Ground detector relay 64: what it is and its variants

Short answer

Number 64 in IEEE C37.2 is the ground detector relay: it operates when the insulation to ground of a machine, transformer or other equipment fails. Since the number describes what is detected and not how, it is used for very different functions: stator and rotor ground of generators, ground detection on ungrounded medium-voltage systems, ground on the substation DC system and, on some drawings, restricted earth fault. Its IEC 61850 node is PHIZ.

What 64 protection covers

The 64 monitors the insulation to ground. It does not wait for the fault to carry a high current: it detects that some point of a system that should be isolated from ground, or grounded through a high impedance, is already in contact with it. It protects generators, medium-voltage systems with an isolated neutral and DC systems against the first ground fault, which on those systems produces almost no current but sets up the second one, which is destructive.

ANSI/IEEE C37.2: 64, ground detector relay.
IEC 61850: logical node PHIZ (ground detector).
IEC 60617: the relay symbol with the ground indication; on drawings it carries the suffix.

Where it is applied: the variants of number 64

Variant	Equipment	Level	How it measures	Guide
64G, 64S (95 %)	Generator stator with high-impedance neutral	MV	Neutral overvoltage (59N)	Neutral overvoltage 59N
64S (100 %)	Neutral end of the stator	MV	Third harmonic or low-frequency injection	100 % stator ground 64S
64F, 64R	Rotor field winding	Field DC	Injection or resistive bridge	Rotor ground fault 64F
64B, system 64	MV buses and systems with isolated neutral	MV, industrial LV	3V0 from a broken delta	This guide
DC 64	Substation DC system	125 V or 48 V DC	Detection bridge	DC ground fault detection
64 / 87N (REF)	Transformer winding	MV and HV	Ground differential	Restricted earth fault 87N

On solidly grounded systems a ground fault produces a high current and is protected by overcurrent, distance or differential; there the 64 is not used as a system detector.

Measuring principle

On a medium-voltage system with an isolated neutral, the three phase-to-ground voltages are balanced with the system healthy, and their sum is zero. When one phase touches ground:

The voltage of that phase to ground drops almost to zero.
The other two rise towards the phase-to-phase voltage.
The neutral shifts: a residual voltage 3V0 appears which, with a bolted fault, is about three times the phase voltage.

That voltage is measured with three voltage transformers connected in grounded wye on the primary and broken delta on the secondary: with the system healthy, the broken delta gives almost zero; with a ground fault, it gives 3V0. The voltage relay connected to it is the ground detector. A loading resistor is usually fitted across the broken delta to damp ferroresonance.

High-resistance grounded systems. In industrial plants it is common to ground the neutral through a resistor that limits the ground fault current to a low value. There the fault produces both neutral current and residual voltage, so it can be detected by either one. The system can keep running with an alarm or trip, depending on the plant philosophy and on how much current is allowed to flow.

On generators and DC systems the principle changes: neutral voltage, third harmonic, signal injection or a resistive bridge. Each case has its own guide.

The characteristic

For the ungrounded system detector:

3V0 threshold: above the natural unbalance of the network and the VT errors.
Time delay: so that it does not operate on transients or on the connection of unbalanced loads.
Action: normally alarm, not trip, because the system can keep running while the fault is located.

On the phasor diagram, the fault shows as the shift of the neutral point to the vertex of the faulted phase, with the two healthy phases stretched towards the line voltage. You can reproduce it with our free phasor diagram tool.

Ground detection on an ungrounded system

Healthy system. Balanced voltages and 3V0 almost zero
Ground fault on one phase. The phase drops to zero and the healthy ones rise towards the line voltage
Broken delta. 3V0 appears
64 relay. 3V0 exceeds the threshold for the set time
Alarm. The faulted circuit is located before a second fault

Setting criteria

Threshold: with margin above the residual voltage of the healthy system; on high-resistance grounded systems the aim is to cover most of the winding or of the network.
Time: coordinated with the directional ground protection that locates the circuit, if there is any.
Alarm or trip: according to the system philosophy and the owner’s standards. On generators connected through a step-up transformer, the stator 64 usually trips.
Loading resistor across the broken delta, as recommended to avoid ferroresonance.

Interaction with other functions

67N or sensitive directional ground: locate which feeder has the fault on ungrounded or compensated systems. See directional ground overcurrent 67N.
59N: the same measuring principle; on generators it covers most of the stator.
51G and SEF: on grounded systems a ground fault does produce current and is protected by overcurrent. See ground fault 51G and sensitive earth fault SEF.
VT supervision (60): a blown fuse in the broken delta or in a VT can simulate or hide a fault.
Lockout relay 86: on generators, the stator 64 and the tripping 64F act through the 86.

How each manufacturer implements it

Siemens SIPROTEC (DIGSI): ground fault detection by displacement voltage, and stator and rotor functions in the generator series.
SEL (AcSELerator QuickSet): residual voltage elements and, in the generator relays, 100 % stator elements and rotor modules.
Hitachi Energy and ABB Relion (PCM600): in the REG670, stator and rotor ground protection by injection, plus the third-harmonic method.
GE Vernova Multilin (EnerVista): neutral voltage elements and, with an external module, low-frequency injection.
Schneider MiCOM and Easergy: residual voltage and generator functions.

IEC 61850 logical node

PHIZ represents the ground detector. It publishes the start (Str) and the operate signal (Op). If the 64 alarms, the signal goes to SCADA and not to a trip; if it trips, it goes through PTRC to XCBR by GOOSE or a physical output. When the 64 is built with a neutral overvoltage element, some manufacturers model it on PTOV; check the ICD file to see which node publishes the signal.

How it is tested

In general, with secondary injection:

The threshold and time with 3V0 injected directly or with unbalanced phase voltages.
The broken-delta connection and its loading resistor, with primary injection or with service voltage.
The alarm signal up to SCADA, or the trip up to the breaker.
On generators, the tests specific to each variant.

Typical mistakes

Broken delta with one phase reversed, which gives 3V0 with the system healthy.
No loading resistor across the broken delta, with a risk of ferroresonance. See ferroresonance in voltage transformers.
Ground alarm ignored for weeks, until the second fault arrives.
Confusing the suffixes on the drawing and testing another function.
Low threshold that alarms on the natural unbalance of the network.

Do you need to review the ground detection of your system or your generator? At Tiza Group we review the events and settings remotely, worldwide. See our remote relay configuration service or contact us.

References

IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations: number 64, ground detector relay.
IEEE C37.101, Guide for Generator Ground Protection: stator and rotor ground.
IEEE C37.102-2023, Guide for AC Generator Protection.
IEEE 142, Recommended Practice for Grounding of Industrial and Commercial Power Systems: ungrounded and resistance-grounded systems.
IEC 61850-7-4, Compatible logical node classes and data object classes: PHIZ node.

Frequently asked questions

What do 64G, 64F, 64R and 64B mean?

They are suffixes that indicate where the ground is detected: G for the generator or its stator, F for the field and R for the rotor (both refer to the same fault), and B for the bus or the system. The suffixes are not uniform between companies; read the description on the drawing before assuming which function it is.

Why does a ground fault on an ungrounded system only give an alarm?

Because with an isolated neutral the first ground fault produces almost no current: only the capacitive current of the network flows. The system can keep running while the fault is located. The risk is a second fault on another phase, which would be a short circuit, and the overvoltages on the healthy phases.

Is the 64 the same as the 59N?

The 59N is a neutral or residual overvoltage element, and it is one of the ways to build a ground detector on ungrounded systems or on a generator stator. In those cases drawings may call it 64 or 59N. The 64 is the name of the function; the 59N is the measuring principle.