Peterb, DanDel and Alehman are all correct in what they say.
If you have downstream circuit breakers that have an instantaneous trip then the upstream protective device, say an air circuit breaker (ACB) should either have its instantaneous trip set to off (preferable) or it should be set to above the level of the prospective fault current so that it can not operate. This is to allow discrimination with the downstream device, pure and simple.
A typical situation will be an MCC where motor starters and feeders (to DB's say) all use moulded case circuit breakers (MCCB's). Depending on the trip unit fitted, MCCB's may have adjustible long time, short time and instantaneous trip settings. Generally with MCCB's the instantaneous trip can not be set to off. Some makes also have a so called "reflexive trip" where the contacts are forced apart by very high fault currents. The upstream device will be the incomer to the MCC and will typically be an ACB (in North American terminology, a power breaker?). This ACB will also have a trip unit with adjustible long time, short time and instantaneous trip settings. The instantaneous setting needs to be set as described, and you need to set the long time and short time settings to co-ordinate with the downstream MCCB's.
You can get the situation where the upstream breaker is an MCCB. Even though it should not be possible to set its instantaneous setting to off, you can still achieve discrimination with downstream devices provided the upstream MCCB is selected in accordance with discrimination charts provided by the manufacturer. Some manufacturers such as Merlin Gerin are really good at providing the necessary information to make a correct selection. I imagine CH is as well?