管道防止杂散电流的方法

管道防止杂散电流的方法

某地铁工程采用金属电缆桥架，要求桥架每个支架对隧道本结构钢筋之间的绝缘电阻不小于10kΩ。为此支架固定时采用绝缘膨胀螺栓，绝缘测量时采用万用表，若绝缘电阻表（兆欧表），因10kΩ这一数值相对不大，难以读数。当桥架过成一体后，再测桥架与隧道主结构钢筋之间的绝缘电阻，会出现“假象短路”，然而测量桥架与隧道主结构钢筋之间却往往存在一个电位差。出现这种电位差的原因是：

    1、大地不是等电位体在电力工程中，总是把大地人作零电位，但实际并不如此。变压器中性点接地后，从大地的同一点测量各相对地电压，若三相对地电压相等，再从A点测量L1对地电压B点测量L2对地电压、C点测量L3对地电压，此时三相电压值往往是不等的。因为大地的A、B、C各点的电位通常是不同的，大地各点的电位是自然电场和人工电场的合成结果，因此大地不是等电位体。

    2、混凝土不是完全的绝缘体桥架支架虽然用尼龙膨胀螺栓作绝缘固定，但桥架支架与混凝土之间还是有接触的，桥架长度越长，桥架与混凝土之间的接触面也越多，两者之问的绝缘电阻也越小。

    3、铜、铁之间存在电位某地铁采用耐腐蚀的铜接地极，作为工作接地，一个地铁车站只以一个接地点，接地装置与隧道主体结构钢筋相互是绝缘的，于是铜接地极与隧道钢筋之间产生一个电池效应，与铜接地极相连的桥架和主钢筋之间就出现一个电位差。

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.