Mains and Filament Grounding

Grounding is the cause of many problems in amateur made equipment, but if thought about logically, there is no need for it to cause any problems whatsoever. Colloquially, the term "grounding" refers to the mains ground safety bond to the metal chassis, and also to the 0 V signal wiring, but the two are quite distinct.

Ground Safety Bonding

The three wires leaving a domestic supply are line, neutral, and ground. Neutral and ground are connected together at the substation or possibly at the electricity supply company's cable head within the house. this means that if line contains ground, a fault current flows, determined by the ground loop resistance, which is the entire resistance around the loop, including the resistance of the line wire.

The purpose of the ground safety bond is to provide a sufficiently low resistance path to earth that if the line wire of the mains should come into contact with the exposed metalwork (which then would be a shock hazard), the resulting line to earth fault current is sufficiently great to rupture the fuse quickly. The time taken for the fuse to rupture is proportional to the ground loop resistance, so there is no such thing as an ground loop resistance that is too low.

Although exposed tubes may appear to conform with safety standards because the electrodes are insulated by a vacuum and the glass envelope, it the envelope is broken, the secondary layer of insulation also disappears. To ensure conformity, tubes on the top of the chassis should be enclosed by a perforated metal cover.

If you build an amplifier on a chassis with exposed metal, then the construction must be such that all hazardous voltages must be insulated from, and totally enclosed by, ground-bonded metalwork. the ideal place to achieve this bonding is close to the entry of the power cable. The bond should be made using a solder tag bolted to the chassis with a shakeproof (star) washer between the washer and the chassis because this bites in to the metal of the chassis and the tag to provide a gas-tight joint. If the chassis is anodized aluminum, the surface anodizing must be thoroughly scraped away underneath the tag to ensure a good bond.

The nut and bolt should be prevented from loosening using further shakeproof washers and/or locknuts. the ground bond bolt should never pass through plastic, such as the mains input socket, because plastic quickly creeps and causes the bond to loosen.

The ideal ground bond would weld the incoming ground wire from the mains cable directly to the chassis, but this is not very practical. A perfectly reasonable alternative takes the incoming ground wire directly to a solder tag, where it should form a mechanical soldering joint. The tag is then screwed to the chassis with a shakeproof washer either side of the chassis, and another shakeproof washer above the ground tag, followed by a flat washer (to prevent the tag rotation when the bolt is tightened), then secured with a locknut. the nut and screw should be firmly tightened with a large screwdriver and a spanner after soldering otherwise the secure bond to the chassis prevents the iron from heating the tag.

A thick cable should be used to bring mains ground to the bond point in order to reduce ground resistance. Although it is permissible for 3 A rated equipment to have 0.5 Ohm of resistance from the pin to the mains plug to the chassis (not measured directly at the bond point), reducing this resistance to o.1 Ohm, or less, by using 2.5 mm mains cable, reduces the likelihood of hum and further improves safety.

0 Volt System Grounding

it is the 0 V signal ground connection to chassis that causes the hum due to hum loops between multiple grounds, not the safety bond. Hum loops are circuits within ground paths that can have hum currents induced into them by mains transformers. Because there is resistance in the circuit, an unwanted voltage is developed, and this causes the audible hum. To remove the hum, the loop must be broken, and this is often done by removing the ground wire from within the mains plug of one of the affected pieces of equipment, but this is extremely dangerous. The loop should be broken by removing the 0 V signal ground bond to chassis from one of the pieces of equipment. Fortunately, most modern equipment is double insulated, so hum loops do not often occur.

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