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POLARIZATION CELLS
Polarization cells
are electrochemical devices which are able to resist the flow of DC
current to a few milli amperes whilst allowing AC currents and high DC
surges to pass. They can be fitted across isolating joints, or used
in their own right to provide isolation, i.e. to separate an area
of an earthing system which would cause excessive current drain, whilst
maintaining a fault current path. Polarization cells consist of
stainless steel or nickel plates in a potassium hydroxide electrolyte.
Passage of DC current causes polarization and the build up of a back EMF
due to the formation of hydrogen gas on the cathode plates and oxygen on
the electrode plates. These films are dissipated when an AC or
high DC voltage is seen across the cell. When the fault current
subsides, the cell returns to steady state characteristics. Polarization
cells are available in current ratings from 1kA upwards. Standard
available ratings are 5, 25, and 50 kA. Cell cases are made of
transparent acrylic sheet, whilst terminals and bus bars can be of
brass, copper or stainless steel. The whole cell can be housed in
custom built galvanized ventilated enclosures.
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