作者: Franco Bua, Francesco Buratti and Antoni Klajn
源: 电能质量手册编辑安杰洛Baggin, 约翰·威利 & 儿子, 有限公司
The following case study refers to a steel factory in the north of Italy where the entire electronic equipment in the plant’s laboratories was operating apparently in a crazy way: all device functionality was seriously affected by some unidentified problem which forced the factory managers to ask for expert help.
The causes of all the problems experienced can be found in the constructive aspects of the earthing system.
In the case of a large current flowing into the ground system of a big plant (Figure C10.1, the voltage drop between two different points of the ground system, due to that current, could be not negligible (Figure C10.2), causing problems related, 例如, to reference ground, as the whole net no longer has the same potential all over.
因此, the first solution proposed to this problem was simply to split the ground system of the sensible electronic appliances (Figure C10.3) from that of machinery causing disturbances (in this case some VSDs).
The datasheets of the sensible electronic appliances were then checked: the manufacturer had prescribed connection of its appliances only to a so-called clean earth.
This solution was then adopted: a new ground system was installed, far from the existing one near the boundary of the plant, and used only for the connection of sensible electronic appliances. 其结果, every problem disappeared.
A further analysis of the problem was to determine if the implemented solution was correct or not: 答案是这取决于观点.
如果有人可能同时触摸连接到两个不同系统的两个裸露导电部件, 该解决方案不能被认为是正确的: 人会受到两个接地系统之间的电压差的影响.
只要两个系统是独立的, 这种情况很危险，因为电压干扰变成了接触电压.
如果保证没有人可以同时触摸连接到两个不同系统的两个外露导电部分, 解决方案显然是正确的.
真正的问题是没有人能够做到这一点. 因此应考虑到不可能将两个接地系统分开的可能性: how can the problem be solved in this situation?
In principle two different solutions exist and can be adopted:
- To operate at the level of the appliances, choosing them with a higher immunity level.
- To work at the level of the installation.
The first one is of course the easiest solution, even if it cannot be considered a solution but just circumventing the problem. For this reason it is better to concentrate on the second solution, and to add just a small philosophical comment on the first one.
A system engineer could probably argue that it his duty to feed appliances, not to select them on the basis of some kinds of issues. This is partially true, but if we consider that engineers usually have to select equipment on the basis of International Protection (IP) grade against pollution, 为什么不根据电磁IP等级来选择设备以防止电磁污染? 概念完全一样.
国际标准除特殊符号外，没有对低噪声等电位联结作出任何规定或建议。 (图 10.31).
用于处理安装级别问题的方法来自良好实践, 每个系统工程师都应该知道:
- 必须以防护措施相关要求为准.
- 仅在无法同时触及外露导电部件的情况下才允许使用独立接地系统.
- 将敏感电路和干扰电路分开.
- 避免并行.
- 采用生产线平衡.
- 使用盾牌和双盾牌.
- 采用差异化导体.