Ground Fault Protection (GFP)

Different type of ground fault protections

(see Fig. F27)

Three types of GFP are possible dependind on the measuring device installed :

Residual Sensing RS

The “insulation fault” current is calculated using the vectorial sum of currents of current transformers secondaries. The current transformer on the neutral conductor is often outside the circuit-breaker.

Source Ground Return SGR

The insulation fault current is measured in the neutral – earth link of the LV transformer. The current transformer is outside the circuit-breaker.

Zero Sequence ZS

The insulation fault is directly measured at the secondary of the current transformer using the sum of currents in live conductors. This type of GFP is only used with low fault current values.

Fig. F27 – Different types of ground fault protections

• 

  RS system

• 

  SGR system

• 

  ZS system

Positioning GFP devices in the installation

Type / installation level	Main-distribution	Sub-distribution	Comments
Source Ground Return (SGR)	Possible		Used
Residual Sensing (RS) (SGR)	Possible	Recommended or required	Often used
Zero Sequence (SGR)	Possible	Recommended or required	Rarely used