ESD Grounding Straps with No Chassis Ground

Question

I've got water pump device that needs to pass ESD testing (IEC 61000-4-2, 8kV contact, 15kV air discharge).

On this device (as circled in red), there are these four metal inlets.

At 2kV with an ESD gun this device is hard resetting, on any contact with the metal inlets. So I'm assuming the ESD is triggering the reset pin (or some other condition) on the microcontroller.

This is a friend's device, and built by a hobbyist of sorts. He used a single layer board with no signal GND plane of sorts, and we are going to redesign the board to 4 layers with TVS, ferrites, and series resistors as needed to protect the microcontroller lines. There'll be as large a possible signal GND plane on this board for the TVS to shunt the ESD transients.

So in the ideal, that will fix our ESD issues.

BUT, what about ground straps from each metal inlet to provide a low impedance path for the ESD transient before it even approachs the board?

The enclosure is all plastic, and there is no chassis GND.

On the four metal inlets, where could you ground those if you just have the one PCB in the system?

I'm thinking we would have an isolated area on the PCB that runs straight to signal GND with these four straps from the metal inlets?

How are folks making these types of connections to chassis ground (if we had a chassis ground)?

I'm thinking of soldering a wire to the metal inlet -- is there something off-the-shelf that works better that's commonly used for these types of connections?

Should all four straps run to one central location in the far corner of the PCB that feeds to the signal GND?

I was brainstorming a wire with a ferrite in series that would dissipate the ESD as it comes in from the metal inlets and goes to our PCB signal GND?

Answer 1

With ESD the current discharge pathway from the ESD event needs to be controlled. On connections to the board TVS diodes could be used to shunt the current away from the board.

Source: https://www.sunrom.com/p/tvs-diode-12v-smbj12ca

The best way to handle ESD with a DC connection is to have another ground or chassis ground or shield ground that can take the current from any of the inputs and shunt it to mains ground (since it appears this device can be plugged in). It is not ideal to have ESD shunted on the negative (or ground) side of the DC connection because the inductance of the wire nH's to mH's can cause common mode voltages to make the entire ground voltage rise and cause problems for electronics.

So make sure you have TVS diodes (or diodes) on connections to the outside world from your PCB and especially on connections to a microprocessor. Another thing to try is put a conductor on the switches/pump-inlets that runs to mains ground.