DeviceNet Problems

Greetings,

Is the fuse that is failing an input or output fuse? Or is it the PLC common power supply?

Are you using a common power supply for your entire system?

Pretty much all PLC inputs are optically coupled to the field device and only require a minimum of amperage (microamps or milliamps) therefore they do not need to be and normally are not fused in most applications.

The only way an input device could cause a power supply fuse to fail is if there is field or terminal wiring that is "cross-wired" (shorted) to a separate voltage source other than that which is normally referenced and used by the PLC.

The ampacity of most PLC outputs are limited by the OEM manufacturer usually with an on-board fuse (surface-mounted or in-line board mounted), so the output(s) should not be causing your power supply fuse to fail.

If the on-board fuses are failing, the field device or wiring is shorted or grounded or it requires more current than the PLC output is rated for.

Also, if the output power is shorted to an external voltage source of a higher magnitude, the on-board fuse and/or the common power supply fuse will fail.

If all of your wiring is clear of shorts and grounds and it is not mixed with any external power source(s) and the power supply fuse is failing, you need to check the current (power) rating of your power supply against the total amperage your field devices require.

To do this, go into the field, interogate all field devices attached to your PLC outputs, record the amperage requirements of each device, add all of the recorded ampacities, and compare that number to the rating of your power supply.

If the power supply current rating value (number) is lower than the total amperage required by your field devices, install a larger power supply with at least 10% headroom (higher ampacity rating). Example: If your field devices pull a total of 4.5 amps then install a power supply with at least 5 amps of capacity.

If you are going to be expanding your I/O in the future, calculate the circuit loading for future expansion device requirements and compensate by upgrading the power supply to meet all of your requirements. That way you only have to pay for the upgrade cost one time.

Hope this helps,

Shockhiscan

Thank you for your lengthy explanation.

Will be checking for ampere surge.

Just one additional question. If there is a short in the circuit, definitely the fuse will blow immediately after connection right?

What happened here previously was, it was running fine for about 4 days all of sudden the fuse just blow. Lucky the PLC didn't smoke.

Can this be a short circuit phenomena? However, will be checking on the total amperage?

Thanks.