ABB 5SHY3545L0010 3BHB013088R0001 子传输电路通常布置在环路中 

副变速器在相对较低的电压下运行。把所有的都连接起来是不经济的配电站因为该设备更大且更昂贵。通常情况下，只有较大的变电站连接到这种高电压。

在电流被送到较小的变电站之前，电压被逐步降低。子传输电路通常布置在环路中，这样单个线路故障不会在很短时间内中断对许多用户的服务。

回路可以是“常闭的”，其中一个电路的损失不会导致中断，或者是“常开的”，其中变电站可以切换到备用电源。而子传输电路通常进行架空线路在城市地区，可以使用埋地电缆。低压次输电线路使用较少的路权和更简单的结构；地下工程没那么难。

The secondary transmission operates at a relatively low voltage. Connecting it all together is not an economical distribution station because the equipment is larger and more expensive. Typically, only larger substations are connected to this high voltage.

The voltage is gradually reduced before the current is sent to a smaller substation. Subtransmission circuits are usually arranged in loops so that a single line failure does not interrupt service to many users in a short period of time.

The loop can be "normally closed," in which the loss of one circuit does not cause an interruption, or "normally open," in which the substation can switch to backup power. While sub-transmission circuits are usually carried out on overhead lines in urban areas, buried cables can be used. Low voltage sub-transmission lines use fewer right-of-way and simpler structure; Underground works aren't that hard.