Electrical Installation Safety Trainer
AD 1 - MI 3399
39
Figure 1-6: Line impedance (Z
LN
) measurement
– standard method
1.4.2 Fault Loop impedance
The Fault loop impedance should be low enough in order to enable the potential fault
current to interrupt installed protection device within prescribed time interval in case of
faulty load.
After pressing the START key the instrument loads the installation between L and PE
terminals (see Figure 1-7) and measures loaded (Figure 1-7: switch position
1
) and
unloaded (Figure 1-7: switch position
2
) voltages. The result is obtained by the
following formula (simplified form).
𝑅
𝐿𝑂𝑂𝑃
= 𝑹
𝑳−𝑷𝑬
=
𝑼
𝑼𝒏𝒍𝒐𝒂𝒅𝒆𝒅
− 𝑼
𝑳𝒐𝒂𝒅𝒆𝒅
𝑼
𝑳𝒐𝒂𝒅𝒆𝒅
/𝑹
𝑳𝒐𝒂𝒅
If phase delays are measured, loop impedance Z
L-PE
can be calculated as:
Z
L-PE
= R
L-PE
+ j
L
L-PE
𝒁
𝑳−𝑷𝑬
=
𝑼
𝑼𝒏𝒍𝒐𝒂𝒅𝒆𝒅
− 𝑼
𝑳𝒐𝒂𝒅𝒆𝒅
𝑼
𝑳𝒐𝒂𝒅𝒆𝒅
/𝑹
𝑳𝒐𝒂𝒅
=
𝜟𝑼
𝑰
𝒕𝒆𝒔𝒕
where
Δ
U
– Measured voltage drop
I
test
– Test current
Summary of Contents for MI 3399
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