Is it necessary to install a fuse on a 24V DC relay?

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Of course it should be added; at each level of automatic control, the fuse core must be selected based on the load, and there is no strict requirement regarding its inclusion. If a card-slot type is used, it’s best to have an indicator light that lights up once the fuse blows, to make it easier to locate it. It is generally determined by the actual power of the relay coil.

This post was last edited by txcc1111 on 2015-6-21 22:02. Is it necessary? The coil power is only 0.53W – can a fuse be used with 0.53W as well? Additionally, the relay comes with a fault indication function; it is an intermediate relay used by the PLC for isolation purposes

The coil circuit of the relay is generally driven directly by power supplied from the DO card; the DO output channels have built-in overload protection, and it is unlikely that these internal cables will become grounded. Additionally, the power consumption of the coil is low, so there is no need to install fuses here. However, fuses must be installed on the field side/MCC side terminals, as there are \"power loads\" on the outside and a possibility of grounding.

Check the PLC user manual to see if a fuse needs to be added to the COM terminal. Whether it is relay output, transistor output, or thyristor output, the current is generally between 5-10A (see the manual). To extend the lifespan of the PLC contacts, a surge absorber, which is essentially a rectifier diode, should be connected in parallel with inductive loads. On some circuit boards, there is a small-power integrated circuit with a fuse resistor connected in series to it. I have also encountered many cases where the PLC outputs were damaged and stopped functioning when connected to inductive loads with very low power; this is certainly due to the presence of a short-circuited load. This depends on factors such as the usage environment. Of course, there are no strict rules regarding this; most people install it without using a fuse, as everyone’s approach to installation varies. For my own PLC wiring and programming, I give top priority to the equipment and protection. Different people have different views.

That makes sense. There’s another situation: if it’s over 300 points, it’s probably better to simplify it.