attack time

Attack time is the interval that elapses between the moment an input signal crosses the activation threshold of a device or circuit and the moment the device produces its specified output response. In protection relays, compressors, limiters, and dynamic range processors, this parameter directly controls how quickly the equipment engages once a fault or signal condition is detected. It is measured in milliseconds, microseconds, or cycles depending on the application and the speed requirement of the system.

The practical value of attack time depends entirely on what is being protected or processed. In audio compression, a typical attack time of 10 to 100 milliseconds allows transients to pass through before gain reduction engages, preserving punch and articulation. In electrical protection systems, such as overcurrent relays, attack times of 20 to 200 milliseconds are common, but instantaneous relays may respond in under 5 milliseconds. Fast attack is essential when protecting against catastrophic faults; slow attack is valuable when filtering out noise or preserving signal character.

Interaction with detection and response time

Attack time differs from the total response time of a system because it excludes the detection delay (the time needed to recognize that a threshold has been crossed) and any intentional delays built into the control logic. A device might detect an overvoltage condition in 1 millisecond but be programmed with a 50-millisecond attack time before the output relay closes or the control signal begins to ramp. This distinction matters when coordinating protection zones or tuning audio dynamics processors.

Circuits with very short attack times can introduce instability if the threshold is noisy or the input signal fluctuates rapidly. Hysteresis (a difference between the activation and deactivation thresholds) and filtering are often used to prevent chatter or hunting, where the device continuously engages and disengages. Conversely, if attack time is too long, the device may fail to contain a transient or limit a fault before damage occurs.

Attack time is always paired with release time or decay time, which governs how quickly the device returns to its quiescent state once the input falls below threshold or the fault condition clears. Together, these two parameters define the dynamic behavior and selectivity of the circuit, making them critical variables in the design and commissioning of protection schemes and signal processors.