其它开关卡，例如7700型20通道差分复用模块，采用的是闭锁型继电器，无论是成对还是单独使用一个闸刀，偏移电压都是相同的。
Other CARDS, such as the Model 7700 20-channel Differential Multiplexer Module, use latching relays, and the offset voltage is the same whether the poles are used as a single pole or as a pair.
由于零差保护的特殊性，零差继电器本身就是一只灵敏电流继电器或动作功率很小的电压继电器。
Due to the specialty, REF relay is a current relay with very high sensitivity or a voltage relay with very low acting power.
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