Specifications

Operations – Ground Fault Sensing Schemes

Ground Fault Sensing Scheme

The following are brief descriptions of the ground fault sensing

schemes as they relate to the Siemens Electronic Trip Unit.

Detailed technical and application information of the ground

fault sensing schemes is contained in NEMA Standard No.

PB 2.2 “Application Guide for Ground Fault Protective Devices

for Equipment”.

Residual (3-Phase, 3-Wire): Under normal system conditions

(without ground fault), the vector sum of the phase currents

being monitored by the trip unit is zero.This is also true under

the condition of an overcurrent phase-to-phase fault and

phase-unbalance condition.When a phase-to-ground fault

occurs, the vector sum of the phase currents is directly

proportional to the magnitude of the fault.The trip unit’s

microprocessor uses this vector sum data in the execution

of the ground fault protection function.The trip unit utilizes

the internal breaker current transformers. No external current

transformers are required.

Residual (3-Phase, 4-Wire): In the 3-Phase, 4-Wire Residual

scheme a fourth current transformer is connected in the

neutral conductor to “Sense”normal neutral currents. Under

normal system conditions, the vector sum of the currents in

all phases equals the neutral current.This is also true under

the condition of an overcurrent phase-to-phase fault and

phase-unbalance condition.When a phase-to-ground fault

occurs, the fault current returns via a path other than the neu-

tral.Therefore, the vector sum of the phase currents no longer

equals the neutral current.This current differential is detected

by the trip unit and used in the execution of the ground fault

protection function.

Residual Sensing: Circuit BreakerWiring for Ground Protection

(3-Phase, 4-Wire System Shown)

Source Ground: In this scheme, the phase currents are not

used in detecting and processing ground faults.The trip unit

executes the ground fault protection function based on data

from a ground current sensor.This sensor is located on the

neutral connection to ground at the service entrance, and is

connected to the neutral transformer input terminals on the

trip unit.

Source Ground Current

Zero Sequence Ground Fault Protection