TIME – CURENT CHARACTERISTIC CURVES (TCC) BASICS AND TUTORIALS

The TCC curve depicts the time required for a circuit breaker to open automatically versus the current through it. The curve is principally a function of the type of trip unit and its settings.

The inverse-time characteristic is intended to protect conductors. The inverse-time characteristic derives its name from the inverse proportionality of time to operate versus magnitude of current flowing through the circuit breaker. In other words, opening is faster when the overcurrent condition is higher.

The curve may conveniently be separated into three regions:

Long-time
In which opening is timed in minutes up to a maximum of 1 hour or 2 hour—depending on the circuit breaker size and the degree of overcurrent—to provide an inverse-time characteristic. The provided time delay allows intermittent or cyclical loads above the pickup current to be carried without causing an interruption. It trips on sustained overcurrent to protect conductors and other equipment.

Short-time
In which opening is timed in seconds or tenths of seconds. Overcurrents might be in the range expected in the case of a motor locked rotor or an arcing ground fault. Time delay in this region allows for starting and inrush transient currents or for selective coordination with supply-side or load-side devices.

Instantaneous
In which opening is not intentionally delayed and is timed in milliseconds. Typical operation is a result of short circuit from a bolted fault.



Time current curves are excellent engineering tools when time and current are the primary factors. In the short-circuit region, other factors (e.g., power factor, instant of fault initiation, number of poles or phases in the fault, behavior of other equipment in the circuit) can also affect reaction and clearing time.

Commonly available curves drawn in accordance with NEMA AB 1-1986 show a wide band of clearing time for multiple pole interruption that includes the effect of these factors.

The continuous-current rating may be fixed or adjustable. Some designs may require replacing all or part of the trip unit to change the continuous rating. Overcurrent trip characteristics are a function, multiple, or percentage of the continuous-current rating.

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