Surge protection for control and instrumentation signals (C&I)

While studying Weidmuller's surge protection catalogue for control and instrumentation signals, I got really confused so that I can't easily explain my question.Is surge protection for analogue signals different from binary signals?Is surge protection for such signals different from the surge protection for power supplies? I can't understand the need for separating analogue and digital signals in surge protection application, and also seperating control signals and power signals!

Answer

Is surge protection for analogue signals different from binary signals

Weidmuller refer to open and closing signalling contacts as binary signals and protecting these may indeed be very different to protecting an analogue signal input or even a digital signal input. Power lines will be different again.

So, for a typical analogue signal you might be vary wary about placing protection components across it that might alter a dc level or the signal amplitude. For this reason you might choose a simple low power zener diode.

For an AC power line a simple zener diode is inappropriate because every half cycle it will conduct and turn into smoke so, you might choose a much more powerful device like a bi-directional TVS or a MOV (metal oxide varistor).

With either of these solutions and applications you might also want to use a fuse although it is less likely to be implemented in an analogue signal line because there may be many such lines and many fuses will create a big space usage headache.

For serial comms lines like RS485 you may also choose a bi-directional TVS but probably not a MOV. You might also use unidirectional TVSs from each data line to ground. Depending on the situation the TVSs may indeed be zener diodes with two back-to-back zeners making a bi-directional zener. As data speed increases self-capacitance of protection devices can become a problem so what may be easily suitable for 9600 bps will probably not be suitable at 100 Mbps.

GDTs (gas discharge tubes) might be used in such applications as telephone lines but be aware that they are not reliably fast in offering their protection due to the time to initiate an internal glow discharge. They probably will not be used in AC power line applications because if they do activate they can take hundreds of amps and potentially destroy wiring and cause a fire (taken to extremes).

So, different applications require different protection.