Simple wiring question......

Re: Simple wiring question......

http://www.cdxetextbook.com/electrical/princ/comp/shielding.html

maybe I misunderstood
but my understanding is that a wiring shield or drain
should be connected only on one side

in the industrial control circuits
the shielded cables are grounded at one end to prevent the high voltage currents in the cabinets from inducing noise into the control circuit

drain wire Telecom Definition
A wire wrapped around or part of a shield within a cable that reduces the resistance from any point on the shield to ground. A drain wire serves to complete an electrical circuit from the shield, thereby carrying extraneous electrical noise to ground and away from the circuit or system the shield is intended to protect. See also ground, ground wire, and shield.
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shield Telecom Definition
A covering, sheath, or screen designed to protect a signal traveling over a conductor from electromagnetic interference (EMI) due to the unwanted coupling of extraneous signals.The shield is in the form of a conductive metal foil, screen, or braid that surrounds a conductor or group of conductors. A dielectric material insulates the conductors from each other and from the shield, which essentially is an outer conductor that intercepts extraneous signals and conducts them to ground. Therefore, the shield must be continuous and must be grounded via a ground wire, or drain wire.To ensure electrical continuity, the shield must be bonded at any splice points and the shield must be bonded to the ground wire, strap, or rod. Coaxial cable, screened twisted pair (ScTP), and shielded twisted pair (STP) are examples of shielded transmission media in telecommunications applications. See also coaxial cable, dielectric, EMI, ground wire, ScTP, and STP.
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Signal Ground
Power supplies, circuit boards and signal pathways in most electronic equipment are also connected to the same earth ground as the chassis for reference voltage and safety. There are exceptions; for example, medical equipment that is attached to a patient is generally isolated from earth ground even when plugged in, because the time between a short circuit and the breaker tripping could prove fatal. Ethernet adapters and other networking equipment are also isolated from earth ground in order to prevent unbalanced ground loops from causing interference (see ground loop).
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here


ground loop Telecom Definition
A complete circuit made up of the earth, represented by two different grounding points, and another conductor such as the shield of a cable grounded to devices at both ends. A surge of electricity results in the cable when the grounding points of different devices are at different voltage levels, and the data line connects the devices' ground together. See also ground, surge, and voltage.
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Re: Simple wiring question......

In a guitar you have asymmetric cables. One hot inside, one shield. Since an electric circuit must be formed to the amplifier you need two conductors. In the case of asymmetric cables one is the hot wire, the other is the shield. So the shield must be connected to the amp's input shield and to the pickup/pot in the guitar.

The situation is different when you have symmetric cables like you use for microphones. These have two hot wires inside and a shield outside. Here the circuit is formed with the two hot wires and the shield is not part of the circuit. So the shield in only there for -well- the shielding, and it does that job if it is grounded anywhere. So in this case it is sufficient to only connect the shield on one side of the cable, as long as that side is grounded. There is another complication in that you can induce hum if the shield is connected on both sides and both sides of the cable are in equipment pieces, and both equipment pieces are grounded, but lousy cables cause different ground potential. That is called a ground loop and it causes the 50/60 Hz cycle to become audible. In that case you deliberately make sure that the cable's shield is only grounded on one side.

That has nothing to do with electric guitars since the guitar it not grounded by itself except through the one cable, and in any case the cable is asymmetric so you can't lift the shield. (in theory you could use a symmetric cable for the guitar, though)
 
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