4.1 ANSI Device Numbers (27, 50, 51, 59, 86, 87)

Why Device Numbers Matter on a NETA Job

Quick Answer: Every relay, lockout, and meter on a one-line is labeled with an ANSI/IEEE C37.2 number. Knowing the number tells you what the device does, what test to perform, and what the trip target is before you ever touch the test set.

ANSI/IEEE C37.2-2022 (the active standard; no 2026 revision exists) assigns 95 function numbers plus 22 acronyms. A NETA Level 2 technician meets the same handful on almost every acceptance and maintenance job, so memorize the core set first.

The Core Device Numbers You Must Know

Suffix Letters That Change the Meaning

A bare number tells you the function; the suffix tells you the application.

• N — Neutral. 50N and 51N operate on residual or neutral current (3I0). They are sensitive to ground faults a phase element may miss.
• G — Ground. 51G uses a dedicated ground CT (often a window/zero-sequence CT) rather than the residual connection. The distinction matters during a 50G/51G commissioning test because the shield and ground conductors must be routed back through the window or the scheme will misoperate on through-current.
• P — Phase. 50P/51P are the phase overcurrent elements; modern numeric relays label each phase explicitly (50P-A, 50P-B, 50P-C).
• T / G / B / M on 87. 87T = transformer differential, 87G = generator differential, 87B = bus differential, 87M = motor differential. Each zone has its own restraint characteristic and CT requirement.
• S — Stalled / start. 66/48S-style suffixes appear on motor relays; 46 is phase-balance (negative-sequence) protection.

Why 50 Versus 51 Matters for Pickup Testing

A 50 element is an instantaneous trip. Once current crosses the pickup setting, the relay issues a trip in roughly 1.5 to 4 cycles — there is no intentional delay. Field pickup testing is therefore a single-point operate/no-operate check: inject 0.9 x pickup and confirm no trip, inject 1.1 x pickup and confirm a fast trip, and read the actual drop-out.

A 51 element is time-overcurrent. It follows an inverse-time curve (see Section 4.2), so pickup is verified at several multiples of setting current — typically 2x, 3x, 5x, and 10x — and the measured operate times are compared to the manufacturer curve and the coordination study. A common Level 2 mistake is to test a 51 at one current only; you have missed whether the time dial is correct.

A combined 50/51 relay houses both elements. The instantaneous element is usually set above the maximum through-fault (so it never operates for an external fault), while the time element covers overload and downstream backup. When you inject a current above the 50 pickup, expect an instantaneous trip; below the 50 pickup but above the 51 pickup, expect a timed trip. Reading the wrong target is a frequent cause of misdiagnosed relay tests.

86 Lockout — the Master Trip Relay

The 86 relay is not a protective relay in the sensing sense; it is a lockout/master trip auxiliary. When any protective element (50, 51, 87, etc.) operates, it energizes the 86 coil, which then seals itself in through a contact and trips all required breakers. Because it locks in, the breaker cannot be reclosed until an operator pushes the reset button (or sends a remote reset command). The 86 is why a faulted transformer stays isolated after a trip — without it, autoreclose schemes (79) could re-energize damaged equipment.

Worked Scenario: 50/51 on a 480 V Feeder

A 480 V feeder carries 600 A load and is protected by a 50/51 relay with a 1200:5 CT. Settings: 51 pickup = 1.25 x FLA (750 A primary, 3.125 A secondary), 51 time dial = 0.3 (standard inverse), 50 pickup = 9000 A primary (37.5 A secondary, set above the 7500 A maximum through-fault). During a close-in bolted fault at 25,000 A:

• The 50 element operates first, tripping the feeder in ~2.5 cycles.
• The 51 element also starts timing but the breaker is already open, so its target may not pick up.
• The 86 lockout seals in and blocks autoreclose.

If you mis-set the 50 pickup to 6000 A (below the 7500 A through-fault), the relay will trip for a downstream fault — a coordination failure your injection test will catch if you sweep both elements.