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Protection & Relaying

ANSI device numbers · 51/50/87/27/49 · differential · numerical relays

Breakers are dumb — they trip when current exceeds a setting. Relays are smart — they decide WHEN and WHY. Every protection device has an ANSI device number. Coordination studies plot the curves and verify selectivity.

ANSI/IEEE Device Numbers

Every protective device has a number. The IEEE C37.2 standard assigns 1-99 (some up to 999) to specific functions. Memorize the dozen most-used; the rest are looked up.

Device #	Function	Where used
21	Distance relay	Transmission line protection
25	Synchronism check	Generator paralleling, ATS closed-transition
27	Undervoltage	Motor protection, generator dropout
32	Reverse power (directional power)	Generator protection (motoring), prevent backfeed
37	Undercurrent	Motor loss-of-load protection (e.g., loss of cooling)
40	Loss of field (excitation)	Synchronous motor / generator protection
46	Negative sequence (current unbalance)	Motor protection — phase loss, unbalanced load
47	Phase sequence / phase reversal	Verify correct phase rotation on incoming feed
49	Thermal overload (machine)	Motor + transformer protection
50	Instantaneous overcurrent	Universal — fastest trip on high faults
51	Time-overcurrent (inverse-time)	Universal — coordinated overcurrent
50G / 51G	Ground fault (50 = inst, 51 = time)	Detects ground faults; required by NEC 230.95 for large 480V services
50N / 51N	Residual neutral overcurrent	Ground fault on Y-grounded systems
59	Overvoltage	Generator protection, capacitor protection
67	Directional overcurrent	Looped systems where fault current can flow either direction
79	Auto-reclose	Distribution feeder breakers; reclose after temporary fault
81	Frequency (under or over)	Generator protection, load shedding, intentional islanding
87	Differential	Transformer (87T), bus (87B), motor (87M), generator (87G) — fastest, most selective protection
87L	Line differential	Transmission line — pilot protection

Inverse-Time vs Definite-Time vs Instantaneous Tripping

Tripping characteristic	Description	Where used
Instantaneous (50)	No intentional delay — trip in < 1 cycle when current exceeds setpoint	High-fault region — clears bolted faults fastest
Definite-time (51 with definite-time setting)	Fixed delay regardless of current magnitude (after pickup)	Backup protection — coordinates above downstream device's clearing time
Inverse-time (51)	Higher current → faster trip. IEC and IEEE curves: standard inverse, very inverse, extremely inverse	Universal time-overcurrent. Coordinates naturally with downstream OCPDs at all current levels
Pickup current	The current threshold that "starts" the timing element	Set above maximum normal load current with margin
Time dial	Multiplier on the curve — shifts curve up/down	Coordinated with downstream; lower TD = faster operation

Differential Protection (87)

Differential measures current entering a zone vs current leaving. If they don't match, current is going somewhere it shouldn't — internal fault. Trips immediately. The fastest protection available, with no coordination delay needed because it only operates on faults INSIDE its protection zone.

Differential type	Protected zone	Operation
87T Transformer differential	Inside the transformer windings	CTs on primary + secondary. Compensates for turns ratio + winding configuration. Trips on any internal fault.
87B Bus differential	Inside the switchgear bus	CTs on every bus connection. Sum should be zero. Trips on any bus fault — clears in < 1 cycle, prevents catastrophic arc flash.
87M Motor differential	Inside the motor windings	CTs on phase + neutral connections. Detects winding-to-winding fault.
87G Generator differential	Inside generator windings	Same principle — most generators have 87G as primary protection.
87L Line differential (pilot)	Transmission line	Communication channel between line ends compares currents. Telecomm-dependent.

Modern Numerical Relays

Old-school electromechanical relays (cup-and-disk) are being replaced everywhere by numerical relays — microprocessor-based devices that combine many ANSI functions in one box, with communication, event logging, and remote access.

Manufacturer	Common product line	Notes
Schweitzer Engineering Labs (SEL)	SEL-351, 387, 411, 421, 487	Industry leader. Strong cybersecurity. Engineering-friendly programming.
GE / Multilin	F60, F35, MIF II, T60	Strong utility presence. UR family.
ABB	Relion 615, 620, 630, 670 series	European-strong; 60870-5-103/104 native.
Siemens	SIPROTEC 4 / 5	European-strong; integrated DIGSI software.
Eaton	EDR 5000, MP-3000, MP-4000	Industrial focus.

Coordination Study — The Deliverable

A coordination study plots every TCC for every OCPD on a single log-log chart, with the available fault current marked. The result: visual confirmation that for any fault, only the closest device opens.

Component	What's shown
Source impedance line	Available fault current at each bus
OCPD curves	Each device's TCC at its protected location
Cable damage curve	Conductor I²t damage threshold (NEC 110.10)
Transformer damage curve	Per IEEE C57.109 / ANSI
Motor inrush region	For motor branches, plot inrush curve to ensure CB doesn't trip on starting
Selectivity bands	Time gap between upstream and downstream curves (≥ 0.3 sec typical for fuses, ≥ 0.4 sec for CBs)

Worked Example 1 — Atlas DC1 MV Switchgear Protection Scheme

Example 01 · Atlas DC1 spine12.47 kV switchgear protecting TX-A and TX-B + utility incoming

Protection functions on each device

Position	Protection (ANSI #s)	Why
Utility incoming CB (12.47 kV)	50, 51, 50G, 51G, 27, 59, 81	Standard incoming protection: overcurrent, ground, voltage, frequency
TX-A primary CB (12.47 kV)	87T (with TX-A secondary CT input), 50, 51, 50G, 51G, 26 (sudden gas pressure)	Differential primary protection of TX-A — trips on any internal fault. Backup overcurrent.
TX-A secondary CB (480V)	50, 51, 50G, 51G, GFP per NEC 230.95	Backup feeder protection for 480V SWGR
Bus differential 87B	One zone per side (A bus and B bus)	Clears bus fault in < 1 cycle — minimizes arc flash

Why each device matters

87T transformer differential: A winding-to-winding fault inside TX-A would draw fault current from utility but the fault location is inside the transformer enclosure. Without 87T, only the slower 51 element trips → significant transformer damage. With 87T, trip in 1-2 cycles.
87B bus differential: A fault on the 480V bus (e.g., insulation failure from a falling tool) would otherwise wait for transformer 51 to time out (~ 100 ms+). At 50 kA fault current, that's massive incident energy. 87B clears in 4 cycles → 90% reduction in incident energy.
50G/51G ground fault: Detects ground faults on solidly-grounded system before they escalate to phase-phase faults. Sensitivity: 100-1200 A typical setting.

Why DCs invest heavily in protection

A typical commercial building's MV switchgear has 51 and 50G — that's it. Atlas DC1 adds 87T, 87B, 27, 59, 81 because every cycle of clearing time matters: less arc flash, less downtime, less chance of cascading failure into the IT load. The cost is justified by the value at stake.

Worked Example 2 — Industrial Motor Protection (49, 51, 46, 27)

Example 02 · Alternate context500 HP induction motor on 480V — protected by integrated motor relay

ANSI #	Function	Setting	Why
49	Thermal overload	Per NEC 430.32 — 115% of FLA for SF=1.0; 125% for SF=1.15	Protect motor windings from thermal damage
50	Instantaneous OC	~ 130% of locked-rotor	Bolted fault on motor leads
51	Time-OC	120-130% FLA pickup, time dial coordinates with upstream	Backup to thermal overload
46	Negative sequence	Pickup at ~ 5% I₂/I₁	Detects phase loss and unbalance — both very damaging to induction motors
27	Undervoltage	~ 80% of nominal	Drop motor on sustained undervoltage to prevent stall and overheating
37	Undercurrent	Custom per application	Optional — detect loss of load (broken pump shaft, etc.)

One numerical relay (e.g., SEL-710) provides all of these functions plus event recording and Modbus communication. Old electromechanical equivalent would be 4-6 separate panels.

Coordination Plot Example — Atlas DC1 MV Protection

The TCC plot for the MV switchgear protection chain: utility 51 → TX-A primary 51 + 87T → 480V SWGR-A 51 + 87B.

Differential (87B, 87T) clears INSIDE its zone faster than any 51 element — sub-cycle vs hundreds of milliseconds. Critical for arc flash.

Drill — Quick Self-Check

Work each problem mentally; reveal to check. Goal: reflex, not deliberation.

Drill 1 · Top ANSI numbers

What are 50, 51, 87?

Drill 2 · 87T speed

Why is 87T (transformer differential) faster than 51?

Drill 3 · Pickup vs time dial

Two settings on a 51 element?

Drill 4 · Negative sequence (46)

What does 46 detect?

Drill 5 · Atlas MV

What protection does Atlas DC1's MV switchgear use?

Instrument Transformers — CT + PT

Protective relays and meters can't measure thousands of amps or thousands of volts directly. CTs and PTs step these down to safe, standardized values (5 A and 120 V respectively).

Current Transformers (CTs)

Parameter	Description
Ratio	Primary:secondary, e.g., 1200:5 means 1200 A primary → 5 A secondary at full load
Burden	Load on the secondary (relays + wiring + meters). Specified in VA. Higher burden = more saturation risk.
Accuracy class	For metering: 0.3, 0.6, 1.2 (% error at rated burden). For relaying: C100, C200, C400, C800 (relaying class — voltage at saturation).
Polarity	Marked terminals (H1 + X1). Critical for differential protection — wrong polarity = immediate trip on energization.
Saturation	At very high primary current (faults), CT iron core saturates → secondary output stops following primary. Causes incorrect relay operation. Sized to avoid saturation at maximum fault current.
Open secondary danger	Never open a CT secondary while energized! With no burden, voltage rises to thousands of volts → arcing + insulation failure + lethal. Always short-circuit before disconnecting.

Potential Transformers (PTs / VTs)

Parameter	Description
Ratio	Primary:secondary, e.g., 14400:120 means 14.4 kV primary → 120 V secondary
Burden	Same concept as CT but secondary is voltage-limited not current-limited
Accuracy class	0.3, 0.6, 1.2 metering. Various relaying classes.
Connection types	Wye-wye (most common), open-delta, V-V (used when delta primary system has no available neutral)
Capacitive Voltage Transformers (CVTs)	Used at very high voltages (≥ 138 kV) — capacitive divider + tuning circuit. Cheaper than full magnetic PT.

Ladder Logic — Relay Programming Basics

Ladder logic is a graphical programming language designed to mimic the wiring diagrams of relay-based control panels. PLCs use it; modern protective relays often use a similar logic syntax.

Symbol	Meaning
`--\| \|--`	Normally open contact (input). True when input is energized.
`--\|/\|--`	Normally closed contact (input). True when input is NOT energized.
`--( )--`	Output coil. Energized when the rung's logic is true.
`--(L)--`	Latching output coil. Stays energized after one true cycle.
`--(U)--`	Unlatching coil. Resets a latched output.
Rungs in series	AND logic — all conditions must be true
Rungs in parallel	OR logic — any condition true energizes output

Boolean Algebra — The Math Behind Ladder

Operation	Symbol	Truth	Ladder equivalent
AND	· or &	1·1 = 1; else 0	Series contacts
OR	+ or \|	0+0 = 0; else 1	Parallel contacts
NOT	Bar over var	NOT(0) = 1; NOT(1) = 0	Normally-closed contact
NAND	NOT(AND)	NOT(1·1) = 0; else 1	Series of NC contacts
NOR	NOT(OR)	NOT(0+0) = 1; else 0	Parallel of NC contacts
XOR	⊕	1 if exactly one input is 1	(A·NOT B) + (NOT A·B)

If You See THIS, Think THAT

If you see…	Think / use…
"51"	Time-overcurrent. Universal coordinated protection.
"50"	Instantaneous OC. Fastest trip on high fault.
"87T"	Transformer differential. Internal-fault protection. Sub-cycle clearing.
"87B"	Bus differential. Critical for arc flash reduction.
"50G", "51G"	Ground fault elements. NEC 230.95 requires for 480V services ≥ 1000A.
"49" thermal overload	Motor protection. NEC 430.32 sets the limits.
"46" negative sequence	Detects phase loss / unbalance on motors. Very valuable — prevents motor damage from single-phasing.
"27" undervoltage	Motor protection (drop on UV) or generator protection.
"81" frequency	Generator protection or load shedding logic.
"25" synchronism check	ATS closed-transition. Generator paralleling.
SEL-351 / SEL-787 / SEL-787-3	Schweitzer relays — feeder, transformer, multi-phase. Industry standard for new installations.
"Coordination study"	Plot of all TCCs. Verify selectivity at all fault levels.
"Pickup" + "time dial"	The two settings on every 51 element.