Flashover of insulators in transmission and distribution systems may cause
costly outages for power companies and their customers. Industrial and/or
coastal pollution of external insulation is a major cause for such events at the
normal power frequency voltage of the systems. The power companies are
now facing increasing competition resulting in pressure to lower the cost and
to increase the system reliability. Different methods have been applied in the
past to overcome or reduce the problems with flashover on insulators.
Methods which should provide reliable data under real physical conditions. In
this paper several measuring methods to evaluate the pollution levels on
outdoor insulators are described. According to this comparison, Leakage
Current Measurement ‘LCM’ method is a reliable method for measurement
leakage current in outdoor insulators and surge arresters.
Keywords
Reliability, Leakage Current Measurement, Site Pollution Severity, Flashover

 Flashover of high voltage insulators results in the reduction of reliability of power
systems and irretrievable losses to the power networks. The possibility of flashover in
contaminated environments depends on the type of pollution and duration of the time that an
insulator is placed in a polluted environment. The occurrence of pollution-born flashover on
insulator surface generally consists of the following stages [1, 13]:
• Settling of pollutants on the insulator surface
• Compounding soluble pollutants with rainwater and formation of a conductive layer
• Formation of leakage current
• Insulator surface’s getting hot and formation of a dry area
• Partial discharge and the occurrence of flashover.
Therefore, inappropriateness of insulation designing will be resulted, flashover and
consequently in system outages. Hence, identifying different factors causing insulation
surface pollution and dealing with their unfavorable effects have an important role in
increasing the reliability of the network [1, 13, 15, 17, 21]. The major consequence of
pollution is the reduction of insulation in high-voltage transmission lines and substations.
During recent years, many studies have been carried out to improve methods of pollution
measurement, including IEC815, IEC507, IEC383, IEC1106, ANSI C290, DE0441,
VDE0218, and ASTMD2302. For this purpose, the relationship between Equivalent Salt
Deposit Density ESDD/ Non-soluble Salt Deposit Density NSDD and LCM in high-voltage
insulators have been studied [1, 6, 11]. The performance of spiral-shaped insulators and their
electric resistance against pollutants is rather weak [20]. In a study carried out on pollution
measurement, analysis of performance of ceramic insulators under polluted (estimation)
circumstances is based on ESDD and LCM [12]. The relationship and the confirmation
coefficient between flashover voltage and leakage current have been analyzed in [10].
Preparing the pollution map affecting insulation based on the values obtained from pollution
measurement with ESDD and Directional Dust Deposit Gauges (DDG) can also prove useful
in designing insulator of high voltage substations and transmission lines [13]. Among other
issues related to the methods of insulation pollution measurements are the relationship
between ESDD and resistance of high voltage insulator’s surfaces [2, 8, 9] and estimation of
Salt Deposit Density (SDD) content in terms of leakage current peak [3]. In this paper, the

LCM method to find site pollution severity affecting insulation has been explained and its
capabilities have been compared with those of other common techniques. In the second part,
the significance of pollution measurement is studied and then, in part three, common methods
of pollution measurement are spelled out. Identifying pollution severity based on leakage
current measurement and comparing it with other methods are presented in parts four and
five, respectively.