Safety-Earthing-GNDing

EARTH and GROUND & Safety
(written/compiled by 41Hz-user Scratchy, it contains some info from sound.westhost.com)


I encourage everyone to fully understand why they are doing something a certain way, and not just because some guy on a forum told them to do it that way. Knowledge without understanding can be a dangerous thing.

I know it's a lot of information to absorb, but it's better to be safe than sorry.
URL with complete/original article but without new 41hz additions: Earthing your equipment (while you're there, the other articles (link to index on top and bottom) might prove useful too. ed.)

(items mentioned here prevent possibly a annoying humm/buzz, hiss or other weird problems. ed.)

• Do not use the earth connection as a mounting pint for any other panel or component - it must be dedicated to the task of providing a safety earth point. If a component mounting bolt is used, at some stage it may be disconnected by a service (or other) person, which means that the appliance is unsafe until everything is (hopefully) put back where it belongs - this does not always happen.
• You should fix all safety issues, as you never know: you might NOT be the only person opening this amp, even 5 or 10 years down the road!

If you read the article (by using the link above), and build the PS section as described, then even forgetting to hit the power switch would be safe from prying fingers (note the fuse holder used in the drawing below is fully enclosed):

• Note that required sleeving over all mains connections are not shown. These are ESSENTIAL for electrical safety, and usually just means the required heatshrink tubing is placed over the wire(s) before attaching and soldering. Make sure that soldering does not heat the tubing, or it will shrink before it is properly located. To this end, make sure that the tubing can be located at least 25mm (1") and preferably more, away from the solder connection.
• Rubber boots are available for IEC chassis mount connectors, and large heatshrink can be used to completely encase the fuseholder. Don't forget to feed the wires through the heatshrink or rubber boot before soldering!

As well as proper safeguards against accidental contact with the mains, it is also extremely important to keep mains and low voltage wiring well separated.
This means either physical separation, or reinforced insulation between the two sets of wiring.
(remember that because of sometimes high voltages and usually high frequencies of a T or D-Class amplifier, the output can be considered quite dangerous too! (example: +/- 60Vdc rails then gives max. 120 Vac-peak-to-peak (Vac-pp)! ed.)

(also try not to run un-shielded signal or output lines (and especially mains lines) in parallel ; they WILL influence each other, but this a separate topic. ed.)

If physical separation is used (and this is the most common and easily achieved), make sure that wherever possible, the minimum distance is around 25mm.
It should not be possible to squeeze or otherwise coerce the primary (mains) and secondary (low voltage) together under any circumstances. All wiring must be secured using cable ties, and suitable chassis anchors may be needed in some cases to ensure that all wiring remains properly separated.

With most toroid-transformers, all leads come out of the over-wrapping in (more or less) the same general area. Normally the insulation provided is sufficient to ensure safety (double insulated), but some additional heatshrink tubing will not go astray if the leads are close together.

The diagram above shows the location for an optional loop breaker (optional as in: if you want to connect Earth to Pgnd, use it. ed.).
Full details of this are in the article 'Earthing Your Hi-Fi' (URL above).
The loop breaker allows the internal electronics to "float" during normal operation, and it effectively disrupts any earth/ground loop induced hum when two or more pieces of equipment are connected together.

• It is extremely important that all input and output connectors are isolated from the chassis, and this applies whether the loop breaker is included or not.
• Secondly, it is best not to connect PGND directly to the chassis ground/earth - NOTE THE LOOP BREAKER in the diagram above.

(loop-breaker can be a resistor 10-20ohm-5W, parallel with a 100nF cap, check through URL above)

• If you are using a pre-amp then the AGND source (RCA input gnd) is most likely already connected to Earth.
• The ferrite bead connects the AGND to the PGND and is meant to isolate noise between the 2 grounds. By earthing the PGND, you have created 2 ground points which is a potential ground loop - not a good thing.

The only exception is if a double insulated mains transformer is used, but these are rare. Should the transformer be of "conventional" construction (not a toroidal), then the transformer body - the steel core - must be connected to chassis directly. Do not use any loop breaker circuit to isolate the transformer core, as it is unnecessary and dangerous to do so.Since you are using a Toroid transformer, isolating the 0 Volt line (center-tap) from earth can be considered safe.

• Make sure the PGND is completely isolated from the chassis ground/earth.
• This includes the copper heat slug on the Tripath chip/MosFETs/other (mica, thermal pad, etc.)
• With everything unplugged (including the RCA inputs, you should measure infinite ohms between PGND and the ground screw of the chassis.
• Yes, it's OK to tie all three RCA AGNDs together. (usually, they are connected on the PCB = Agnd starpoint, try not to do it elsewhere too)

Finally
When connecting Earth to PGND or PCB to chassis (i.e. not insulated), you in effect will create a one turn transformer winding that puts out a low voltage ~ 0.33 Volts at high current, which is flowing through your input cabling and preamp.
Never route an earth wire to the main (star) earthing point on a chassis in such a way that it forms a partial (or full) turn around a transformer. It is better to relocate either the star earth point or the transformer to ensure that no earth conductor can create a partial turn.

There may often be conflicting requirements, but there is usually no reason that proper earthing for minimum hum and maximum safety should be mutually exclusive. Both are important, and both must be accommodated in the final design.