Short-Time Protection

Short-Time Protection Switches

MicroLogic 5.0P
Trip Unit

MicroLogic 6.0P
Trip Unit

Short-time protection protects equipment against short circuits.

Short-time protection is standard on 5.0P and 6.0P trip units.
The short-time pickup (Isd) (A) sets current level (below instantaneous trip level) at which circuit breaker will trip after the preset time delay.
The short-time delay (tsd) (B) sets the length of time that the circuit breaker will carry an overcurrent above the short-time pickup current level before tripping.
The I²t on/I²t off option provides improved selectivity with downstream protective devices:
- With I²t off selected, fixed time delay is provided.
- With I²t on selected, inverse time I²t protection is provided up to 10 x Ir. Above 10 x Ir, fixed time delay is provided.

Intermittent currents in the short-time tripping range which do not last sufficiently long to trigger a trip are accumulated and shorten the trip delay appropriately.
Short-time protection can be zone-selective interlocked (ZSI) with upstream or downstream circuit breakers. Setting tsd to the 0 setting turns off zone-selective interlocking.
Short-time protection uses true RMS measurement.
Short-time pickup and delay can be adjusted to provide selectivity with upstream or downstream circuit breakers.

NOTE: Use I²t off with ZSI for proper coordination. Using I²t on with ZSI is not recommended as the delay in the upstream device receiving a restraint signal could result in the trip unit tripping in a time shorter than the published trip curve.

MicroLogic Trip Unit Short-Time Delay Values

Setting	Short-Time Delay
I²t off (ms at 10 Ir) (seconds)	0	0.1	0.2	0.3	0.4
I²t on (ms at 10 Ir) (seconds)	–	0.1	0.2	0.3	0.4
tsd (min. trip) (milliseconds)	20	80	140	230	350
tsd (max. trip) (milliseconds)	80	140	200	320	500