Isolator / Changeover Switch
Switchgear
The visible-break safety switch: an isolator disconnects a dead circuit so it can't come alive while someone works on it; a changeover switch selects between two sources — the classic mains/generator handle on Indian homes and shops.
Schematic symbol
Switch (SPST)
How isolator / changeover switch appears in a circuit diagram.
Types & variants
Key specs
Current rating
AContinuous capacity: 32/40/63 A domestic, 100–800 A industrial, with matching short-time withstand.
Poles
—DP for single-phase (isolates L and N), FP/TPN for three-phase with neutral.
On-load capability
—Critical distinction: off-load isolators must NEVER be opened under current (they arc and weld); switch-disconnectors are rated to break load.
Breaking capacity
kAFor switch-disconnectors: the fault current they survive making/carrying until protection clears.
Changeover type
—Centre-off (break-before-make) is mandatory for generator use — both sources must never parallel.
Markings
Handle positions marked I/O (or MAINS–OFF–GEN on changeovers), ratings on the label (A, V, utilisation category like AC-22A/AC-23A — the code that says whether it can break load).
Standard values
Domestic: 32/40/63 A DP changeover for genset/inverter feed. Industrial: 63–400 A TPN switch-disconnectors as panel incomers. Substation: gang-operated air-break isolators either side of breakers.
How to choose
1) Rating ≥ the circuit's protective device. 2) For anything operated by non-electricians, choose on-load rated (AC-22A+) — someone WILL pull it under load. 3) Generator changeover: centre-off break-before-make, sized for the larger source, neutral switched too (4-pole for 3-phase). 4) Lockable handles for maintenance (lockout-tagout). 5) An isolator is not protection — it has no trip; pair with MCB/MCCB.
Pinout & package
Input terminals top, load bottom (changeover: two source inputs + one output). Enclosed steel/plastic boxes for wall mounting, DIN-rail modules for distribution boards, and pole-mounted gang-operated blades for HV.
Example circuits
- House with inverter: mains → changeover → selected source feeds the essential-loads DB
- Shop generator: MAINS–OFF–GEN handle ensuring the genset never backfeeds the grid (lineman safety!)
- Machine-side lockable isolator beside every industrial motor for maintenance
- Substation: isolator–breaker–isolator arrangement with earth switch for safe line work
Common failures
Arc-damaged contacts from opening off-load types under current, overheated terminals from loose connections (browned casing near a terminal is a fire warning), broken handle mechanisms, and corroded contacts in outdoor units raising resistance.
How to test
Dead-test: continuity through each pole closed, open-circuit when open, and on changeovers, verify the two sources can never bridge in any handle position. In service, IR-thermometer terminals under load — hot spots mean loose or pitted contacts.
Substitutes
A switch-disconnector replaces an off-load isolator (never the reverse). An ATS replaces manual changeover for automatic generator start setups. An MCB of adequate rating provides isolation + protection in small boards (if marked suitable for isolation).