ground

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published: Jun-17-2015, updated: Mar-16-2017

What does it mean ….Ground

There are basically 3 types of ‘Ground’ which is referred to in electronics
Safety ground,
Common signal path/return path,
The ground we actually walk on.

1: Safety ground is the ground we all know as the ground pin in the wall outlet. It is there for one reason only and that is safety, to protect us against electrical shock when an appliance has metal parts (or conductive parts) that can be touched from the outside.  The reason being that in case  of certain failures the enclosure/outputs may come in contact with electrical voltages inside.

Safety ground is NOT needed for the so called ‘double insulated’ appliances which have just 2 pins.

Safety ground does little to nothing against common mode ‘garbage’ being emitted by the power supply and/or internal circuitery.  Even 30cm of safety ground wire (the well known yellow/green wire in the mains cord) is a substantial resistance for high (RF range) frequencies as it basically is an inductor. The impedance (virtual resistance) of an inductor rises with the frequency so for signals in the audio range these wires are 0 Ohm, only for RF signals >10MHz the inductance of mains wires become an obstacle. Safety ground can help lower L(ow) F(requency), i.e. audio range, common mode ‘garbage’ OR increase the problems depending on several circumstances. The article would become too long and too technical if I were to elaborate on all kinds of circumstances so will leave it with this basic explanation.

When you want to find out if equipment is safety grounded or not you can simply measure that with a multimeter in resistance mode. There should be very little resistance (max a few Ohms) between the ground prong of the plug and the enclosure (bare metal parts, paint usually is a good insulator).

All equipment that has a 3 prong connector must (should) be connected to a 3 prong wall outlet. connecting such a device via a home made extension cord with a 3 prong socket and 2 prong plug is anything but smart as there is a risk of getting an electrical shock or appliances can emit more common mode garbage as is allowed. Also you run a higher risk of getting audible ‘noises’ when you connect a 3 prong audio device that is connected to a 2 prong outlet. More on this in the ‘how to setup my equipment for lowest leakage currents’ part. Using safety ground correctly doesn’t guarantee optimal results with the least amount of audible ‘garbage’ but may lower it to levels that are below the interference limits of the equipment used.

2: Common signal, ground plane, return wiring. Most ‘communication signals’ be them audio, video, digital, or otherwise between equipment need a common (also called reference or return wire). This is true for wiring in USB, firewire and coax, though caox could be an exception depending on the way it is made. Only true symmetrical signals (using signal trafo’s) do not need a common but often still have one to act as an extra screen or to connect the ‘common’ of both sides to prevent static electricity problems that could potentially cause failures.

Symmetrical (balanced) audio signals are generally used when long lengths of wire is used and or in environments where lots of electric interference is expected. It is rather pointless to use balanced home equipment unless it’s the only way it can be connected.

We speak of a ‘ground-lift’ if there isn’t a connection between safety ground and ‘reference ground/common’ of the used PCB. This can be beneficial but also be counterproductive depending on the circumstances. It’s why the ground-lift usually is ‘switch-able’. Common or reference ground (as in ground planes on a PCB or a sealed enclosure) can be effective against entering or exiting common mode garbage but ONLY when designed properly AND above all connected properly ! PCB design and cable termination, including screens, is of paramount importance and MOST DIY efforts fail in multiple aspects, as do a lot of ‘audio only’ firms by the way, simply by lack of knowledge in this particular discipline. Having a proper ground plane can prevent a lot of problems. A PCB (Printed Circuit Board) with just a few traces can behave well but when not designed properly can be problematic. A way around this ( but may be difficult to do properly) is to use a single ground point where all ground reference wires converge. The best way is to have a well designed ground plane. A ground plane is literally all the copper that isn’t used for the signal traces  that is connected to the common/reference ground. The better this plane, the lower it’s resistance, the better it works. The better PCB’s with 2 layers often have planes on top and bottom side connected with so called vias (metallizations from top to bottom plane). An even better way is to have boards with multiple layers (4 or more) where at least one layer is a complete un-interrrupted plane.  If you knew how many designs fail EMC testing in the first attempt and need alterations in the (PCB) design just to pass minimal criteria is rather big. Digital has forced the designers community to pay more attention to these aspects. It’s actually a separate branch of science in the electronics industry and not understood by MANY electronics designers b.t.w. HF currents (common mode that is) IS actually measured with a clamp that is connected to a special EMI receiver (picture google: EMI receiver). The currents in USB are small (unless the load draws a lot from the power wire) and you will need a sensitive probe and won’t tell much about anything going on inside the cable as one measures all wires at the same time. a multimeter won’t show much above a couple kHz already (if not sooner) while the nasty ones are above the MHz range.

3: the Ground we walk on also is part of the equation and actually can be of influence on certain equipment that is sensitive to HF signals (>100MHz) that can cause problems by ‘detection’ of these signals which may become audible. The ground we walk on has a certain resistance (impedance) for HF signals. For signals in the audible range the ground we walk on is the same as safety ground, just not electrically connected.

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