Q: How do I know which CT is for my meter and which is for my relay?
A: CTs are specialized. Meters need precision at normal loads; relays need it during huge faults.
1. Metering Classes :
• 0.2s / 0.5s: Accurate down to 1% current. Used for revenue billing.
2. Protection Classes :
• 5P20: 5% error at 20 times rated current. Used for relays.
Mistake: Never use a Meter CT (0.2) for protection (it will saturate); never use a Protection CT (5P) for billing (not accurate enough).
My CT has S1, S2, S3, S4, S5. How do I find the correct ratio?
These are "Tapped" CTs. Changing connections changes the secondary turns and ratio.
Testing Tips:
• Identify Common: Usually S1 is common.
• Verify Resistance: S1-S5 (highest turns) will have the highest resistance.
• Ratio Math: If S1-S2 is 100/5 (Ratio 20), S1-S4 should be 300/5 (Ratio 60).
Pro Tip: Never short-circuit an unused tap while testing another. Mark unused taps as N/C in your report.