mpec SA380-IT, Руководство пользователя

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The Resistance of the earth loop is measured in a manner very similar to "Resistance to Earth of Monitored Circuits".
Equivalent Circuit for Earth Loop Measurement:
I = Measurement Pulse Current.
p
Loop Earth = Loop Earth terminal.
FE = Functional Earth circuit terminal.
V = Measured Voltage (High Impedance).
m
R = Earth Loop Resistance.
earth
U = Voltage due to stray Earth Currents.
stray
Measurement Procedure:
When the Earth Loop Resistance is not being measured, the current source is disconnected from the measurement circuit.
When measuring Earth Loop Resistance, a "pulse test" method of earth loop resistance measurement is employed.
A series of constant-current, rectangular pulses are injected into the monitored circuit. This will cause a constant current to flow
through any resistance in the earth loop path.
The constant current generates a voltage across the earth loop resistance. It is this voltage that is acquired by the measurement
circuitry. Voltage is limited to 3.5 V in the open circuit case.
Unwanted voltage will also be picked up by the measurement circuitry due to stray ground currents flowing in the earth loop.
Unwanted stray AC components above 3.5Hz are removed using digital filtering techniques.
The unwanted stray DC component is removed through the use of a bi-directional current source, similar to resistance to earth
measurement, the constant measurement pulse presents as a differential signal across the measuring element, whilst the stray DC
current appears as a common signal across the measuring element.
Measurement takes approximately 300 milliseconds.
By taking the difference of the measured voltage for the positive and negative measurement pulse signals, the "common" current is
eliminated allowing the calculation of Earth Loop Resistance (R
)
earth