The annealed transformer core is composed of silicon steel sheets. In order to reduce the eddy current, there is a certain insulation resistance between the sheets (generally only a few ohms to several tens of ohms). Since the inter-chip capacitance is extremely large, it can be regarded as a path in the alternating electric field. The entire stack of core laminations can be clamped to ground potential with a single grounding in the core.
If the annealed transformer core or its metal components are grounded at two or more points (multiple points), a closed loop will be formed between the grounding points, and the magnetic flux of the key chain will induce an electromotive force and form a loop, resulting in local Overheating, even burning the iron core. The transformer core is only grounded at one point, and it can be normally grounded. That is, the core must be grounded and must be grounded at one point.
The failure of the iron core is mainly caused by two reasons. First, the short circuit caused by poor construction process, and secondly, multiple grounding due to accessories and external factors.