Then goes to a 100k resistor just in case I short out a heating or thermistor wire while changing the nozzle. That is then bonded to the frame. I verified that the frame is a ground via the PSU.

I did not ground the build plate at this time and hope that will not be needed. Don't forget that ESD discharges are cumulative on the semiconductors protect circuits.

I have done what you suggested. Is it better if I remove some of the paint at the area where the ends of the cable connect to the printer's frame and PSU?

My other printer is older and  a much higher % of conductive materials were used in its construction.   I never get zapped when I use that one.

I also agree with Joan in that some effort may be required to electrically connect the conductive parts on this printer.  The paint they use is tenacious enough to insulate some of the screws.  I also had to remove paint to get all the legs electrically connected. 

I love it when I see working people using their trade to seek out the why things happen and creating solutions instead of accepting excuses.  I'm fortunate to work a team of smart people that spend the time to understand why a thing  happens,  before putting any effort into changing it.  It's a mindset that is rapidly being lost in the general public.  That's why I pay a lot of attention to this forum. Because I get to see so many people here that have avoided the "if its broke just buy a new one" mentality so many of our contemporaries have accepted.   I love it. 

I've managed to mitigate most of the ESD issues but I still get the occasional LCD display waver and alien speak when I touch the selector knob. Fortunately it recovers quickly, but it's still concerning.

Nice pic,  I had forgotten what a beast that Keithley is.

I connected the extruder cooling find with a flexible wire following the ombilical from the controller board to the 617 input.

I used the first layer cal to test. So one layer only with the zigzag and the stamp.

There is of course some fluctuations near the zero point but no real changes during the bed heating and extruder heating up.

There is no difference during the levelling pinda process either.

However the moment it started to extrude the filament things happened in both nanoCoulomb mode and in voltage mode.

In Coulomb mode it went overflow half way through and with an extra 10nC for a total of about 40nC.

In voltage mode it went to about 50V and to >100V in another.

Putting my hand on the spool (PLA) or rubbing the filament greatly increased the readings.  Looks like the filament accumulates electrostatic charge and delivers it to the floating head to accumulate.

Could it be that with dry filament it is possible to have a high enough voltage to cause a spark to conduct an ESD to the control board via breakdown in the thermistor or heater? On a long print this might cause resets or errors at random intervals.

OLYMPUS DIGITAL CAMERA OLYMPUS DIGITAL CAMERA OLYMPUS DIGITAL CAMERA

"the issue with 3D printers is that we don't fully know whether you are discharging to the printer,  or whether the printer is discharging to you... "

This is an interesting observation. I would assume the controller board ground is directly linked to the PSU so that would be OK.

But considering the extruder assembly is floating could there be a possibility that during a print the spool, filament and extruder do accumulate a charge from static electricity from all the rubbing, much like a van de Graff generator?

I'll see if I can get my Keithley 617 electrometer connected to the extruder and see if it's collecting a charge.

"the earth in the mains lead should be grounded to the power supply case, but there is no intentional equipotential bonding from the powersupply to any of the frame, that I am aware of. and I have never checked to see if the 0volt connections on the power supply, are connected to earth"

Just checked (Canada plugs, like US) the frame is bonded to the earth/ground from the house plug. But the printer is on 4 rubber feet so that is why I installed a ground wire to the metal table.

Also of note the LCD display is housed in a plastic housing, so the moment you approach the reset or control knob you could inject an ESD directly in the electronics.

Also of note I had to scratch the black coating on the frame to get a contact. Not an issue for a ESD to punch through but the bonding between the PSU and the frame might be marginal in some cases if assembled loosely.

they decided that Our Leather tool cases had to be replaced by more 'Professional' plastic tool / brief cases... 

and that we all had to wear 'Corporate Clothing' 

Net result, as we walked to our jobs in customer premises, we charged up with MASSIVE static electricity charges as the plastic briefcase rubbed against our clothing,  So much so, that if we reached out to open a metal door frame, a spark would jump from our fingers to the metal of the door. 
this happened so frequently that we used to carry our keys in our hand, and let the static discharge from the keys tot he door, so it didn't hurt so much...  

Lady Switchboard operators were particularly deadly to early electronic telephone switchboards,  We suspected they created static charges when their clothing rubbed on their stockings and the static discharged tot he telephone systems as they worked... 

the issue with 3D printers is that we don't fully know whether you are discharging to the printer,  or whether the printer is discharging to you... 

Earthing yourself before touching the printer will dissipate your static charge, but make you a better target for the printer to discharge to, if it is creating the charge... 

Prusa don't directly earth their printer components 

the earth in the mains lead should be grounded to the power supply case, but there is no intentional equipotential bonding from the powersupply to any of the frame, that I am aware of. and I have never checked to see if the 0volt connections on the power supply, are connected to earth

Luckily, for me, I have not noticeably had a discharge problem with my printers,,, 

maybe you can check continuity on your printer and let us know... 
regards Joan

"but I bet if you removed the electronics and used a meger, you could find paths where and ESD could make it the controller. "

I have a Megger to about 1200V but I did not dare to do that (printer work well= too scared to do that....). I use a KeyTek ZapMaster for ESD testing. Typical human body ESD is about 4 to 8kV, 150ns long decay  with smaller than 10ns risetime with about 2.5A peak current (ANSI/ESDA/JEDEC JS-001-2017, easy to find on the net). That is why I suspect the extruder heater or the thermistor as potential path to the controller as such a spark could easily jump in there. The steppers are another potential source but they have a much larger capacitance in situ to absorb the charge.