In a dramatic day's developments, the BSE stopped trading for a brief period after 2.30 p.m. after the Sensex dived over 2,000 points amid speculations that the exchange closed trading after the circuit ceiling (the maximum allowed swing) was triggered.

"Sebi has asked for explanation from BSE," a Sebi official said.

The exchange spokesman explained: "It was only a technical snag. It was not a circuit filter. It was just a coincidence (that the shutdown occurred when the Sensex fell over 10%)."

As per rules, the circuit gets triggered only if the benchmarks fall (or rise) 15 per cent after 2.30 p.m. or one-hour before the close. Before 2.30 p.m., the trigger is 10 per cent.

There was no such stoppage by the National Stock Exchange though its officials also asked brokerage houses to pay up the margin money so that they could continue trading unobstructed.

When is trading halted

Trading will be halted if index drops 15 per cent.

According to data on the BSE website, "In case of a 10 per cent movement of either index, there would be a 1-hour market halt if the movement takes place before 1:00 p.m.

In case the movement takes place at or after 1 p.m. but before 2:30 p.m. there will be a trading halt for half an hour. In case the movement takes place at or after 2:30 p.m. there will be no trading halt at the 10 per cent level and the market will continue trading.

"In case of a 15 per cent movement of either index, there will be a 2-hour market halt if the movement takes place before 1:00 p.m. If the 15 per cent trigger is reached on or after 1:00 p.m. but before 2 p.m., there will be 1-hour halt. If the 15 per cent trigger is reached on or after 2:00 p.m. the trading will halt for the remainder of the day.

"In case of a 20 per cent movement of the either index, the trading will be halted for the rest of the day."