FR2570193A1 - Method and device for measuring the resistance of earth connections by using as reference the capacitance between a conducting body and the surface of the ground - Google Patents

Abstract

The measurement is made using the capacitance of an aerial with respect to earth, by determining the voltage produced by a current pulse sent into a circuit consisting of the earth connection under test, the neutral and the earth connection of a low voltage transformer. The measurement is carried out by using an aerial amplifier 3 measuring the resistance of the local earth connection 5 and of the current regulated pulse generator 4. The aerial amplifier is reset to zero before the measurement, thus avoiding interference from residual 50 Hz signals.

Description

Already known methods allow this resistance to be measured
comparison with two established auxiliary lands
by two stakes.

This operation requires the disconnection of the
land of the general installation and moreover it is not
always easy to plant stakes at a sufficient distance
health so that the measurement is not disturbed by the
surface conductivity of the soil.

The process described in this patent uses the variation of transient current in capacity 1,
Fig 1, consisting of an antenna 2 and the ground surface /
measured by amplifier 3, variation occurring during
of establishment in the earth-neutral loop of a neck
rant produced by the generator 4.

i The latter generates pulses with a duration of I00
microseconds, current regulators, therefore independent of
the resistance of the loop.

The short duration of these pulses makes it possible not to per
Turbine safety devices in service such as
earth leakage breakers.

This method only measures the earth resistance 5, at
the exclusion of other resistances which could be found
worm in the loop such as earth resistance 6 of
transformer 8 or resistor 7 possibly intro
pickup between neutral and transformer earth.

On the other hand, the characteristics of the amplifier 3
whose input is zero impedance and whose reference
is connected to earth, allow independent measurement
pending of any capacity 9 such as a hangar or a metal frame joined to the ground.

The devices for implementing the com process
take the pulse generator, the year amplifier
and a reset and sample system.

The generator consists of an IO transistor (Fig 2)
mounted in constant current due to its resistance
transmitter there and powered by a DC voltage I2, its
base receiving pulses at constant voltage supplied
by a conventional device.

 The voltage of the DC supply must be higher than the peak voltage of 50 Hz that can be encountered between neutral and earth.

 The antenna amplifier consists of an operation amplifier -1 I3 counter-reacted by the resistor 4 and whose ref,; - enclosure is fixed to the earth ground by the resistor I5; it is associated with a peak voltmeter made by diode 16 and capacitor I7.

 The sampler IB whose analog input is connected to the peak voltmeter by the capacitor 19 is controlled by pulses positioned 50 microseconds after the establishment of the current in the loop.

The measurement sequence includes four phases: - resetting the peak voltmeter and the input of
the sampler by two electronic switches 20 et2l - * start of the current pulse in the loop.

- measurement and storage of the voltmeter voltage from
peak by sampler 18 - end of current pulse.

 The duration is a total of I50 micro-seconds.

 This device is made up of a conventional monostable system.

 The short duration of the whole of this sequence makes it possible to be little influenced by the 50 Hz; the recurrence is approximately 25 milli-seconds, not synchronized on the 50 Hz, which makes it possible to statistically eliminate the residual by the filter 22.

 The voltage stored by the sampler sent to the measuring device 23 through the filter is linked to the earth resistance by a law which depends on the characteristics of the operational amplifier; this law represents the intersection of the exponentials of load of the capacity between the antenna and the ground and a straight line whose slope is the maximum speed of variation of the output voltage.

Claims (5)

 I) Method for measuring the resistance of an earth connection characterized by the measurement, by means of a capacitance of an antenna with respect to the ground, of the voltage caused by a current pulse sent in a circuit constituted by the earth to be measured, the neutral and the earth of the low-voltage trainer train4.  2) Method according to claim 1, characterized by the use of the antenna amplifier 3 measuring the resistance of the local earth 5 and the pulse generator regulated in current 4, the measurement being preceded by a reset antenna amplifier to avoid disturbances due to residuals at 50 Hz.  3) Device for the implementation of the earth measurement method, according to claims i and 2 characterized by the use of a pulse generator 4 with a duration of I00 Micro-second, regulated in current connected between earth and neutral, which does not disturb the safety devices such as earth-leakage circuit breakers. 4) Device for implementing the measurement method  ground according to claims 1 and 2, characterized by the use of an antenna amplifier I3 with zero input impedance measuring a current whose amplitude is proportional to the ground resistance; device making the system insensitive to neighboring metal masses and to other resistances which could be found in the neutral earth circuit.  5) Device for implementing the earth measurement method according to claims 1 and 2 characterized by a lB sampling and reset system  20 and 21 a short time before the measurement, constituting a rapid sequence making the whole very insensitive to 50 Hz and whose non-synchronous recurrence allows after fi.ltrage to read on the voltmeter 23 a voltage independent of disturbances due to 50 Hz of the network in service.