Rules say trading should be halted for an hour if there is a fall of 10 per cent in the Nifty or Sensex before 1.00 pm and for 30 minutes if the limit is breached between 1 pm and 2:30 pm.
These norms are similar if indices rise/fall by 15 per cent and 20 per cent. on Friday, after the crash (9:49 am-9:51 am), NSE halted trade for less than 15 minutes, before resuming at 10:04 am.
Brokers said NSE might be right in not halting trade for longer, as disturbing normal market trading due to an erroneous trade could have caused more damage.
NSE controls 90 per cent of India's (average daily) equity derivatives market, worth Rs 1.5 lakh crore ($28 billion).