Galvanic isolation

Galvanic isolation is a principle of isolating functional sections of electrical systems, thus preventing the movement of charge-carrying particles from one section to another, i.e. no direct current flows between the sections. Energy or information can still be exchanged between the sections by other means, e.g. capacitance, induction or electromagnetic waves, or by optical, acoustic or mechanical means.

Galvanic isolation is used in situations where two or more electric circuits must communicate, but their grounds may be at different potentials. It is an effective method of breaking ground loops by preventing unwanted current from flowing between two units sharing a ground conductor. Galvanic isolation is also used for safety, preventing accidental current from reaching ground through a person's body.

Overview

• Transformers couple by magnetic flux. The primary and secondary windings of true transformers are not connected to each other (but autotransformers do not provide galvanic isolation). The voltage difference that may safely be applied between windings without risk of breakdown (the isolation voltage) is specified in kilovolts by an industry standard. The same applies to transductors. While transformers are usually used to change voltages, isolation transformers with a 1:1 ratio are used in safety applications.
  
  If two electronic systems have a common ground, they are obviously not galvanically isolated. The common ground might not normally and intentionally have connection to functional poles, but might become connected. For this reason isolation transformers do not supply a GND/earth pole.
• Optocouplers transmit information by light waves. The sender (light source) and receiver (photosensitive device) are not electrically connected; typically they are held in place within a matrix of opaque, insulating plastic.
• Capacitors allow alternating current (ac) to flow, but block direct current; they couple ac signals between circuits at different direct voltages. As capacitors can and do fail by going short circuit, connecting together circuits that should be isolated, there are special ratings for capacitors used for safety isolation, such as "Class Y" .
• Magnetocouplers uses giant magnetoresistance (GMR) to couple from AC down to DC.

Applications

• Optocouplers to decouple a function block from another connected to the power grid or other high voltage, for safety and equipment protection.
• Optocouplers to transmit a signal between circuit blocks at different potentials.
• Transformers to allow the output of a device to "float" relative to ground to avoid potential ground loops.
• Isolation transformers to increase the safety of a device, so that a person touching a live portion of the circuit will not have current flow through them to earth.
• Power sockets intended for electric razor supply must employ a residual-current device or an isolation transformer (to prevent an electric shock if the razor should be dropped into water).
• Boats connected to improperly grounded shore supply systems should employ a galvanic isolator to isolate from any stray currents in the marina (either from the shore mains or neighbouring boats). Without the isolator, a galvanic corrosion path may be created damaging metal equipment below the waterline. Zinc anodes may help to prevent this, but often prove insufficient when the boat stays in the marina for extended periods.