an output completely. If I have each output fused individually, then chances are I'll only burn up one. This way I can swap it over to another output address and keep on rollin'.

I guess it all depends on the application, but that is how I usually do it.

This is what Siemens is saying about their DC transistor Output cards.

"DC outputs :
Fusing to protect external wiring and DC power source only is acceptable. Transistor outputs will typically not be protected by a fuse unless additional current limiting is included. The transistor bond wires will open faster than a commercial fuse. If short circuits are expected, a 3 ohm resistor in series with each output, or a DC power source that instantly limits maximum current to 8A, will allow an external fuse time to act. External fuse for this case should be 1A or less with an I*I*T rating of 5.5 or less."

I like to use output cards with electronic fusing.

Sounds good. I agree with you guys on fusing the all the outputs all the time. But I normally don't have to use isolating relays to switch loads. So that's why I was questioning leaving the fusing out. I think I"ll just go with fusing the whole card and size it accordingly. Thanks, Patrick