管道防止雜散電流的方法

某地鐵工程采用金屬電纜橋架，要求橋架每個(gè)支架對(duì)隧道本結(jié)構(gòu)鋼筋之間的絕緣電阻不小于10kΩ。為此支架固定時(shí)采用絕緣膨脹螺栓，絕緣測(cè)量時(shí)采用萬(wàn)用表，若絕緣電阻表（兆歐表），因10kΩ這一數(shù)值相對(duì)不大，難以讀數(shù)。當(dāng)橋架過(guò)成一體后，再測(cè)橋架與隧道主結(jié)構(gòu)鋼筋之間的絕緣電阻，會(huì)出現(xiàn)“假象短路”，然而測(cè)量橋架與隧道主結(jié)構(gòu)鋼筋之間卻往往存在一個(gè)電位差。出現(xiàn)這種電位差的原因是：

? ? 1、大地不是等電位體在電力工程中，總是把大地人作零電位，但實(shí)際并不如此。變壓器中性點(diǎn)接地后，從大地的同一點(diǎn)測(cè)量各相對(duì)地電壓，若三相對(duì)地電壓相等，再?gòu)腁點(diǎn)測(cè)量L1對(duì)地電壓B點(diǎn)測(cè)量L2對(duì)地電壓、C點(diǎn)測(cè)量L3對(duì)地電壓，此時(shí)三相電壓值往往是不等的。因?yàn)榇蟮氐腁、B、C各點(diǎn)的電位通常是不同的，大地各點(diǎn)的電位是自然電場(chǎng)和人工電場(chǎng)的合成結(jié)果，因此大地不是等電位體。

? ? 2、混凝土不是完全的絕緣體橋架支架雖然用尼龍膨脹螺栓作絕緣固定，但橋架支架與混凝土之間還是有接觸的，橋架長(zhǎng)度越長(zhǎng)，橋架與混凝土之間的接觸面也越多，兩者之問(wèn)的絕緣電阻也越小。

? ? 3、銅、鐵之間存在電位某地鐵采用耐腐蝕的銅接地極，作為工作接地，一個(gè)地鐵車(chē)站只以一個(gè)接地點(diǎn)，接地裝置與隧道主體結(jié)構(gòu)鋼筋相互是絕緣的，于是銅接地極與隧道鋼筋之間產(chǎn)生一個(gè)電池效應(yīng)，與銅接地極相連的橋架和主鋼筋之間就出現(xiàn)一個(gè)電位差。

?

Methods for preventing stray current in pipeline

A subway project uses a metal cable bridge, and it is required that the insulation resistance between each support of the bridge and the structural reinforcement of the tunnel is not less than 10k Ω. For this reason, insulation expansion bolts are used for fixing the support, and multimeter is used for insulation measurement. If the insulation resistance meter (megger) is used, it is difficult to read because the value of 10K Ω is relatively small. After the bridge is integrated, the insulation resistance between the bridge and the main structural reinforcement of the tunnel will be measured again, resulting in "false short circuit". However, there is often a potential difference between the bridge and the main structural reinforcement of the tunnel. The reason for this potential difference is:

1. The earth is not an equipotential body. In power engineering, the earth is always regarded as a zero potential, but it is not so. After the neutral point of the transformer is grounded, measure the voltage of each phase to the ground from the same point on the ground. If the voltages of the three phases to the ground are equal, measure the voltage of L1 to the ground from point a, L2 to the ground from point B, and L3 to the ground from point C. at this time, the three-phase voltage values are often unequal. Because the potentials of points a, B and C on the ground are usually different, and the potentials of points on the ground are the combination of natural and artificial electric fields, the ground is not an equipotential body.

2. Concrete is not a complete insulator. Although the bridge support is insulated and fixed with nylon expansion bolts, there is still contact between the bridge support and concrete. The longer the bridge length, the more the contact surface between the bridge and concrete, and the smaller the insulation resistance between the two.

3. There is potential between copper and iron a subway uses corrosion-resistant copper grounding electrode as working grounding. A subway station only uses one grounding point. The grounding device and the reinforcement of the main structure of the tunnel are insulated from each other, so a battery effect occurs between the copper grounding electrode and the reinforcement of the tunnel, and a potential difference occurs between the bridge connected to the copper grounding electrode and the main reinforcement.