6
VI. The Principle and Application of Detection
1. Testing Principle.
Non-Contact Grounding Resistance Online Detector
’
s
fundamental is measuring return circuit resistance.
firstly Sensor send out a drive pulse signal to be measured ground return circuit, then measured ground return
circuit responses a pulse electromotive force E, under the influence of electromotive force E there will generate
current I in the be measured ground return circuit. Sensor measures E and I, and immediately gets resistance of be
measured ground return circuit through the formula: R=E/I.
2. Return Circuit Resistance Definition.
As picture: return circuit is a composite value includes A spot grounding resistance, grounding down lead
metallic conductor
’
s resistance, metal overhead line
’
s resistance, connection resistance(contact resistance)between
grounding down lead and metal overhead line, B spot grounding resistance. For the formative above-mentioned
return circuit
’
s grounding system, Detector can be installed directly for monitoring, if the grounding system not
form a return circuit, it need adding auxiliary grounding electrode to form a return circuit, further install the
Detector, see the after-mentioned single point grounding system. If the Detector measured out return circuit
’
s
composite value of grounding network A and grounding network B is 5.0
Ω
, that is: R
A
+R
B
+R
C
+ R
D
=5.0
Ω
(R
C
and R
D
respectively represent resistance of metal overhead line and grounding down lead ), actual grounding
resistance value of grounding network A and grounding network B
’
s in parallel resistances less than or equal to
2.5
Ω
, accordingly judge actual grounding resistance value of grounding network A and grounding network B
parallel connection whether qualified. If grounding network A and grounding network B
’
s in parallel resistances
less than engineering standard required value, so grounding network A and grounding network B are all qualified.
3. Metal Return Circuit Connection Resistance Testing.
If grounding network A and grounding network B up and down are connected together on the ground, then
Detector measures out metal return circuit resistance
’
s value is very small general speaking, only few of an Ohm,
this is the connection resistance of metal return circuit, namely equipotential resistance, isn
’
t grounding resistance.
Therefore the Detector is also convenient to test metal return circuit
’
s connection resistance.
In the large-scale grounding grid, such as converting station ground connection, oil depot ground connection,
building construction ground connection and so on, they are all an integral large-scale grounding grid under the
ground, meanwhile there are many grounding down lead extends to the ground, and connects the other on the
ground, as the following picture. To such large-scale grounding grid, its grounding grid
’
s maximum diagonal
distance is about a few hundred meters to thousands of meters, testing its grounding resistance is very difficult and
troublesome. If it exist unqualified grounding resistance, that problem is appearing at the connection position of
grounding down lead and grounding grid (direction of arrow of the welding position in the picture), engineering
reform is to excavate grounding down lead
’
s position, then again weld grounding down lead. Actually it
’
s