Contamination Severity
The contamination is separated into the following two general classes:

a) Industrial: The industrial contamination is caused by particles driven by the wind and deposited on
the insulator surface. The contamination may be dust from road or rural areas, cement, fly ash, limestone,
etc. These materials contain salt and form a conducting layer when wetted.

b) Sea: Salt-water spray driven by the wind can also contaminate insulating surfaces. This forms a conducting
layer. In this case, the contamination and wetting occurs simultaneously.

The degree or severity of the contamination is specified in two ways
— Industrial contamination, with the amount of salt on the surface, is normally specified in units of mg/cm2.
— Sea contamination, the amount of salt per volume of water, is normally specified in grams per liter or in kg per cubic meter of water.
Contamination Site Severity
To standardize for industrial contamination, the equivalent salt deposit density (ESDD) is used, defined as the equivalent amount of NaCl, which would yield the same conductivity as the contaminant. Another method of measuring industrial contamination severity, used in Germany, is to measure the conductivity of the moistened layer. 

The appropriate measurement unit is the microsiemens, mS. IEEE Std 4-1995 gives the method of collecting the contaminate and the measurement of contamination level.

The general site severity and its definition per IEEE Std 1243-1997 [B9] and CIGRE Technical Bulletin 63 [B10] in terms of the ESDD. The layer conductivity is approximately 100 times the ESDD and the salt spray salinity is 140 times the ESDD. 

For example, the equivalent layer conductivity and the equivalent salt spray salinity for an ESDD of 0.05 is 5 mS and 7 kg/m3, respectively.

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