Questions about grounding a cab

[jimmy2x2x]

After fitting an IEC socket to my cab I have started to notice an occasional buzzing from the speakers when I touch certain parts of the cab.

There are various metal parts on the cab that are not currently grounded, control panels, cocktail legs, glass clips, large speaker/ air intake grill etc.

If I make a ground loop, connecting all of the metal parts (or as many as practicle) would it be best to connect it to the ground pin on a mains plug (in UK here)?

Not really sure about this, never done this before.

Thanks

[mgb]

Yeah, everything metal should be grounded together and bassically you want it to go back to the ground of your power plug.

[BobA]

Yeah, everything metal should be grounded together.

The purpose of this is to GET RID OF A GROUND LOOP that can cause interference when grounds are at different potentials.

[MonMotha]

Here's the thing to remember about "ground".  The word "ground" is commonly used, especially in the US, for two things: power "common", and earth ground.  I'll attempt to carefully distinguish between them using these terms here, even though many people just say "ground".

Earth ground (often just referred to in Europe as simply "earth") is a safety thing.  All the externally accessible metal bits of your cabinet should be tied together and connected to earth ground to prevent touch hazards between these metal chunks and, well, the earth.  "Earth ground" is the 3rd prong on your IEC inlet.  It's somewhat preferable to run dedicated, heavy gauge wires from each metal chunk back to a central point, but that's often impractical, and just daisy chaining them together is fine, too.

Power common, aka "the black wire", is something else.  It's simply the return path for electric current from your DC supplies.  All your signals and such are also referenced to this "common" line.  This includes not just button signals but also your video signal as well as single-ended (non-balanced) audio lines like the line level audio outputs from your PC.

There's two common schools of thought on how to address this.

A common thing to do is connect these two "grounds" (earth ground and power common) together AT ONE POINT ONLY.  No other connections are allowed as this results in "ground loops" which can cause audio hum, interference patterns in video, etc.  Your PC will do this internally in its power supply.  If any other connections exist, and there's even the SLIGHTEST bit of potential difference between them (which can happen for lots of common reasons), current will want to flow on your signal wires between all these connections to equalize things out.  That's what causes the interference and creates the "ground loop".

The other option is to provide no path at all between the power common line and earth ground.  This will work fine as long as absolutely no paths exist.  However, the way switch mode supplies, like PC power supplies, are built, there's a small amount of current that wants to "find" earth ground, and it's surprisingly easy to create a "weak path" to earth such as through a person if they happen to touch both a connector and a chunk of metal.  It'll also tingle (quite a bit) should you do this, but it's not particularly hazardous.

One thing to note is that arcade monitors will generally provide a tie between power common and earth ground.  This is in a generally undesirable place and is difficult to avoid (since, for safety reasons, the monitor frame really should be grounded), but it's in a place that doesn't generally cause many problems.  I have actually seen commercial arcade designs that don't ground the monitor frames for this reason, though.

The Wikipedia article on "electrical grounding" will explain some of this in more detail.  You can kinda think of the "power common" within your PC and/or cabinet as a "technical ground" as would exist in a TV/radio studio or a "signal ground" that might exist on a circuit board.

[SavannahLion]

If I may....

I'm just adding to what MonMotha has said. He just overlooked a couple of minor details.

First off... Check to see if you even have an "Earth Ground" on your socket, your power strip, or whatever you're plugging your cab into. I spent a mighty long time figuring out why I was constantly getting shocked and getting weird behavior from my PC (in my old apartment) until I realized the landlord was too cheap to do a proper (Earth) ground at the socket. I add a ground myself and those problems went away.

Second... Make sure your socket is wired correctly in the first place. I discovered one socket where the ---uvula--- (same apartment) flipped the hot and common lines. Those sockets are polarized for a reason. If your system is tying the Common and Earth together and you plug in to this socket. Good Luck. I caught it while looking for groundless sockets with one of those chintzy plug "checkers" from Rat Shack.

[mgb]

You guys are kinda over complicating it.
First in ac wiring the power common, or return, also known as the neutral is "the white wire"

What the op is talking about earth grounding his cab so that there are no ground loops which create noise, etc.

This is accomplished by having all metal components on a single ground loop.
there is no question about whether he has a ground at the cabinet because as he stated, he installed an iec socket which would have hot, neutral and ground.
whether his house wiring has a seperate ground is not really a topic of discussion for this group.

[MonMotha]

And you will NEVER hook the AC neutral (yes, sometimes ALSO referred to as "common", and which, btw, is not white in Europe but usually blue) to the DC common, which is what I was referring to (I thought clearly, but apparently not) as "power common".  The AC neutral shouldn't go anywhere other than the power supply's (the box that turns line voltage AC into low voltage DC, so we're clear on that one) neutral terminal.

To make this clear for the OP, the AC neutral (blue in Europe, white in North America) should NEVER be connected to the earth ground/safety ground (green, green/yellow, or bare) in an appliance nor should it EVER be connected to the DC common (black wire) on a new design or a design where a dedicated earth ground (green wire) is available.

Of far more interest in a discussion like this is the DC/signal common, which would be black.  Note that the Europeans happily use brown for their single-phase hot line, so there's no confusion like we end up with in North America.  Ending up with multiple points of connection between this DC/signal common line (the black wire) and earth ground (the green, green/yellow, or bare wire) can cause all sorts of problems with hum and interference patterns, as mentioned.

Likewise, failing to connect all the exposed metal chunks to earth ground (and NOT willy nilly to DC common) can cause all sorts of weirdness, not to mention possibly being a hazard and likely being in violation of local codes and guidelines.

What I'm getting at is that many people assume that "DC power common" (the black wire) and "earth ground" (the green wire and big metal chunks) can be used interchangeably since they are often just referred to as "ground" and they're usually, at some point, connected together.  This train of through is generally the cause of problems.  Ensuring that there's one and only one connection between the two (inside your PC's power supply) will often fix strange ground related problems.  Ensuring that all metal chunks are connected to the earth ground is a good idea for various reasons, including avoiding hum and video interference.

[jimmy2x2x]

Thanks for all your replies, if I am understanding this correctly:



Cable from pin 3 on my mains plug 'Earth Terminal' in the above diagram and daisychain to all metal parts?


Another question, if a metal part - (legs on a cocktail cab for example) is only fixed into the wooden frame of the cab (an insulator?) and has no conductive path to any other part of the cab, would this need to be included in the daisychain too?

Thanks

[mgb]

Any metal part that seems to be isolated from ground can work like an antanae. So bringing all of those parts together to a common ground takes care of that.
like MonMotha said, this ground has nothing to do with your dc common.

@Monmotha, sorry, I thought you were referring to the AC neutral before

[SavannahLion]

I was  talking about the wall socket where you get power from. If you have a miswired wall socket all the work of ensuring your wiring is correct in the cab will be for naught.

People do weird ---steaming pile of meadow muffin--- with their wall sockets.

[mgb]

That is true.

[eerie]

hello, I'm new here, greetings everyone. also, I'm from spain so sorry for my english

I have a problem with the grounding in my arcade. the frame of the monitor, hantarex 9110 25", has a voltage of 130v. I can't find the reason, no wires touching metal or something like that, everything seems to be connected fine. as my arcade wasn't grounded at all, I connected the frame, the cpo and the coin door to an earth wire and that wire directly to the third pin in the socket which goes to the wall that is correctly earthed. when I turned on the arcade, a fuse blew (F1) and my house GFCI also tripped. so I assume that I have to find a solution for those 130volts not being on the frame before grounding anything up. the arcade works fine with the jamma board but I was trying to solve this before connecting the pc and jpac as I think that maybe something could be damaged. what do you think? do I leave it as it is? it seems to work fine, maybe just taking care not to touch the live parts that I already know...

many thanks

[MonMotha]

It sounds like your monitor needs an isolation transformer on its AC power lines and you either don't have one or it's broken.

[DaOld Man]

+1 on what monmotha said.

isolation transformer on ebay

[eerie]

It sounds like your monitor needs an isolation transformer on its AC power lines and you either don't have one or it's broken.

I have a 220/130v transformer which feeds the monitor, I think that should do the function... or maybe not? would adding a second transformer really do anything?


thanks

[DaOld Man]

I dont think adding a second transformer is necessary, unless the first one is bad.
you can check it if you have an ohm meter, disconnect the transformer from the monitor and read from each lead to the metal case or part of the transformer. it should read very high resistance, maybe in the megohms. If it reads low resistance from either secondary lead to the metal part of the transformer, then the secondary winding is shorted to the metal part of the transformer. The transformer is bad. You should get the same results on the primary side, if both of the primary wires are disconnected from the supply line.
primary on a transformer is power in (line) and secondary is power out to the monitor (load).
If the GFIC circuit breaker from the supply is tripping, you have either a direct short, or part of the circuit bleeding to ground. Most likely the insulation of the windings in the transformer have broken down, or a lead coming out could be rubbing against the transformers case, perhaps worn through the insulation and copper touching steel.
Side note: Neither lead of the secondary should touch either lead of the primary. this is why you use the isolation transformer, to remove the common connection between the primary and secondary circuits.

[MonMotha]

It sounds like your monitor needs an isolation transformer on its AC power lines and you either don't have one or it's broken.

I have a 220/130v transformer which feeds the monitor, I think that should do the function... or maybe not? would adding a second transformer really do anything?


thanks

Many step-up/step-down transformers sold to consumers are autotransformers and do not provide the isolation needed.  You can check this by using a meter on resistance setting between one of the primary (input) taps and one of the secondary (output) taps.  It should be very high resistance (megohms) or totally open circuit to indicate an isolating transformer.  You can get isolating step-up and step-down transformers, but they're less common, so be sure to know what you buy.  Putting a separate isolation transformer between the autotransformer and monitor would also work.

[eerie]

hi, I did the checkings and indeed I have one terminal of the primary "connected" to another pin in the secondary so it's just a common transformer and not an isolation one. I'll go look for one tomorrow and let you know if I got my problems solved. many thanks guys

[BKahuna]

To make this clear for the OP, the AC neutral (brown in Europe, white in North America) should NEVER be connected to the earth ground/safety ground (green, green/yellow, or bare) in an appliance nor should it EVER be connected to the DC common (black wire) on a new design or a design where a dedicated earth ground (green wire) is available.

This is incorrect. I realize you stated it correctly, elsewhere in your post, but since this is relatively important, I had to point it out. You might want to edit your post to help eliminate confusion. AC neutral in Europe is Blue, not Brown.

[MonMotha]

To make this clear for the OP, the AC neutral (brown in Europe, white in North America) should NEVER be connected to the earth ground/safety ground (green, green/yellow, or bare) in an appliance nor should it EVER be connected to the DC common (black wire) on a new design or a design where a dedicated earth ground (green wire) is available.

This is incorrect. I realize you stated it correctly, elsewhere in your post, but since this is relatively important, I had to point it out. You might want to edit your post to help eliminate confusion. AC neutral in Europe is Blue, not Brown.

Hum, looks like I did swap those.  Corrected.  I should know those better: while I live in the US, appliance wiring often uses the IEC colors.

[DaOld Man]

Good call on the auto transformer MonMotha, I didnt even think about that.
Who wouldve thunk it?