直观分析：

Building maintenance workers （主语）were circulating （谓语动词）through the office space （介词短语作状语）ensuring power had been restored（带有目的的伴随状语）, which it had.（定语从句）

【难点讲解】本句难点不在于现在分词短语 ensuring power had been restored 的语法作用，而在于定语从句：

it 指什么？ —— 指 power

which 指什么？——指 been restored，不表示其他。

which it had  =  it （电力）had been restored （已被恢复）

【注】关系代词 which，可以指代主句中的部分成分，甚至是谓语动词的一部分： been restored。

which 的“特殊指代”讲解：

关系代词 which，除了指代主句中的某个名词或者整个主句的内容外，还可以代替主句中的名词词组、形容词或形容词短语，甚至指代部分谓语动词。因为 which 能够表达比 that 和 who更加丰富的内容，这是其他关系代词取代不了的。

1. She is the perfect accountant which her predecessor was not. 

她是一个完美的会计师，而她的前任不是。

which 不单纯指 accountant，而是整个词组 the perfect accountant。

2. The man was unable to earn enough money, which allowed him to support his family.

那人无法赚到足够的钱，足够的钱才能让他得以养活家人。

（which 指代enough money，不是 money，更不是整个主句：既然赚不到钱，后面的句子就不通了。）

3. He was not sick, which some of the other passengers were.

他没有生病，但有些其他旅客却病了。

（which 指代形容词 sick）

4. She is always careless, which we should not be.

她总是马虎大意，我们可不应该这样。

（which 指代形容词 careless）

5. The two policemen were completely trusted, which in fact they were.

那两个警察完全受到信任，事实上，也真是如此。

（which 指代形容词短语 completely trusted）

6. She was very patient towards the children, which her husband seldom was.  

她对孩子们很耐心，她丈夫却很少这样。 

（which 指代形容词短语 patient towards the children）