METREL MI 3298, Instruction Manual With Exercises

Page: 65 / 232

Power Network Application Trainer
– Earthing / Grounding network impedance measurement
65
2.5 Impulse impedance [Zp]
Grounding system should provide low earth impedance and not simplified low earth
resistance. FFT spectral analyse of lightning impulse shows presence of both high and
low frequency components in the typical lightning waveform. The high frequency is
associated with the extremely fast rising “front” of the lightning impulse while the lower
frequency component resides in the long - high energy -
“tail”.
The transient performance of an earthing system is determining of the system ability to
discharge transient energy into the ground and minimizing earth potential rise and
ensuring that equipment and personnel are safe.
The grounding system appears to the lightning impulse as a transmission line. The soil
acts as a dielectric which under high potential stress at the electrode-soil junction can
actually break-down, decreasing the resistivity of the soil during the surge.
Measurement of earth resistance with low frequencies may not provide results which
are indicative of the ground response to a lightning discharge. In complex installations,
many earths are interconnected and the whole network is measured as one.
Lightning first strokes normally have 1 to 10
μs impulse current rise times. Higher dI/dt
occurs with re-strikes which occur in 75% of lightning discharges, which could have 0.2
μs rise time. Under this condition, the local earth is subject to the full discharge current
before the lightening wave-front has travelled more than 60 metres. This assumes
transmission at the speed of light (3•10
8
m/s). The speed is lower if inductance and
capacitance is considered.
The impulse impedance of an earthing system is a useful parameter, to predict the
behaviour in transient conditions (lightning strike for example), as it provides a direct
relationship between the peak potential and peak current rise.
The impulse impedance (Zp) is defined with the ratio between peak voltage and the
peak current.