Page 215 - Elpress英文样本电气连接压接系统

Earthing resistance Method 2 From economic aspects it can be pointed

Due to the high resistance of the soil (109 x (acc. to EBR-standard U2:80) out that the diameter of the earth con-
resistivity metal) at current bleed in the soil This method normally has a measurement nection has a negligible role when cal-
a strong electrical field is formed in the earth error of more than 2%, but practically is culating the earthing resistance in deep
connection, which diminishes in strength easier to perform than Method 1. earthing. This means that when using
with distance from the earth connection. At The summary gives this method: Elpress deep-earthing systems with cop-
a certain distance, this field can be neglected - Probes and electrodes are placed as per line, the cost will be lower than when
(removed earth). illustrated, 90° from the main direction of using, for example, conventional systems.
The earthing resistance of the earth connec- the earth connection. What is important in practice in terms of
tion is usually measured with the same type - The position of the probe/electrode is equal the cable diameter is what currents the
of instrument used to measure the resist- when measuring both an individual earth system is dimensioned for and what rules
ance of the soil. However, this measurement connection as earth connection system, i.e. and requirements apply.
requires only one voltage probe and a cur- at least 80 m from the earth connection. Examples of applicable requirements:
rent electrode (auxiliary earth connection). - Measurement of an earth connection is car- the lightning protection standard states Cu-
The location of probes and electrodes varies ried out with an open earth conductor clamp. line min 25 mm², EBR prescribes min. Cu-line
between different measurement methods. - Measurement of the resulting transition 35 mm² for earth connections in overhead
The two methods that follow are a method resistance on multiple earth connection line networks and min. 50 mm² for earth
of accurate technical measurement and a systems is carried out with the clamp closed connections in ground cable networks.
more practical, simplified method. and with the measuring line connected on

Corrosion
the top of the earth conductor clamp.
With the help of the conductivity and The service life of an earth connection
the maximum earthing resistance, which depends on its resistance to corrosion
is required by, among other things, the (rust). The prerequisite for all types of
high current regulations, it is possible to corrosion is an electrolyte fluid that al-
estimate how much line may be needed lows the transport of positive metal ions
according to the formula: from anode to cathode. At the anode,
l = p / R metal atoms are dissolved in the electro-
l = length in metres lyte, forming free positive ions - oxida-
p = soil resistance in ohmmeter tion- and at the cathode these ions are
Measuring earthing resistance of the R = earthing resistance in ohms. neutralized and deposited on the metal
earth connection - Method 1 In the discussion of the advantage of surface - reduction.
Method 1 deep-earth connections compared to sur- In galvanic corrosion caused by contact
(acc. to the lightning protection standard face-earth connections, it should be men- between two metals, the corrosion rate
SS 4870110). This method has a measure- tioned here that the earthing resistance is proportional to the galvanic tension
ment error of +/ - 2%. of a horizontal surface-earth connection between the metals. A base metal has
The summary gives this method: is twice that of a similar line length in a higher negative potential than a nobler
- The probe and auxiliary electrode are deep- earth connection, i.e. metal and therefore forms the anode in a
placed in a straight line from the earth con- R0 = 2 x p / l corrosion process.
nection to be measured as illustrated. Parallel connection There is also a clear correlation between
- If the ground is layered, measurement Parallel connection of several earth sockets the corrosion rate and ground resistance.
should be carried out in two directions. The is often necessary for practical reasons to The rate of corrosion depends on the
largest value is used. achieve a sufficiently low value of earthing composition of the soil. Influencing fac-
- The reliability of the measurement result resistance during earthing. In order to limit tors are the pH of the soil, temperature,
depends on the location of the probe/ reciprocal connection between individual oxygen content, moisture content and re-
auxiliary electrode. Note the distance table earth connections, deep-earth connections sistivity. These factors affect the corrosion
below. These provide normally acceptable shall be installed a distance apart of 1.5 current lk, which is directly proportional
measurement accuracy. times the depth of the earth connection l. to the rate of corrosion. lk can be deter-
earth connection - probe = 0.5a-0.6a Resulting earthing resistance: mined by direct measurement with an
earth connection - electrode = a Rres = k x Rm Ammeter or calculated, if the transition
a ≥ 40 m if l ≤ 4 m where Rm is the mean of the resistance resistance Rö between the two electrodes
a ≥ 10 x l if l > 4 m value of the earth connection and k a 11
is known, according to formula:
reduction factor, the value of which is
obtained from the following table. lk = Ug / Rö
No. of parallel earth connections k for a = 1.5l Ug = galvanic voltage
2 0.60 In some cases, Rö can be measured with
3 0.40 a resistance bridge of the same type used
5 0.25 to measure the earthing resistance of an
10 0.13
earth connection. The rate of corrosion is
often expressed in μm/year where 1 μm
represents 1/1000 of 1 mm and denotes
the thickness of the corroded away outer
metal layer for 1 year. The table below
indicates some practical values as guide
values for different soil resistivity.

Resistivity Corrosion
Measuring earthing resistance of the p < 1 Ωm 100 μm/year
earth connection - Method 2. p < 1-10 Ωm 100-30 μm/year
p < 10-100 Ωm 30-4 μm/year
Parallel connection. p > 100 Om negligible

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