Grounding Housings of electronics

[healhustler]

Regarding the grounding lugs on the outside of electronic cases, I’ve read somewhere that it’s better to connect these lugs to the bonding system rather than the battery negative, claiming that there is less noise in the electronics, but friends still claim battery negative is correct. Internal device power remains battery positive & negative, of course. To be clear, I’m speaking about the ground connections on the outside of chart plotters, sounders, etc..

Thanks in advance for any feedback.

 

[O C Diver]

I've always run mine to the bonding system. If they wanted it to the battery negative, think they would call it a battery negative lug.

Ted

 

[psneeld]

The bonding system, but the systems are eventtually tied together.

Many units are never grounded snd work fine and for years.... other than bleeding of RF or once thst internally produce high voltage....am curious to know why a big deal (if any) to ground them.

 

[diver dave]

It is rarely necessary to bond/earth the case of low voltage electronics. The huge exception is a SSB/HF auto tuner.
THe equipment I design for power utility use most always has external bonding, but that is for human safety, equipment survival for lightning, and fault survival.

If you had a metal box up on a fiberglass masthead, I would bond that for lightning safety. But the Garmin, etc at the helm? not necessary.

 

[what_barnacles]

connecting electronic equipment to the bonding system could induce stray currents into the one thing you dont want them to exist.

 

[angus99]

I've always run mine to the bonding system. If they wanted it to the battery negative, think they would call it a battery negative lug.

Ted

I know the OP is asking about chart plotters, etc., but our Magnum inverter does call for a massive case-bonding to the DC negative bus. (Just in case someone doesn’t start with the first post and thinks case-bonding doesn’t apply to inverters.)

 

[psneeld]

connecting electronic equipment to the bonding system could induce stray currents into the one thing you dont want them to exist.

Well, if I'm not mistaken, thats what tbe scew on tbe back is for with the little earth symbol... or many power cords just have a bare wire that goes to ground.

As recommended by the manufacturer.

On LORANS, if you didnt connect that ground, it seemed to be way more squirrely....but as I posted, it doesnt usually seem to matter....but I dont think it is bad either.

 

[Hamrow]

It is rarely necessary to bond/earth the case of low voltage electronics. The huge exception is a SSB/HF auto tuner.

Well, if I'm not mistaken, thats what tbe scew on tbe back is for with the little earth symbol... or many power cords just have a bare wire that goes to ground.

As recommended by the manufacturer.

On LORANS, if you didnt connect that ground, it seemed to be way more squirrely....but as I posted, it doesnt usually seem to matter....but I dont think it is bad either.

Yes that little earth symbol (horizontal lines forming an upside down triangle) are typically called Earth or chassis ground. in cars & aircraft, these would go to any nearby metal frame, but boats are a bit different due to the risk of galvanic issues.
in full disclosure, i've been in the electronics industry for many years but somewhat new to the boating industry... but having said that, i would not connect a chassis ground directly to a battery.
for example, one of my Garmin boxes has a screw on the back designated as a chassis ground but the manual says it is usually not necessary to connect it to anything.
The common factor on both systems mentioned above (SSB and LORANS) is that they were both very far below VHF in terms of frequency. they were both roughly in the range of say 2-12 MHz, vs VHF starting up at 156 MHz.
Very low (HF) systems are inherently more problematic with electromagnetic interference so it makes sense that proper grounding or adding noise capacitors would be more necessary for those systems.

 

[what_barnacles]

Well, if I'm not mistaken, thats what tbe scew on tbe back is for with the little earth symbol... or many power cords just have a bare wire that goes to ground.

As recommended by the manufacturer.

On LORANS, if you didnt connect that ground, it seemed to be way more squirrely....but as I posted, it doesnt usually seem to matter....but I dont think it is bad either.

"that goes to ground"

Right, ground, not the bonding system.

 

[diver dave]

My loran c was really flaky when my cabin fluorescent light was on. I think all loran is now dead?

 

[twistedtree]

Lots of room for confusion with terminology. To some people, "ground" means battery negative, where to other people, "ground" means the bonding system.

The green wire lugs on electronics should be wired to the bonding system, not the battery negative system. The two systems are connected together at only one point to prevent current cross flow, but really should be viewed as totally different systems, just like the ground and neutral in an AC power system.

The primary reason for the green wire case ground/bonding is for RF shielding. Without it, the case of the electronics is a less effective, or ineffective shield for RF radiation. Radiation emitted by one device can in turn be picked up by another device and cause miss-operation, noise, or other interference. The shielding keeps inside radiation inside, and outside radiation outside.

How much of a risk is there without the shield bonded? Probably not much other than with the fore mentioned SSBs. But do you really want electronics that might go wonky on you from time to time with no apparent reason? I don't.

 

[psneeld]

"that goes to ground"

Right, ground, not the bonding system.

Earth ground versus neg terminal ground for current carrying circuit.....

 

[diver dave]

Bonding has nothing to do with rf shielding. The shield works because of wall conductivity and/or high permeability. Running a wire to this wall does not affect that. A metal box floating in space is a fine shield.

 

[twistedtree]

Bonding has nothing to do with rf shielding. The shield works because of wall conductivity and/or high permeability. Running a wire to this wall does not affect that. A metal box floating in space is a fine shield.

Then why are cable shields and equipment chassis grounded and not left floating?

 

[diver dave]

Cable shields “extend” the box shield. Otherwise what you have is a shield penetrated by unfiltered signal source.

 

[twistedtree]

Cable shields “extend” the box shield. Otherwise what you have is a shield penetrated by unfiltered signal source.

But why does everyone recommend they be tied to ground if it does nothing?

Any why does the signal wire RF penetrate the cable shield (I assume this is what you are saying) when the shield is isolated from a box, and not when it is connected?

 

[diver dave]

But why does everyone recommend they be tied to ground if it does nothing?

Any why does the signal wire RF penetrate the cable shield (I assume this is what you are saying) when the shield is isolated from a box, and not when it is connected?

I'm not sure how much shielding theory and EMC this forum will accept.

BUT, A shield requires no connection to earth(dirt) to function. There are plenty of shields inside your cell phone that work very well and see no wires to earth.
Same with a handheld VHF.

In regards a wire having to penetrate a shield:
Let's say you have a perfect shield, but, of course you need to power the device and get inputs and outputs from this shielded device.
One cannot violate the shield by running a bare, unshielded wire into the shield. WHat you would have is an antenna, picking up energy on one side of the shield wall, and transferring it the to other side of the shield wall. And, that effect is bilateral (or commutative??). So, what is done is one of several things.
1. A filter is fitted at the shield wall that suppresses unwanted energy in the wire of interest. Lumped or discrete inductive and capacitve elements typically, or,
2. The wire is shielded itself, and will need to connect to the wall of the product shield. This, in effect, extends the product shield to the wire.

In no case does a "wire" connected to the shield WALL going to earth, or any other place, make a shield work better.

Now, there are occasions where a bond wire to the shield makes a product work better. It won't be due to shielding improvements, but more likely an antenna counterpoise, or other effect.

HF/SSB, and LoranA/C, Omega, etc are low frequency devices, and the associated marine antennas can benefit from a counterpoise. Which COULD be done by "grounding" the case of the receiver, or tuner.

One of the "other effects" is a static bleed, due to antenna charge buildup. Those you want to bleed to earth, via that thumbscrew/wingnut. That can reduce corona, st elmo's etc, either will cause receiver noise.

 

[twistedtree]

In no case does a "wire" connected to the shield WALL going to earth, or any other place, make a shield work better.

This is the only part of what you say that I would disagree with. Analog Devices has a good paper on the subject.

An un-grounded shield has a significant shielding effect. No doubt about that. But any RF "inside" the shield will induce a voltage in the shield. That in turn makes the shield an antenna that radiates to the outside. Conversely, any "external" RF will similarly induce a voltage in the shield, which in turn is coupled to the "inside". Now these coupled signals are significantly attenuated, but the shield is still an antenna. By grounding the shield, you provide a path to drain the induced energy from the shield. That's why it's often referred to as a drain wire, and it increases shielding accordingly.

Now as many, many people have demonstrated, you can often run just fine without grounding the shield/drain, But doing so provides an extra measure of protection. Prudent? Perhaps. Overkill? Perhaps. But the designers of all these products didn't install those ground lugs for decoration, and tell you how to hook it up just for kicks.

 

[diver dave]

AN-347? I’ll respond to that one in a bit.

 

[diver dave]

http://www.analog.com/media/en/technical-documentation/application-notes/41727248AN_347.pdf

This is worth a read. In the very beginning, the drawing of a Faraday Shield is showing that with a "perfectly conductive" shield, you cannot have any voltage gradient inside the shield. Therefore, its a perfect shield, and grounding (or not) is not a factor. As you add wiring, the situation degrades. Now, the problems of grounding the ends of the wire shield can become complex. Current loops, length of the cable, and frequency of the interference become important.

I did see one thing, that has stuck in my mind. In the technical museum in Munich, there is a HV generator, with a Faraday cage up near the ceiling. They ask for a volunteer to sit inside the shield. Then, the operator yells Actung, and you hear a loud bang, as the lightning bolt goes around the person and shield, on its way to a little model Bavarian town, hitting the church steeple. The shield is not grounded, there is a human inside, and there is tremendous voltage and current shot at him. He walked out a little deaf maybe, but unscathed