MI 3290 Earth Analyser
Tests and Measurements
107
S&T Current Source
Start
Initial Test
Igen = on
55 Hz
Igen > 50 mA
Check
H - auxiliary earth rod
Stop
yes
no
yes
yes
no
Igen = ±10%
no
M
ea
su
rin
g
Lo
op
Figure 11.80: S&T Current source flow chart
During the measurement a sinusoidal current (55 Hz) Igen is injected into the earth through
an auxiliary probe (H). The resistance of the auxiliary probe (H) should be as low as possible
in order to inject a high test current. The resistance Rc can be decreased by using more
probes in parallel. A higher injected current improves the immunity against spurious earth
currents. The voltage drop is measured with the help of MI 3295M (high sensitive 55 Hz V -
meter). As the test current is usually only a small fraction of the highest fault current, the
measured voltages must be up scaled according to following equation:
𝑈
𝑠,𝑡
= 𝑈
𝑚
(MI 3295M)
⋅
𝐼
𝑓𝑎𝑢𝑙𝑡
𝐼
𝑔𝑒𝑛
(MI 3290)
where:
U
s,t
.................................... Calculated Step or Touch Voltage in case of an fault current
U
m
..................................... Test voltage drop MI 3295M V-meter
I
fault
.................................... Set fault current voltage (maximal earth current in case of a fault)
I
gen
.................................... Test current injected between H (C1) and E (C2) terminal
Test can be started from the S&T Current Source window.
Figure 11.81: S&T Current Source menu
Figure 11.82: Example of S&T Current
Source menu result
S&T Current Source Measurement procedure:
Select the S&T Current Source.
Connect the test leads to the instrument and to the test object.
Press the Run key to start the measurement.
Wait until the test result is displayed on the screen.
Press the Run key again to stop the measurement
Save results (optional).
Notes:
Consider displayed warnings when starting the measurement!
MI 3290 is only a current source! For the voltage measurement Um and for the step,
touch calculation the user must use the MI 3295M instrument.
