If an injector stays on all the time, 100% duty cycle, it is said to be static. Injectors should never go static because control of fuel delivery is lost and this can cause catastrophic engine failure. Static injectors, or 100% IDC is normally an indication that the injector is too small for the engine. Injector duty cycle should usually not exceed 80%. Once 80% IDC is reached a bigger injector should be installed.
In some instances low fuel pressure or a failing fuel pump can cause injectors to become static. On the other hand, there are some injectors that will go static if the fuel pressure is too high.