About the Isolator & DS Rating
Isolator (disconnector) rating selection determines the correct voltage, current, and short-circuit withstand rating for isolators used to create visible isolation points in a solar plant's DC and AC circuits — enabling safe maintenance work by locking out energy sources.
How to calculate it
- 1DC isolators (string, AJB, DCDB): rated for the maximum DC system voltage (1,000 V or 1,500 V DC) and maximum string or output current × 1.25.
- 2AC isolators (HT disconnector): rated for system voltage × 1.05 and load current × 1.25. Short-circuit withstand for 1 second (Icw) must match the prospective fault current.
- 3DC isolators must be specifically rated for DC breaking — standard AC isolators will arc and fail on DC circuits. Look for IEC 60947-3 DC ratings.
- 4Number of poles: 2-pole for DC (one pole per polarity); 3-pole for 3-phase AC; 4-pole for 3-phase + neutral.
- 5For HT (> 1 kV): use SF6 or vacuum interrupters; verify that the isolator is not used to interrupt load current (HT isolators are break-before-make — they are not designed to interrupt current).
Worked example
DC main isolator at the DCDB output: system voltage 1,000 V DC, total output current 300 A. Required: ≥ 1,000 V DC rated, ≥ 375 A (300 × 1.25). Select: 400 A, 1,000 V DC 2-pole DC isolator with IEC 60947-3 DC Category 2 rating. Verify: must be DC-specific — a 1,000 V AC isolator will fail on DC.
What you can calculate
- Voltage class
- IS 9921
- Creepage
- Current rating
Frequently asked questions
Why can't I use an AC-rated isolator on a solar DC circuit?
AC current naturally passes through zero 100 times per second (at 50 Hz) — this zero-crossing extinguishes the arc when contacts separate. DC current has no zero crossing — the arc persists and can sustain indefinitely, burning the contacts and potentially causing a fire. DC-rated isolators use special arc-chute designs, longer contact travel, and often magnetic arc-blow technology to extinguish the DC arc. Using an AC isolator on a DC circuit is a code violation and a fire risk.
What is the difference between an isolator and a circuit breaker?
A circuit breaker is designed to interrupt fault current and load current — it has active arc-quenching mechanisms. An isolator (disconnector) is designed only to create a visible break after the circuit has been de-energised — it has little or no arc-breaking capability. The correct sequence: open the circuit breaker first (to interrupt current), then open the isolator (to create visible isolation for maintenance). Never open an isolator under load.
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