Q: I see P1/P2 on the primary and S1/S2 on the secondary. What does it actually mean?
A: Polarity tells us the relative direction of current. Since AC changes direction, we need a reference to know which way is "forward."
Subtractive Polarity:
Rule: Current IN to P1 = Current OUT from S1 at the same instant.
P1/P2: Primary terminals. S1/S2: Secondary terminals.
Analogy: Imagine a mirror. If you walk into P1, your reflection walks out of S1.
Why is Polarity so critical? What happens if reversed?
Q: If magnitude is the same, why does direction matter?
A: For directional and differential protection, reversed polarity is a disaster.
The "Betrayal" Effect (Differential Protection):
Correct: 100A In - 100A Out = 0A difference (Relay happy). ✅
Reversed: Relay sees 100A In and -100A Out → looks like 200A internal fault. ❌
Result: Instant false trip. Huge economic loss due to downtime.