Proper grounding metal parts

Thread Starter

Raymond175

Joined May 28, 2019
57
Hello,

I have a question about how I can properly ground all the external metal parts of my pinball machine. I'm using the Mean Well SP-320-48 power supply that requires the green wire to be connected to the GND terminal (see picture below), which will ground the power supply itself. But now I need to also ground the legs, side rails, coin door, etc.

I have my ground, neutral and line connected using insulated fork type cable lugs. Should I now "just" add a second fork type cable lug to the GND terminal that connects to all my metal parts, so that these are all connected to ground via the wall socket?

1612207982214.png

Kind regards,
Raymond
 

ElectricSpidey

Joined Dec 2, 2017
1,651
Yes

Personally I would bring the AC lines to a barrier strip first then put a fuse in the hot line and connect the metal parts to ground there.
 
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Thread Starter

Raymond175

Joined May 28, 2019
57
Personally I would bring the AC lines to a barrier strip first then put a fuse in the hot line and connect the metal parts to ground there.
My power supply connects the V+ and V- to my power board (PCB) where this is split into multiple power rails that all have a fuse already. But if I understand correctly I now can also connect the GND to my power board and split this into multiple connectors as well, so that I can just plug-in the connectors of the grounding wires?
 

MaxHeadRoom

Joined Jul 18, 2013
23,084
Normally a standard way of grounding a system is to set up a 'star' point ground that you take all the ground wires to including the service ground conductor.
Especially required if needing to prevent ground loops.
Max.
 

Thread Starter

Raymond175

Joined May 28, 2019
57
Normally a standard way of grounding a system is to set up a 'star' point ground that you take all the ground wires to including the service ground conductor.
Especially required if needing to prevent ground loops.
Max.
I understand the star ground and ground planes, but these are for the electrical circuits themselves aren't they? Having a single "common" and to prevent ground loops. But in my case I need to ground metal parts, not circuits, just to "prevent" (bad wording) shocking the player.

Do you say I should connect that to the same ground as the circuits? Because I intended to have separate "earth" (GND via power supply to wall socket) and "common" (GND via microcontroller) rails in my power board (PCB).
 

MaxHeadRoom

Joined Jul 18, 2013
23,084
All depends how much galvanic isolation you have between the LV circuits and HV, there is no problem connecting LV to earth GND and also Reduces the chance of noise issues.
Just connect (bond) all the metallic parts of the machine you are concerned about to the earth conductor.
The star GND can be used for the machine as it is done in industrial machinery.
AKA equi-potential bonding.
Max.
 

Tonyr1084

Joined Sep 24, 2015
5,956
"Ground" is meant to redirect any uncontrolled electrical energy from harming a human who may be in contact with metal parts. In many cases ground is used to manage noise from other electronic equipment with regards the electrical circuitry. However, the main purpose of the "Earth Ground", it is intended to protect human life. Should there be a situation where high voltage (110VAC or higher) comes in contact with metal framing, legs, side-rails - whatever, it might not blow the breaker, but present a live energized surface that a human could touch.

In high school electronics class we had a wave generator with a bad ground. I was running it with an oscilloscope. Inadvertently I rested one hand on top of the scope and the other on the generator. I got a full dose of 110VAC from frame to frame. The wave generator ground was bad and the primary of the main transformer was shorted to its core. Live energy from the transformer core to the metal frame, to my right hand, through my body to the frame of the properly grounded oscilloscope. In that electronics class students did all sorts of dangerous practical jokes just for a laugh, not realizing the danger they put people into. One trick was to cut a piece of solder and wedge it in between the prongs of an electrical plug. When you plugged it in it was a dead short. You jumped and everyone laughed, even though there was hot lead and tin spewed all over things. Someone thought it was funny to plug a TV in that I was actively working on. I got a damned good shocking and the teacher grounded every student in the class. No more messing with line voltages.

Sorry - I got off on a tare. Ground all metal parts if there's a remote chance they could become energized.
 

Hymie

Joined Mar 30, 2018
976
The chassis/enclosure of the Meanwell PSU will be connected to the earth terminal of the unit – therefore you could use the conductive mounting of the PSU to provide the earth connection to your pinball machine chassis.
 

Thread Starter

Raymond175

Joined May 28, 2019
57
The chassis/enclosure of the Meanwell PSU will be connected to the earth terminal of the unit – therefore you could use the conductive mounting of the PSU to provide the earth connection to your pinball machine chassis.
There you have a very good point indeed... For some reason I keep forgetting the most obvious things lately, it must be the time we live in now...
 

metermannd

Joined Oct 25, 2020
189
I don't know that I would go to the trouble of grounding ALL the exposed metal, especially if there is no existing wiring near the parts in question.

The legs are made to come off for transport from the factory to the distributor, so the chance of any wire near the leg mounts, even on a electromech machine, is fairly low. Same goes for trim / back cover on the backbox, as that too is made to be separated from the main cabinet for more efficient transport.

The absolute bare minimum I would ground is the coin door itself (not the frame) and the power switch mounting plate.

Grounding the playfield glass supports is a good idea, sure, but keep in mind the ground wire attachment has to remain clear of the playfield or there may be difficulty in raising / lowering it.
 

Thread Starter

Raymond175

Joined May 28, 2019
57
I don't know that I would go to the trouble of grounding ALL the exposed metal, especially if there is no existing wiring near the parts in question.

The legs are made to come off for transport from the factory to the distributor, so the chance of any wire near the leg mounts, even on a electromech machine, is fairly low. Same goes for trim / back cover on the backbox, as that too is made to be separated from the main cabinet for more efficient transport.

The absolute bare minimum I would ground is the coin door itself (not the frame) and the power switch mounting plate.

Grounding the playfield glass supports is a good idea, sure, but keep in mind the ground wire attachment has to remain clear of the playfield or there may be difficulty in raising / lowering it.
The legs are bolted into a large metal bracket on the inside of the cabinet which is swarmed with cables all around, for example see the following random picture from the internet (where you can see the ground also):
1612254838633.png

And of course it doesn't make sense to ground every metal part, but since its "just" a wire, why not ground all the obvious parts?
 
I wish I had a better Idea of what your dealing with. I have recently been doing some reading on groundig in a medical environment and you don't want the grounds coming undone.

I rebuilt a 13kVDC at 1.5A and a 100 kV DC at 0.1A. There were so many loose connections and failed components.
I did use locktite 222 on the threads.

Fork terminals are not a good idea. Ring terminals with external tooth lockwashers or Keps nuts are preferred. press in studs are preferred. These are all designed to prevent loosening.

one thing you could do, is to use DIN rail https://www.asi-ez.com/member/~PR005-1m.asp and ground terminal blocks. e.g. https://www.asi-ez.com/searchresults.asp?find_spec=ground terminals

usually, a group of terminals requires 2 end stops per group, end covers, partition plates and terminal blocks. Ground blocks generally don't - they just bite into the rail and are self supporting. An end cover finishes up a terminal and a partition plate makes a little space between ajacent terminals.

Your big ground bus can be pieces of DIN rail. You can fan out to attach to external plates.

You really care about 120V shorting to an external metal plate. Short circuit protection. Also leakage protection.

You might be concerned about RFI, EMI and static and connect accordingly. Shielding reduces RFI.

Static protection is different thing. See https://www.electronicdesign.com/po...ts-the-difference-between-hbm-cdm-and-mm-test

inputs need to be protected. An example might be a keypad.
 

Ian0

Joined Aug 7, 2020
2,162
The legs are bolted into a large metal bracket on the inside of the cabinet
In that case, earth the bracket. It's one wire for four legs. If the leg is detached it is insulated, if it is screwed in it is earthed.
It's important to make sure that if any "foreign object" is inserted in the hole where the leg goes, then it can't touch anything live. The relevant standard defines how big the "foreign object" has to be - a probe 100mm long and 3mm wide.
The standard makes a distinction between parts that can be removed by hand and those requiring a tool to remove them. The probe test is carried out when all parts that can be removed by hand have been removed.

If there are any parts that can be classed as "double insulated" or "reinforced insulation" meaning 5mm clearance, which cannot be compromised, between live and parts that can be touched, then they don't have to be earthed, but there's no harm in earthing them to be on the safe side. (5mm applies to 230V mains)
 

Thread Starter

Raymond175

Joined May 28, 2019
57
Fork terminals are not a good idea. Ring terminals with external tooth lockwashers or Keps nuts are preferred. press in studs are preferred. These are all designed to prevent loosening.
This is exactly what I have on the elements that need grounding. I only wanted to use the fork type on the power supply terminal themselves, because there doesn't seem to be enough space for the ring types there, but I might have a better look later.
 

Ian0

Joined Aug 7, 2020
2,162
Fork terminals are not just "not a good idea" - the standard requires ring terminals so that they cannot be removed without undoing the nut.
Some countries (Canada, I think) require the incoming earth to go on first, followed by a nut, then all the rest of the earths, then the lock nut.
 

Thread Starter

Raymond175

Joined May 28, 2019
57
In that case, earth the bracket. It's one wire for four legs.
I indeed have the grounding attached to one of the 8 screws that keep the leg bracket into place, so not to the legs themselves. The metal pieces are connected by a braided ground stap:

1612261249289.png
 

Thread Starter

Raymond175

Joined May 28, 2019
57
Fork terminals are not just "not a good idea" - the standard requires ring terminals so that they cannot be removed without undoing the nut.
Some countries (Canada, I think) require the incoming earth to go on first, followed by a nut, then all the rest of the earths, then the lock nut.
I'll look into it, it makes sense!
 
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