I have a 3D printer with 230 V AC servo motors for x-y, hotbed and a 24 V DC circuit for the rest of the stepper motors, printhead heaters and a 5 V system/CAN-BUS interface for sensors etc total power consumption 3200 W 230 V AC and 800 W 24 V DC.

Now the -24 V -5 V etc. is connected to the frame and the ground from the 24 V PSU this is also connected to ground and the servo driver is on ground.

If the servo driver or something else in my house develops a ground fault there will be 230 V on my printers frame and all the electronics only briefly as the ground fault protection should pop out.

But how can I protect my DC system?

I thought about making the DC PSU floating but this still gives 230 V AC through the system as the servo's are still connected to ground via the driver and disconnecting this would be dangerous if they develop a ground fault.

Having a 2F capacitor bank on the power supply for acceleration and braking the steppers.

There is a circuit protection for the back EMI of the steppers Build in at every motor if they should be suddenly stopped by a collision or something.

And there are fuses on the heaters and fuses between the hotbed and optocouplers+ssr on the hotbed.

Hotbed is electric isolated from the frame and grounded separately.

Is there a simple solutions for this problem?

  • 1
    \$\begingroup\$ Welcome! Please draw a schematic and/or block diagram of how everything is connected. Please also indicate in it how a ground fault would leave your chassis with 230 V AC on it. \$\endgroup\$
    – winny
    Apr 19, 2023 at 21:21
  • \$\begingroup\$ Is this a US or European style setup? Under the US style setup it shouldn't be possible to get 230V chassis to earth potential (maximum difference is 120V), but in the European style, there is the possibility of a hot - protective earth short somewhere in the house, connecting the protective earth (and thus the chassis in this setup), to a supply at 230V from earth potential for the short time it takes the RCBO to disconnect. \$\endgroup\$ Apr 19, 2023 at 21:54

1 Answer 1


Unfortunately, people tend to conflate AC mains ground (nothing but a safety shield) and electroics ground (Vss aka common; normal return current path).

With AC mains power, the entire system is wired "as an isolated system". Both neutral and live wires are insulated from ground. Current should never move on safety earthing "ground" except during a fault condition, which should be quickly ended by the circuit breaker or RCD~=GFCI. Generally with AC mains, chassis is attached to the safety earthing "ground".

With low voltage circuits, you can go either way: run the low voltage as an isolated system, or pick a voltage and call that "common" and attach it to chassis.

Obviously, wiring as an isolated system protects the system better. On the Illinois Central (Metra) and South Shore electric interurban rail operations, propulsion DC+ is 1500V trolley, and propulsion DC- is the rails (and therefore the chassis). As a result, all control wiring, particularly that jumpering between cars, is wired "isolated from chassis" and with 1500VDC rated insulation. That way, if a single railcar in a set runs up on rusty rail and loses contact with the rails, it doesn't try to return propulsion current through the control wiring. This is a sensible approach sometimes.

Now, with certain arrangements of 230V power, there is a "gotcha" with the relationship between supply neutral and earth. In TN-C-S "grounding-not-grounding", they use a single shared wire for both Protective Earth and Neutral (PEN). John Ward does a nice job covering it in this video.


And if you want to see what you can progressively do about it, look at the state of EV charging on TN-C-S.


North America flirted with this arrangement for dryers and ranges, on the logic that the plug is rarely disturbed so a PEN failure is unlikely... but from 1966 to 1996 it was proven to be dangerous and was outlawed.


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