Cost and Safety are Among the Pros and Cons of PLC IO Fusing
Opinions vary on whether or not to fuse individual inputs and outputs when wiring a programmable logic controller (PLC) module in an industrial control panel. Different opinions stated by many end users include fabrication panel cost and size to accommodate the additional large number of fuses versus the time to troubleshoot problems in the field if an input shorts and damages the module. As always, personnel safety takes top priority.
The first place to start with any design engineering project is to follow end-user client standards on fusing. After client standards are followed, design firms may also have specific standards that apply in this case. Each needs to be followed starting with the client standards.
Other considerations must include following the manufacturers’ PLC input/output (I/O) module requirements to ensure a safe, functional and warrantied installation. Fusing I/O should also take into account the voltage of the system; 120VAC inputs — and potential faults — represent a higher risk to personnel and components than 24VDC systems. After all the above steps are considered and followed, some clients and design engineering consultants may not have set standards for PLC I/O fusing. Both sides of the argument are considered here.
Consider the Pros
When installing individual I/O protection, fuses and circuit breakers may both be used. However, fuses are the preferred choice as they interrupt faults by opening faster than breakers. They are also less costly and can be installed using blown indicator fuse blocks for easy fault identification and fuse replacement.
Fusing individual points versus grouping several together allows for a single I/O point interruption from a fault, whether from a field wire short, device failure or panel wire problem. In the case of a single main panel circuit breaker fault, the entire I/O module or modules would lose power and could cause the untimely and potentially costly down time loss of a plant production line.
Individual I/O fuses can save a lot of maintenance fieldwork and plant downtime versus if you have a short with multiple field devices sharing a single field power source. You can also have a power wire fall off a device due to vibration and short against a metal case or conduit. One short in the insulation of a single wire could cause an entire module to lose power. Trying to find a single short among multiple field devices could be time consuming and costly in both idle plant personnel and loss of product production.
Some industries, such as food or pharmaceutical plants, may not allow glass fuses in panels. This restriction is somewhat rare, and an exception can usually be allowed through the use of proper PLC enclosure styles and panel Installation locations away from the product lines.
Care should also be taken to research the manufacturers’ recommendations on external fusing if the I/O module selected has internal fuse protection. Even with internal module fuse protection, additional input fusing may be required or preferred by client standards.
And the Cons
Fusing every PLC input can significantly increase the cost of a PLC panel. Depending on the number of I/O points, you could have a large number of fuse terminal blocks and glass fuses. This also results in a larger panel enclosure. In locations where stainless steel enclosures are required, the larger enclosure cost could be significant. Only installing a main branch feeder or single fuse or circuit breaker per input module can greatly reduce the cost associated with the panel design.
Two field wires per discrete field device are required if fusing every PLC I/O point. Depending on the number of instruments, this would add significant costs for wire and conduit, plus the installation time to mount the conduit and terminate the wires.
Fusing every PLC I/O point may not be necessary in all applications. If the inputs are strictly used for monitoring and not for line control, having an entire module lose power due to a short may not affect the plant operations. The short could be fixed as time allows. However, this could still damage the PLC module.
Situations where a non-critical fault occurs and blows an input fuse can occur when fusing every PLC I/O point. Since the input does not interrupt plant operations, maintenance may leave the fault until they have more time to investigate the trouble. This could lead to a dangerous short somewhere in the panel or field for an undetermined amount of time.
Conclusion: Fuse each PLC input
This article in not intended to serve as a definitive answer to the question of how or when to fuse PLC inputs. It is intended to raise questions during the design engineering phase to ensure a safe installation that meets client and integrator standards.
Taking into account the pros and cons listed above, the safest option for plant personnel is to fuse each individual PLC input. This is not a one-size-fits-all answer but tries to balance the different requirements between cost and safety. As in any design engineering application, the number one priority must always be safety.
As a design engineer with over 30 years of experience, my personal recommendation is to always fuse each individual PLC input, regardless of voltage. With many years in the field working with plant maintenance and electrical installation contractors, the time saved troubleshooting field or panel faults is well worth the extra panel costs to add the fuses.
Matrix Technologies is one of the largest independent process design, industrial automation engineering, and manufacturing operations management companies in North America. To learn more about our manufacturing operations management capabilities and manufacturing process control solutions, contact Dan Wood Senior Consultant, Power, Instrumentation, Control Department.
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Tags: Dan Wood / Automation Planning / Control Panel Design / Controls / Electrical Engineering / I/O Fusing / Industrial Automation Engineering / Manufacturing Process Control / Panel Fabrication / PLC / Process Design / Maintenance / Planning
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