The 1st circuit breaker kicks in when the S&P 500 falls by a level of 7% on any given day. The result of this happening is that trading is halted for 15 minutes, unless the time that it occurs is after 3:25 in the afternoon.
Further, the 2nd curb or circuit breaker kicks in when the S&P 500 falls by a level of 13%. The halt is also for 15 minutes if it occurs before 3:25 PM
The 3rd circuit breaker is if the S&P 500 falls by 20% at any time during the day, trading ceases for the remainder of the day.