Every so often a market falls so fast that the exchange simply switches off. It is not a glitch. It is a deliberate safety valve called a circuit breaker, and it is one of the few moments when the rules of the market override the will of every buyer and seller at once. A circuit breaker is a mandatory trading halt that kicks in when prices move too far, too fast, forcing the whole market to pause and cool down.
The idea is borrowed from electrical wiring. A fuse trips before a power surge burns the house down. In the same way, a market circuit breaker trips before a panic can feed on itself into a full-blown collapse. SEBI introduced the market-wide version in India in 2001, and it has been refined since.
The 10, 15, 20 rule
India runs a three-tier system, and the tiers are set at moves of 10, 15, and 20 percent. The trigger is whichever of the Nifty 50 or the Sensex breaches a level first, measured against the previous day's closing value. So if the Nifty is down 10 percent but the Sensex is only down 9.7 percent, the Nifty breach still halts the entire market.
What happens next depends on the size of the move and, just as much, the time of day.
The logic is that the same percentage move is more dangerous early in the day, when there is time for panic to spread, than in the final minutes. A 10 percent fall at 10 am buys the market a 45-minute timeout. The identical fall at 3 pm does nothing, because the day is almost over anyway. After every halt, a 15-minute pre-open call auction runs before normal trading resumes, so prices restart from a fresh, aggregated level rather than mid-panic.
The morning it actually happened
The clearest example in living memory is 13 March 2020. Minutes after the open, the Nifty and Sensex crashed about 10 percent as the Covid panic peaked, tripping the first-level circuit and freezing all trading in India for 45 minutes. It was the first market-wide halt the country had seen in more than a decade. Days like that usually show up in the FII flow data too: foreign funds selling in a rush is one of the more common triggers for a market-wide halt, since their exits tend to be large and concentrated in a short window.
Then came the twist that circuit breakers are designed to allow. During the 45-minute pause, the mood steadied, buyers regrouped, and when trading reopened the market did not resume falling. It roared back, and the Sensex actually closed higher on the day. The halt did exactly its job: it broke the feedback loop of panic long enough for rational buyers to step back in.
Stock-level circuits are different
The market-wide breaker is rare. The version you will meet far more often is the individual stock circuit, known as a price band or circuit filter. Each stock has a daily band, commonly 2, 5, 10, or 20 percent, beyond which it simply cannot move that day. A small-cap that gets a takeover rumour can shoot to its 20 percent upper circuit within seconds and then sit there, with buyers lined up but no sellers willing to trade.
When a stock is stuck at its upper circuit, it can still change hands at that price, but not above it. At the lower circuit, the opposite happens, and holders desperate to sell can find no buyers. That is why penny stocks and heavily manipulated counters so often show a wall of pending orders at the band, a sign of one-way sentiment with no real market on the other side. Stocks in the futures and options segment usually have wider or dynamic bands so that genuine price discovery is not choked off.
Circuit breakers are one of those rules you can invest for years without ever seeing trip at the index level. But they sit underneath every session like a fuse box, quietly promising that no single day of fear or greed will be allowed to run completely out of control.