We are designing a system that consists of several modules that are mounted on a DIN rail. They are connected to each other using an In-Rail Bus system like this:
System design:
The 5 bus line currently are used like this: GND | SIGNAL | 12V | SIGNAL | GND
We use a third party AC/DC supply that is mounted on the DIN rail as well, but without In-Rail connection. So we have one module that only acts as an interface where the 12V and GND from the power supply are connected and then go to the In-Rail. The DIN rail itself will be connected to PE.
Due to the isolated output of the AC/DC supply, the GND/12V are currently floating with respect to PE.
All our modules are enclosed in plastic casings. Several of them expose some kind of connection interfaces that are susceptible for ESD testing. There is a SUB-D connector for example, where the metal casing of the connector is currently connected to GND. Then there are some high impedance connections where push-buttons can be connected to. We used TVS diodes to GND to protect those exposed contacts.
The In-Rail bus lines are protected by a plastic cap after installation, but we put ESD protection there as well.
ESD testing
During ESD testing the functionality of our modules was occasionally disrupted but went back to normal afterwards. Since we only need to pass "criteria B" this would still pass the test since nothing was damaged. However, we though about improving on that.
(Another somewhat strange behavior we observed was some kind of "ESD related voltage build-up", where we only had a actual contact discharge (as indicated by the ESD pistol) every 4 or 5 ESD pulses.)
Potential improvements?!
We thought about using one of the two GND in-rail lines for PE. We could then connect the SUB-D casing to PE and maybe even the TSV diodes, instead of connecting them to GND. This would prevent the discharge moving through the whole whole GND system.
We probably would have to connect PE/GND right where we interface the power supply with the DIN rail to get it to the same potential.
Any thoughts about this or a totally different approach are welcome!
