METREL MI 3399, Instruction Manual With Exercises

Page: 100 / 272

Electrical Installation Safety Trainer AD 1 - MI 3399
100
Figure 2-39: Insulation resistance measurement at the varistor
4 Press the START key.
Expected results
The point of this exercise is to see if the varistor is performing its function in a building
that has gas pipes installed. At low voltage, a varistor has a high electrical resistance
and the current flows normally over the gas pipe. In case of overcurrent (e.g. lightning
strike) the electrical resistance of the varistor decreases and all the current then flows
past the gas pipe thus protecting the building from possible damage (e.g. fire).
Picture above shows normal insulation resistance.