Fusing of Instrumentation signals

[Lc85]

We have a customer who has asked that "all signals entering the panel should be protected by a fuse e.g WSI 6 110Vac".

In our panel we have a pr5225 module and a PR531 for temperature out in the field we have a speed sensor and RTD's. Has anyone seen this before? is it really necessary?

Thanks

 

[DRWeig]

Simple answer: if the customer specifies it, it's necessary. Ask him the reason. They may have a history of people hooking up 120V to the control wires in the field?

My shop builds UL panels. If you could provide some data (what's a pr5225, are these Class 2 circuits, etc...), we might be able to help out more.

Best to you,

Goober Dave

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[Macbeth]

Sounds like they are just trying to:
1. Keep their I/O cards from being damaged due to field side shorting to ground.
2. They are looking for away to isolate for field service.
3. Just a broadband spec that they apply to every thing.

In any case, if the RPM sensor is Hal-Effect over TTL, I would avoid any additional products between I/O and sensor. The extra “weight” of the fuse may degrade the signal. The same goes for the RTD even if it was 4-wire, every junction is one more failure point and additional resistance.