To Fuse or Not to Fuse Individual I/O Points

When designing a PLC control panel, one of the fundamental decisions is how to provide overcurrent protection for I/O modules. Should each I/O module be fused with a single fuse, or should each I/O point be fused individually? 

Here are 11 things to consider when deciding how and when to fuse I/O points.

1. Always follow specific customer project requirements and specifications when they are provided. Note that many times it is appropriate from an electrical standpoint to fuse each I/O module with a single fuse. However, customers may require individual fusing per I/O point (or per field device) so that one fault only disrupts the one point.

2. Review and consult I/O module product documentation for any specific product details and requirements to ensure that the I/O modules are being installed in a compliant manner with manufacturer’s minimum recommendations.

3. Fusing is generally preferred over circuit breakers for I/O protection because fuses interrupt faults more quickly than circuit breakers. Fuses are more effective at minimizing let through energy and therefore offer better protection for downstream devices. Fuses are also generally more cost effective in the necessary size ranges. Additionally, small form factor fuses for I/O are typically used in a disconnecting style terminal blocks which allow the field wiring to be conveniently isolated from the control panel for startup, testing, and maintenance purposes.

4. At a minimum, I/O fusing should be as follows:

  • Analog input modules will have one fuse per module.
  • Analog output modules/points will not be fused since they are generally self-protected.
  • Digital input modules will have one fuse per module.
  • Digital output modules will have one fuse per module.

5. Check the I/O module documentation for on board fusing details and make sure any external fuses coordinate with on board module fuses. If I/O modules are electronically fused or current limited there should not be a need to use external fuses.

6. Be aware that in food production or facilities that limit glass, glass fuses may not be allowed. Ceramic fuses are generally a better choice in most applications.

7. Fuse selection should give consideration to minimizing the number of replacement parts which must be stocked. Since fuses are replaceable items, there should also be consideration to selecting non-overlapping fuse sizes to facilitate proper replacement. Be aware that extremely low amperage fuses are expensive and usually not warranted. Generally, fuses for I/O will be fast-acting type for better protection (whereas power distribution in a panel will require use of time-delay fuses).

8. The 5x20mm fuse size is popular and often used for I/O fusing since it offers a minimum footprint size that fits well with traditional cage-type terminal blocks. However, some customers may require the 0.25 in x1.25 in format. Fuse holders are preferred to be blown fuse indicating type if budget allows, with touch-safe swing-out arms where possible. Indicating type fuse holders require different types depending on voltage level and voltage type.

9. Output modules have an overall module current rating and a per point current rating. For example, a module may have a total current rating of 8 amps, but each output is only rated for 2 amps. Going by the book, you probably don’t want to limit your total current at 2 amps, so you put an 8 amp fuse on the module supply, and 2 amp fuses on each output. There may also be times when you do not want to fuse a single control point, but rather want a whole group fused together for safety or other sound engineering reasons. If several I/O points of any type are connected to the same field device, it may be preferable to protect all of these points with a single fuse so that there is a single disconnection point for the device.

10. Wire sizing could influence fuse sizing. Make sure that the fuse size selected does not exceed the amperage rating of the wire.

11. Be aware of the ramifications for fusing I/O in safety systems. A blown fuse can cause a false system shutdown and add additional points of failure in a system. This is especially true for non-redundant I/O in safety systems.

Do you have published standards and practices? If not, by following the suggestions above, you can be assured that your I/O modules, field devices and control panel wiring will be protected.


Joe Weathers

Joe Weathers

Joe Weathers is a Sr. Designer at MAVERICK Technologies. Joe has 39 years of experience in electrical design for various industries including food and beverage, oil & gas, pharmaceutical and water/waste water.

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  • We are considered a “Heavy Industry” producing Met Coke, an ingredient in steel, and just last week we had a 16 our downtime period ultimately due to a non-fused analog input complication. One of our Quench Cars had an “out of bounds” position indication interlocking parts of the system. Every indication was a particular transmitter was the problem, which is difficult to access since it requires to be well protected from the process, yet it tested okay in troubleshooting.
    After hours of trials, replacement of good parts and additional troubleshooting we discovered that a “sister” input on the same module was the real issue. The wiring had been compromised, burned actually, by the process and caused a partial short to ground but not enough to trip the module power breaker. Apparently, the voltage to the card was pulled down enough to make it appear as the first transmitter was the issue but not trip any breakers or fuses. Our spec is, one total module breaker sized to the maximum expected load or 0.5A whichever is greater and each input is fused to its abilities (0.063A for 4-20mA for example) or 1 amp, whichever is lower. If we need greater than one amp a control/isolation relay or SSR is used and fused for the actual load. All of that is to say we lost tens of thousands of unrecoverable dollars in lost material, labor time, scheduled production and post-production maintenance time for a $20 fuse & holder. Someone saved money – but it was not us.
    To be fair, we must accept much of the responsibility as the controls spec was either not dictated correctly or was not inspected by a person that knows what to look for before it gets delivered. Now we pay our part and retrofit this equipment with what we consider proper fusing and pay again.
    I must be clear in that I agree with Joe’s assessment and recommendations. Then there is the “but”, which I suggest is a competent design review is a must before a system is built or an addition/modification is made; followed by a competent inspection before it is release for shipment/construction (don’t forget the as-built drawings). The price of saving a few hundred or even a few thousand dollars seldom reflects the total cost paid when the excrement hits the impeller late on a Saturday night. Do it right and rest at night.


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