Lightning arrestors can deteriorate over a period of time due to factors such as dust, cracks, moisture ingress, degradation of the zinc oxide elements inside, etc. This can lead to failure of the lightning arrestor. When a lightning arrestor fails, it usually explodes causing a flashover and damage to the other equipment such as PTs, CTs, etc. Hence, it is imperative that the lightning arrestors in the system are kept in a healthy condition.
The usual tests carried out on Lightning arrestors are the Insulation Resistance Tests and the Hipot Test.
Harmonic Test (online test):
When the lightning arrestor is in line, a small leakage current flows through it. This current can be analysed for Harmonics. Online harmonics analysers for lightning arrestors are available. The leakage current is analysed for the presence of the 3rd Harmonic which usually indicates a failure in the near future. An arrestor thus identified can be isolated and sent for repair before any catastrophic failure can take place
The Insulation Resistance Test:
The tests are conducted with a High Voltage Meggar, usually 2500V. The value, usually in the order of megohms, is compared with the previous values and the test values of the manufacturers.
The Hipot test is conducted at about 175% of the rated voltage.
In addition to these tests, a visual inspection of the lightning arrestors for cracks, dust accumulation, broken fitments is also useful.
In the event of system overvoltages or adverse weather conditions such as thunderstorms, the lightning arrestors need to be tested more frequently