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.

This post was written by Joe Weathers. Joe is a senior designer at MAVERICK Technologies, a leading automation solutions provider offering industrial automation, strategic manufacturing, and enterprise integration services for the process industries. MAVERICK delivers expertise and consulting in a wide variety of areas including industrial automation controls, distributed control systems, manufacturing execution systems, operational strategy, business process optimization and more.

MAVERICK Technologies is a CSIA member as of 3/20/2015

No comments
The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by...
The System Integrator Giants program lists the top 100 system integrators among companies listed in CFE Media's Global System Integrator Database.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
This eGuide illustrates solutions, applications and benefits of machine vision systems.
Learn how to increase device reliability in harsh environments and decrease unplanned system downtime.
This eGuide contains a series of articles and videos that considers theoretical and practical; immediate needs and a look into the future.
Big Data and IIoT value; Monitoring Big Data; Robotics safety standards and programming; Learning about PID
Motor specification guidelines; Understanding multivariable control; Improving a safety instrumented system; 2017 Engineers' Choice Award Winners
Selecting the best controller from several viewpoints; System integrator advice for the IIoT; TSN and real-time Ethernet; Questions to ask when selecting a VFD; Action items for an aging PLC/DCS
This digital report will explore several aspects of how IIoT will transform manufacturing in the coming years.
Motion control advances and solutions can help with machine control, automated control on assembly lines, integration of robotics and automation, and machine safety.
This article collection contains several articles on the Industrial Internet of Things (IIoT) and how it is transforming manufacturing.

Find and connect with the most suitable service provider for your unique application. Start searching the Global System Integrator Database Now!

Future of oil and gas projects; Reservoir models; The importance of SCADA to oil and gas
Big Data and bigger solutions; Tablet technologies; SCADA developments
SCADA at the junction, Managing risk through maintenance, Moving at the speed of data
Automation Engineer; Wood Group
System Integrator; Cross Integrated Systems Group
Jose S. Vasquez, Jr.
Fire & Life Safety Engineer; Technip USA Inc.
click me