Ground fault 51G and sensitive earth fault (SEF)

What 51G and SEF protect

Ground faults are the most frequent faults on medium-voltage systems and in industrial plants. Their current depends on how the neutral is grounded: it can be high in a solidly grounded system, or a few amperes if the neutral is resistance grounded. In some cases the current is so low that phase overcurrent does not even notice it.

The 51G detects those faults by measuring ground current directly. SEF goes further: it looks for high-impedance faults where the current is minimal, such as a distribution conductor down on dry ground, which trips nothing and stays energized within reach of people. Both prevent damage to cables and motors, fires and hazards to people.

• ANSI/IEEE C37.2: 51G and 50G, time and instantaneous overcurrent with the ground suffix; the residual version is 51N.
• IEC 61850: PTOC for the time-delayed stage and PIOC for the instantaneous stage.
• IEC 60617: the overcurrent symbol with the earth indicator.

Where it is applied

At high voltage, ground faults are covered by distance, 67N and differential protection; the 51G remains as backup on the transformer neutral.

Operating principle

Residual versus measured. Ground current can be obtained in two ways:

• Residual (51N): the sum of the three phase currents. Each CT has its own error, and during motor starts or external three-phase faults those errors do not cancel completely. The sum shows a false current that limits the minimum pickup.
• Measured (51G): a CT that sees the ground current directly. The core balance CT (zero-sequence CT) encloses the three phase conductors: with no fault, the net flux is zero. The neutral CT measures the current returning through the grounding of the transformer or generator.

SEF. It is the same measurement with a much lower pickup and a long time delay. It requires a CT and a relay input with high accuracy at very low currents. On ungrounded or resonant-grounded systems the fault current is capacitive and a directional function is needed, explained in zero-sequence directional elements.

The characteristic

On the time-current plane, the 51G has the shape of any overcurrent element: a pickup, an inverse-time or definite-time curve and, optionally, an instantaneous 50G stage. Compared with the 51N:

• The 51G can pick up lower, because it does not carry the phase CT errors.
• The SEF zone sits at the far left of the plane, at very low currents, with a long definite time.
• The transformer neutral 51G is coordinated above the feeder ground elements, as backup.

You can plot and compare these curves with our free relay coordination tool.

Setting criteria

• Pickup: above the natural unbalance and the capacitive current of the downstream system, and below the minimum fault to be detected.
• Resistance-grounded neutral: the maximum ground fault current is limited by the resistor; the pickup must be a fraction of that value to protect most of the winding.
• Coordination: with downstream fuses and relays and, as backup, with the transformer neutral 51G.
• SEF: a very low pickup with a long time delay, so that transient unbalances are not mistaken for faults.

Coordination criteria are explained in protection setting criteria, and neutral grounding in neutral grounding resistor.

Interaction with other functions

• Reclosing 79: in distribution, a 51G or SEF trip can initiate or block reclosing, according to the utility’s philosophy.
• 67N: on systems with several ground sources or with an ungrounded neutral, the ground element must be directional.
• 59N: on ungrounded systems, neutral overvoltage detects the fault even when there is no current. In IEEE C37.2 that detection is the 64; see ground detector relay 64.
• 87N: restricted earth fault protects the winding; the neutral 51G is its backup.
• Breaker failure 50BF: initiated by the ground trip like any other.

How each manufacturer implements it

• Siemens SIPROTEC (DIGSI): ground overcurrent with a sensitive current input for SEF.
• SEL (AcSELerator QuickSet): residual and neutral ground elements, with sensitive-input versions for SEF and high-impedance fault detection in some models.
• Hitachi Energy and ABB Relion (PCM600): non-directional and directional ground overcurrent, with a core balance CT input in the 615 and 620 series.
• GE Vernova Multilin (EnerVista): measured ground and sensitive ground elements, each with its own input.
• Schneider MiCOM and Easergy: ground and SEF stages with a sensitive input.

Check which relay input belongs to each element: the sensitive input has a different range, and connecting a phase CT to it saturates it.

IEC 61850 logical node

The time-delayed stage is modeled with PTOC and the instantaneous stage with PIOC, each with its own ground instance. They publish start (Str) and operate (Op). The trip goes through PTRC and reaches XCBR by GOOSE or by a hardwired output. Directional SEF on resonant-grounded or ungrounded systems may be modeled with specific directional earth fault nodes, depending on the manufacturer.

How it is tested

With secondary injection, the test generally verifies:

• The pickup and curve of the 51G and the SEF, on the correct relay input.
• The times of the instantaneous stage.
• The ratio and polarity of the core balance CT and the neutral CT by primary injection, including the shield passing back through the core balance CT.
• That the 51G does not operate with high balanced phase currents.

The difference between the two kinds of injection is explained in primary and secondary injection.

Typical mistakes

• Cable shield grounded without passing through the core balance CT, which cancels or diverts the measurement.
• Core balance CT installed around a single cable, or with a ground conductor inside it.
• Wrong core balance CT ratio entered in the relay, which shifts the whole setting.
• SEF without enough time delay, tripping on switching unbalances.
• Transformer neutral 51G not coordinated with the feeder ground elements.
• Mixing up the sensitive and the normal input in the wiring.

Is your ground protection tripping with no apparent fault, or do you need its settings reviewed? At Tiza Group we review events and settings remotely, worldwide. See our remote relay configuration service or contact us.

Related guides

• Phase and ground overcurrent 50/51
• Neutral overvoltage 59N
• Ground detector relay 64

References

• IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations: devices 50 and 51 with their suffixes.
• IEEE C37.230-2020, Guide for Protective Relay Applications to Distribution Lines: ground protection and high-impedance faults in distribution.
• IEEE 142, Recommended Practice for Grounding of Industrial and Commercial Power Systems: neutral grounding methods.
• IEC 60255-151, Functional requirements for over/under current protection: overcurrent elements.
• IEC 61850-7-4, Compatible logical node classes and data object classes: PTOC, PIOC and PTRC nodes.

Frequently asked questions