METREL A1472, User Manual

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A 1472, A1579 Leakage current clamp
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4
Leakage current clamp operation
4.1 Measuring of leakage currents
4.1.1 Origins of leakage currents
Leakage currents are currents driven by active conductors of a distribution
system or electrical equipment to earth and/or protective conductors.
Leakage currents flow wherever a conductive path (resistance and/or
capacitance) is applied between active and earthed conductors/protected
parts. Fig.1 shows leakage currents that usually flow in electrical
installations in normal operation.
Fig. 1 Leakage currents in electrical installations in normal operation
The relation between leakage current and (insulation) resistance between
line and earth is defined by the Ohm’s law.
 
) ( /
_
 
k R U mA I
ins nom res leak
Eq. 1
I
leak_res
.................resistive leakage current in mA
U
nom
...................nominal line voltage, e.g. 230 V
R
ins
....................resistance in kΩ
The relation between leakage current and capacitance between line and
earth is shown in Equation 2.
   
nF C f U mA I
nom cap leak
     
6
_
10 28 . 6
Eq. 2
I
leak_cap
.................capacitive leakage current in mA
U
nom
...................nominal line voltage, e.g. 230 V
f.........................mains frequency
C(nF).................capacitance in nF
Capacitive and resistive leakage currents are forming the overall (total)
leakage current. For continuous leakage currents of main frequency,
Equation 3 can be used.
 
2
_
2
_ cap leak res leak leakage
I I mA I
 
Eq. 3
I
leakage
.................overall leakage current
Shapes of leakage currents are usually much more complicated. That will
be described later in this article.