Medium-Voltage Feeder Protection Settings: A Step-by-Step Guide to ANSI 50/51, 50N/51N, 67 and 67N

Feeder protection settings must sit inside a feasible window: above maximum legitimate load, unbalance, inrush and transient quantities, yet below the minimum fault that must be detected—and coordinated in time with downstream clearing and upstream backup. ANSI 50/51, 50N/51N, 67 and 67N are not independent menu entries; they share CT/VT performance, topology, grounding and breaker time.

This step-by-step method develops phase and earth overcurrent settings for MV feeders, including directional polarisation, inverse curves, tolerances, distributed generation, worked illustrative calculations and commissioning tests. It is not a settings template: use the approved short-circuit/coordination study and exact relay manual.

Executive rules

Calculate maximum load/emergency/motor-start and minimum/maximum phase/earth faults for every topology.
Convert all quantities on one documented primary/secondary basis with actual CT/VT ratios.
Set pickup from both sides: security above nonfault current and dependability below minimum fault.
Coordinate total clearing time, not relay operate time alone; include tolerances, breaker and arc time.
Use 50 only where a current threshold truly separates the protected close-in zone from downstream through faults/inrush.
Earth-fault settings follow system grounding and residual-measurement method, not phase-fault rules divided by a constant.
67/67N require verified current/voltage polarity, forward definition and valid polarising quantity under weak/close-in faults.
Test boundaries and dynamic waveforms, then validate load phasors before enabling directional/high-speed elements.

1. Standards and nomenclature

Reference	Application
IEC 60255-151:2009	Minimum functional/performance requirements for over/under-current protection, including time characteristics and influencing factors
IEC 60255-1:2022	Common relay/scheme requirements and tests
IEEE C37.112-2018	Active standard defining inverse-time characteristic equations/allowances, including varying current
IEEE C37.2-2022	Device function numbers/acronyms and contact designations
IEC 61869-1:2023 / IEC 61869-2:2012	Instrument-transformer/CT performance framework
Relay manufacturer	Exact curve equation/constants, measurement algorithm, directional polarisation, tolerances and logic

50: instantaneous or definite-time phase overcurrent stage commonly applied without intentional inverse delay.
51: time-delayed phase overcurrent, often inverse-time.
50N/51N: neutral/residual earth overcurrent; suffix naming varies (N/G) and input source must be stated.
67/67N: directional phase/earth overcurrent supervised by direction.

2. Required study data

one-line and normal/contingency/maintenance/generator/island topologies;
source/transformer/cable/line/motor/generator impedances and X/R;
maximum demand, emergency load, overload, cold-load pickup and motor starting profiles;
minimum and maximum three-phase, phase-phase and earth faults by location;
grounding transformer/NGR/reactor/arc-suppression-coil and system charging current;
downstream fuses/relays/contactors and upstream backup characteristics;
CT ratio/class/burden/saturation and residual connection/CBCT data;
VT ratio/fuse/selection and directional polarising availability;
breaker opening/arcing/total clearing tolerance and interrupting duty;
equipment cable/transformer/motor thermal/damage and arc-flash limits.

3. Establish a calculation basis

For CT ratio CTR = I_{primary,rated}/I_{secondary,rated}:

I_secondary = I_primary/CTR and I_primary = I_secondary × CTR.

Record whether relay settings are amperes secondary, amperes primary or multiples of nominal current. For residual from three phase CTs, the relay input measures vector sum; it is not automatically equal to an independently measured neutral/CBCT input during CT saturation/mismatch. Check relay nominal 1 A/5 A configuration and active settings group.

4. Phase 51 pickup window

Define a security floor and dependability ceiling:

I_pickup,51 > K_load × I_{maximum legitimate}, while I_pickup,51 < I_{minimum required fault}/K_dep.

K_load covers measurement/CT error, load forecast, acceptable overload, unbalance and reset margin.
K_dep covers fault-study uncertainty, arc resistance, source outage and relay/CT error.
Check cable/transformer/motor continuous and emergency ratings; pickup must not authorise thermal overload beyond equipment design.
Check motor starting/cold-load profile against inverse-time operating accumulation/reset, not pickup alone.
If the feasible window disappears, improve sensors/protection/topology; do not force a setting.

5. Choose the 51 characteristic

For common IEC inverse curves, a generic form is t = TMS × k / [(I/I_s)^α − 1]; constants k and α define standard, very or extremely inverse families. IEEE C37.112 uses its defined equation/allowances. Use the exact relay implementation, reset/integration and any definite minimum time.

Curve tendency	Typical coordination value	Check
Standard/normal inverse	Broad relay-relay coordination	May be slow at high multiples
Very inverse	Useful where fault current reduces with distance and downstream devices have inverse behaviour	Load/start and low-fault time
Extremely inverse	Can coordinate with fuse/thermal I²t-like characteristics	High-current time, CT saturation and equipment duty
Definite time	Predictable grading in constrained networks	Slower upstream clearing and step count

6. Coordination time interval

At each common fault current, require:

t_{upstream,total} − t_{downstream,total} ≥ CTI.

downstream relay/fuse operate and tolerance;
downstream breaker opening/arcing/clearing and tolerance;
upstream relay negative tolerance/overtravel/reset behaviour;
CT saturation and measurement/filter delays;
auxiliary/interposing/output and communication time;
coordination safety margin/project criterion.

Plot total-clearing curves/bands, not a single nominal point. Check minimum and maximum source, close-in and remote fault, multiple downstream devices, tie/coupler states and evolving/fault-current-changing conditions.

7. Phase 50 high-set element

Find maximum external/downstream through-fault current seen by the relay including topology/DER and study uncertainty.
Add margin for CT transient response, relay overshoot/tolerance and asymmetrical current representation.
Find minimum close-in internal fault in the weakest source state.
Check transformer energisation, motor start/reacceleration, capacitor switching and load transfer.
Set pickup only if a robust gap exists; otherwise use intentional delay, directional/zone logic or disable 50.
Check CT saturation does not cause underreach for high internal faults or false residual.
Coordinate high-set with downstream current-limiting fuses/contactors and breaker interrupting/close-latch duty.

8. 50N/51N earth-fault inputs

Measurement	Strength	Constraint
Residual of phase CTs	No separate sensor; handles higher earth currents	CT mismatch/saturation/lead errors limit sensitive pickup
Core-balance CT	High sensitivity and direct 3I0	Aperture, cable screen/earth routing, range and saturation
Neutral CT	Measures transformer/generator grounding path	Zone/source specificity, ratio/polarity and parallel return paths

Set above maximum healthy residual: load unbalance, charging current contribution, CT error, harmonics/transients and measurement noise.
Set below minimum earth fault after fault resistance and weakest source/grounding path.
Coordinate with NGR/grounding-transformer thermal rating and downstream earth protection.
In low-resistance grounded systems, definite/inverse 51N may coordinate; 50N needs selectivity/through-fault margin.
In isolated/resonant systems, current magnitude alone may be inadequate; use sensitive directional/admittance/wattmetric/intermittent earth-fault methods.

9. Directional phase overcurrent 67

Define forward current direction relative to the protected feeder and CT P1/P2.
Select line/phase voltage polarisation, cross-polarisation, memory voltage or negative-sequence method supported by the relay.
Set characteristic angle from positive-sequence/source/line impedance and relay convention—not by copying another vendor’s angle.
Check close-in three-phase fault voltage collapse and memory duration.
Check reverse faults, parallel feeders, bus coupler, transformer phase shift and embedded generation.
Use minimum polarising voltage/current supervision and define behaviour on VT fuse failure.
Test torque/directional boundary at minimum current/voltage and frequency/phase-error extremes.

10. Directional earth overcurrent 67N

67N compares residual/zero-sequence current with a polarising quantity such as residual voltage, negative-sequence voltage/current or another vendor-specific method. Sign conventions for 3I₀, 3U₀, active/reactive residual power and characteristic angle differ—document the relay equation and primary forward-fault phasor.

Verify phase CT residual/CBCT/neutral CT polarity and VT broken-delta/residual polarity.
Calculate network charging-current direction/magnitude for healthy and faulted feeders.
Use grounding method to choose wattmetric, varmetric, admittance, conductance or negative-sequence direction.
Check intermittent/restriking faults and harmonics in resonant-earthed networks.
Set minimum residual voltage/current and directional sector with measurement/tuning errors.
Test forward/reverse fault at multiple resistance, inception angle and coil detuning states.

11. Worked illustrative phase example

Illustrative only. An 11 kV feeder has 630 A maximum legitimate load, CT 800/1 A, minimum remote phase fault 2.4 kA, maximum downstream through fault 4.2 kA and minimum close-in fault 7.5 kA. A downstream device clears a 3.6 kA fault in 0.35 s; required grading margin is 0.30 s.

Select 51 pickup 1.00 A secondary = 800 A primary after confirming 800/630 = 1.27 covers approved load/overload margin and 2.4 kA gives multiple M = 3.0.
For an illustrative IEC very-inverse equation t = 13.5×TMS/(M−1), at 3.6 kA M = 4.5.
Required upstream relay/clearing target must include its breaker; simplifying here to relay target 0.65 s gives TMS ≥ 0.65×3.5/13.5 = 0.169. Choose next supported value 0.18, then redo with actual breaker/tolerance bands.
At 2.4 kA, ideal relay time = 13.5×0.18/(3−1) ≈ 1.22 s; check equipment damage and upstream backup.
A tentative 50 pickup 5.5 kA lies above 4.2 kA and below 7.5 kA, but accept only after asymmetry/CT/relay/study margins, inrush and minimum internal-fault coverage demonstrate a robust gap.

Do not use the simplified target calculation as a final setting. Plot manufacturer curves and tolerance/clearing bands at all fault locations/topologies and check CT transient performance.

12. Illustrative resistance-grounded earth window

Assume an NGR limits solid earth fault to about 400 A, minimum studied fault including resistance/source tolerance is 320 A, and maximum healthy/transient residual at the selected CBCT is 20 A. A tentative sensitive pickup such as 60 A primary lies above healthy residual and below minimum fault. Finalise only after CBCT accuracy/saturation, cable screen routing, harmonic/transient, NGR thermal time and downstream/upstream coordination. Phase-CT residual may require a higher security floor.

13. Special operating scenarios

embedded generation/reverse power changes phase-fault direction and fault contribution;
island operation reduces minimum fault and changes frequency/voltage polarisation;
bus coupler changes source impedance and direction;
transformer parallel/one-out changes through-fault and earth zero-sequence path;
motor contribution decays with time, affecting instantaneous and BF current criteria;
arc resistance/high-impedance faults reduce current below traditional pickup;
automatic transfer/reclose requires cold-load/motor residual and settings-group coordination;
maintenance grounding/VT selection/CT ratio changes require controlled settings group or outage.

14. CT performance checks

ratio chosen for load resolution and maximum fault input range;
class/ALF or knee point/transient specification suited to protection application;
hot lead plus relay/test-block burden;
asymmetrical fault/X/R and remanence where relevant;
saturation effect on 50 underreach, 51 delay, residual spill and directional phasor;
relay waveform algorithm/CT saturation detector performance;
CBCT aperture, conductor placement, screen/earth return and low-current calibration;
1 A/5 A hardware/configuration and safe shorting/test facilities.

15. Verification workflow

Peer-review study model, cases, grounding and CT/VT data.
Export relay-native settings report and compare primary/secondary conversions.
Plot phase/earth TCCs with tolerance and equipment-damage/starting curves.
Secondary-inject pickup/dropout and operate time at boundary/multiple points.
Dynamic-test inrush, motor start, CT saturation, directional close/reverse and intermittent earth cases.
Verify trip/lockout/BF/AR/ATS/SCADA logic and settings-group selection.
Primary-inject or prove CT/CBCT ratio/polarity and complete trip path where practical.
Check live load phase sequence, current/voltage polarity, power direction and healthy residual.
Test VT fuse/lost polarisation, DC/network/time failure and fail-safe response.
Archive calculation, settings/checksum, test files, waveforms and final margins.

16. Frequent mistakes

Mistake	Consequence	Correction
Pickup = 125% of nameplate	Actual emergency load/min fault ignored	Two-sided feasible window
One short-circuit case	Weak/strong/DER topology missed	All operating states
Relay times coordinated	Breaker/arc/tolerance absent	Total clearing bands
50 always enabled	Downstream fault/inrush overtrip	Require robust current gap
Earth pickup copied from phase	Grounding/charging/CT mismatch	Grounding-specific study
67 angle copied between vendors	Convention/polarity reversal	Equation/phasor verification
Steady injection only	Transient/security defects latent	Dynamic and failure tests

17. Settings-release checklist

All topologies/load/start/fault/grounding cases approved?
CT/VT ratios, polarity, burden and saturation validated?
Phase/earth pickup feasible windows demonstrated?
Curve equation/constants and reset behaviour exact?
Total-clearing coordination/tolerance/damage limits plotted?
50/50N robust selectivity gap demonstrated or disabled/delayed?
67/67N forward convention/polarisation/fuse-failure secure?
DER/island/coupler/settings-group logic covered?
Breaker time/DC and BF escalation included?
Dynamic/boundary/failure tests passed?
Live-load phasors/residual direction verified?
Calculation, native settings/checksum and as-built evidence controlled?

References and further reading

Engineering note: Worked values are illustrative and deliberately incomplete. Final settings require the approved study, exact relay curves/tolerances and verified breaker/CT/VT performance.