"La tensión es alta."
Translation:The tension is high.
You need to consider that the tension in this phrase is not tension in general, it is a specific period of tension. "The tension (from this moment to this moment) was high". That is why it is ser, because being high is what defines that tension.
Tension would need to have a continual or real presence to qualify for está, but instead it is a thing that just occurs and each occurrence has its own qualities that define it, so ser.
Death is very permanent, but it is a state, not a definition, and thus in Spanish you say "Él está muerto", never "es". Why? Because being dead does not define him as a person. On the other hand, you always use "es" with professions, despite how temporary they are, because in Spanish they are considered to be definitive. "Él es actor", never "está".
In the end you can argue that these distinctions (definition vs state of being) are arbitrary, and you would not be wrong, but that's how they are made in Spanish, not through temporary/permanent aspect that learners with the background of English language are so enthralled with.