变压器正常运行时，是不允许铁芯多点接地的，因为变压器正常运行中，绕组周围存在着交变的磁场，由于电磁感应的作用，高压绕组与低压绕组之间，低压绕组与铁芯之间，铁芯与外壳之间都存在着寄生电容，带电绕组将通过寄生电容的耦合作用，使铁芯对地产生悬浮电位，由于铁芯及其它金属构件与绕组的距离不相等，使各构件之间存在着电位差，当两点之间的电位差达到能够击穿其间的绝缘时，便产生火花放电，这种放电是断续的，长期下去，对变压器油和固体绝缘都有不良影响，为了消除这种现象，把铁芯与外壳可靠地连接起来，使它与外壳等电位，但当铁芯或其他金属构件有两点或多点接地时，接地点就会形成闭合回路，造成环流，引起局部过热，导致油分解，绝缘性能下降，严重时，会使铁芯硅钢片烧坏，造成主变重大事故，所以主变铁芯只能一点接地。
During normal operation of transformer, multi-point grounding of iron core is not allowed, because in normal operation of transformer, there is alternating magnetic field around the winding. Due to the effect of electromagnetic induction, parasitic capacitance exists between high-voltage winding and low-voltage winding, between low-voltage winding and iron core, and between iron core and shell. The charged winding will make the iron core to real estate through the coupling effect of parasitic capacitance As the distance between the iron core and other metal components and the winding is not equal, there is a potential difference between the components. When the potential difference between the two points reaches the point where the insulation can be broken down, a spark discharge will be generated. This discharge is intermittent and has a bad impact on the transformer oil and solid insulation for a long time. In order to eliminate this phenomenon, the iron core and the shell Connect it reliably to make it equipotential with the shell, but when the iron core or other metal components are grounded at two or more points, the grounding point will form a closed circuit, which will cause circulation, cause local overheating, cause oil decomposition, reduce the insulation performance, and in serious cases, it will burn out the silicon steel sheet of the iron core, causing major accidents of the main transformer, so the core of the main transformer can only be grounded at one point.