Circuit breakers and fuses are two of the most widely-accepted types of protection. While circuit breakers have the benefit of reusability, the quicker operation of fuses make them an appropriate choice for I/O protection. In addition, fuses are better at blocking let-through energy, providing better protection for downstream devices. Therefore, wherever quickness is a priority, fuses should be preferred over other modes of protection.
The following scheme should be followed during I/O fusing:
However, customer project requirements should take precedence over default operating procedures. Sometimes it may be appropriate to fuse each I/O module with an individual fuse, however customer may require extra protection to ensure the limitation of fault current at one point. The I/O module product description should be consulted thoroughly before making any decision as there may specific requirements that have to be taken into account before installing a protection scheme.
Wiring size is a considerable factor that can affect the rating of a fuse. Therefore, it is necessary to make sure that the amperage rating of the wiring is taken into account. Furthermore, during installation of fuses the cost of replacement must be considered as low-amperage fuses are expensive, especially when multiplied for each I/O point.
Finally, the control/protection engineer should be aware of the consequences of a blown fuse connected to an entire module. A false system shutdown can result from module-based protection which would keep the system offline until the fuse is replaced.
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