PLC Fusing Outputs

I know it's a good idea to fuse the individual outputs from a PLC and possibly better to isolate with interposing relays. But, is ther any regulatory requirement that such fusing be done?

If you find any let me know. I have looked and could not find any.

The only requirement I could think of is if you reduced the wire size from the output to the device (for example, 14awg to the card common and 18awg to the output), then you'd need to fuse it. Other then that it's probably not required as the wire is already protected upstream of the output card.

Christoff84 said:

The only requirement I could think of is if you reduced the wire size from the output to the device (for example, 14awg to the card common and 18awg to the output), then you'd need to fuse it. Other then that it's probably not required as the wire is already protected upstream of the output card.

Click to expand...

Even then it might not be "required" to be fused any more than what is already in place.

In your hypothetical situation, there's no saying how the 14 AWG wire is protected. If it were to say it's maximum amperage load, then yes, it would woefully ideal to fuse the outputs according to the NEC. Then again, if the common feed is say a 4 amp fuse, then you are good to go with very small wires.

This is one of those times where common sense should supersede knowledge or regulations. Of course if you use DC there are ESCP outputs available with most of the better suppliers.

As always you must protect your wiring.
So if the PLC can handle 2 Amps you must use a fuse of haha 2 Amps in front of the outputs not behind. I divide all outputs in parts and have them fused separate.
The fuses have a contact to detect they have been activated.
If any of them fails the Machine will stop.
In europe we have the directive for this. Machineguide)

Copied from the Siemens Product Support Pages.

" Protection of 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."

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The rule of Protection:
If you install a 50 ¢ fuse to protect a 100 $ component, the 100 $ component will blow to protect the 50 ¢ fuse.