Ground-Fault Protection

Presentation

Ground-fault protection provides protection against phase-to-ground fault, which is more sensitive than protection based on phase current only. It is generally used in TN-S systems but could also be used in other earthing systems.

A ground fault in the protection conductors can provoke local temperature rise at the site of the fault or in the conductors.

Ground-fault and neutral protection are independent and can therefore be combined.

There are two types of ground-fault protection:

• Residual ground-fault protection is based on the summation of the phases and neutral current. It detects faults downstream of the circuit breaker.

• Source ground return (SGR) ground-fault protection is based on the signal delivered by an external sensor, source ground return (SGR) current transformer through the MDGF module. It detects faults both upstream and downstream of the circuit breaker.

  The maximum distance between the sensor and the circuit breaker is ten metres.

Availability

Ground-fault protection is available on:

• MicroLogic 6.0 trip units

• 3-pole and 4-pole circuit breakers

External sensors can be used:

• External Neutral Current Transformer (ENCT): measurement of the current on neutral. For information about the installation of ENCT, consult the ENCT instruction sheet.

• Source ground return protection: including ground-fault protection and an SGR sensor installed around the connection of the transformer neutral point to ground.

Ground-fault protection is powered by the current flowing through the internal current transformers of the circuit breaker and it does not require an additional external power supply.

Operating Principle

The ground-fault current is calculated or measured according to the circuit breaker configuration, as shown in the following table.

@writer: the diagram is shared in MTZ MicroLogic B and NS MicroLogic Basic user guides.

The ground-fault protection pickup Ig sets the level of ground-fault current at which the circuit breaker trips when reaching the ground-fault protection time delay tg.

The time delay tg sets the length of time during which the circuit breaker carries a ground-fault within the ground-fault protection pickup Ig range.

The time delay tg can be adjusted to:

• Four setting values with I²t ON. In this case, the tripping curve is an inverse time curve up to 2 x Ir, meaning that the time delay decreases as the current increases. Above 2 x Ir, the tripping curve is a definite time curve with a constant tripping time.

• Five setting values with I²t OFF. In this case, the tripping curve is a definite time curve with a constant tripping time.

Ground-fault protection is based on the true RMS current of phases and neutral.

In order to trip on an intermittent electrical fault, the trip unit accumulates the intermittent currents in the ground-fault tripping range that do not last long enough to trigger a trip. This accumulation leads to shorter tripping times than those set.

Setting the Protection

The ground-fault protection settings are:

• Ig: ground-fault protection pickup

• tg: ground-fault protection time delay

• I²t (tg): ground-fault protection curve (I²t ON or I²t OFF)

They can be set by using the Ig and tg multi-position dials on the front face of the MicroLogic trip unit.

Protection Settings

The ground-fault pickup Ig and time delay tg values can be set independently and are identical for both the residual and source ground return ground-fault protection functions.