接する and 接触: dealing with people, bordering land, and contact
Read 接する through its target: dealing with customers or land bordering along a river. Compare these uses with parts making a sound when they come into contact.
接する: people or land as the target
接する（せっする） can mean to deal with or to border. 「客に接するときは相手の話をよく聞く。」 states that one listens carefully to the other person's words when dealing with customers. 客 identifies people being dealt with, rather than objects making physical contact.
「二つの土地が川を境に接している。」 describes two regions bordering each other along a river. 土地 identifies the subject, and 川を境に identifies the boundary. The same verb describes a spatial relationship here, whereas the customer example concerns dealing with people.
接触: contact as a condition
接触（せっしょく） means contact. 「二つの部品が接触すると音が鳴る。」 states that a sound occurs when the two parts come into contact. Physical contact between parts is the condition in this example.
接触すると means when the contact occurs. It does not report that the parts have already touched or that a sound has already occurred. The parts in this sentence also do not limit every use of 接触 to contact between components.
Read the target and the sentence structure
With 客に接する, read the context of dealing with people. With 土地が接している, read the boundary relationship between areas of land. With 部品が接触する, read contact between parts. Treating all three as the same physical touching would change the customer example's meaning.
Preserve the nonpast statement about listening, the state of the land bordering, and the condition linking contact to sound. After identifying the target, check tense and conditions so that events not stated as having happened are not added.
Does 接する with customers describe bodies touching, like the parts' 接触?
Show the answer and why
In the customer example, it means dealing with people. Listening carefully to what the other person says establishes that context. 接する with land instead describes a boundary relationship, and the parts' 接触 is presented as the condition for a sound.