The exchange has fixed circuit filters for the Sensex. If the index drops 10% before 1 pm then trading to be halted for one hour. If 10% drop is between 1-2:30 pm then trading to be halted for half an hour. If 10% drop after 2:30 pm then no halt in trading.
If in case index drops by 15% before 1 pm then trading to be halted for 2 hours. If 15% drop between 1-2 pm then trading to be halted for one hour. If 15% drop after 2 pm then trading to be halted for rest of the day.
In the case of 20% drop in the index then trading to be halted for the rest of the day.
As per a Sebi circular, the stock exchanges calculate circuit limits on a quarterly close basis. These percentages are translated into absolute points of index variations (rounded off to the nearest 25 points in case of SENSEX).
On March 31, 2009, the last trading day of the quarter, Sensex closed at 9709. A 20% rise from this level, translates to 1,942 points. Hence, the markets hit the upper limit after the index absolute points gain rose over 2,000 points.