帮忙写一句话（汉译英），必须准确！

当一个开关处于合闸位置，另一开关禁止合闸。或者同一时间内，只允许一个开关处于合闸位置。
Only one switch on at a time.
这个看怎么样？

WARNING!
Only ONE switch be on at ONE time!

thank you for your understanding and cooperation.

(楼主给的分很多，大家说的也很多。
我认为主要要简洁。好懂。
我用了一个最简单的形式。用大写突出两个"ONE"，你词多了，反而不好，是吧。

另外，最好加个“谢谢理解与合作”

Today there is a distribution box, there are two main switch, the two switches are not allowed in the closing position, now needs a warning card in English, on this point. Is: When a switch in a closing position, another switch to prohibit closing. Or the same time, allow only a switch in the closing position.

WARNING!
ONLY ONE SWITCH BE ON AT ONE TIME!!
(全部大写的英文字母表示着重强调此句的重要性！）