Craftsman Electrician study guide

What Is an EOCR (Overload Relay)? Main-Current Pins and Trip Contact (10·4·5)

Last updated: 2026-10-05 · 4 min read

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An EOCR monitors current and changes its output contacts according to configured protection characteristics. It is not simply an ordinary relay coil triggered at a universal current percentage. Trip current, delay and reset behavior depend on the device and settings. This page describes the site's terminals and manually triggered simulation model.

Main-current pins (1·2·3 → 7·8·9) — the path the motor current takes through

Top 1·2·3 in → bottom 7·8·9 out. The three pairs are hard-wired through, like copper bars.

Top 1·2·3 (input) — the three main-circuit phases (L1·L2·L3) from the MCCB's output land here.
Bottom 7·8·9 (output) — the same current exits straight down to the MC's main contacts. 1↔7 · 2↔8 · 3↔9 are always hard-wired together — current literally passes through.
While it passes, an internal current transformer (CT) reads each phase's current in real time.

This example uses the path — MCCB → EOCR → MC → motor — every amp the motor draws has to cross the EOCR first.

Trip contact (10 COM · 4 NC · 5 NO) — where the overload signal leaves

At rest 10-4 (NC) closed / 10-5 (NO) open. On trip 10-4 opens / 10-5 closes.

The trip contact is how the EOCR tells the outside world "this motor is overloaded." It's the same single-body SPDT you saw on the 8-pin relay.

10 (COM) — the common pole; the entry the signal flows in through, regardless of state.
4 (NC) — closed at rest, opens on trip. Wire it into the MC coil's leg and an overload breaks the coil circuit → the motor stops on its own. The heart of "overload → auto-stop."
5 (NO) — open at rest, closes on trip. For alarm output — a red lamp (RL) or buzzer hung here fires the instant a trip happens.

See it work

Normally current passes straight through. On trip the red indicator lights and the contact flips.

In the simulator, switch the MCCB on and hold PB1 and the three phases run POWER → MCCB → EOCR → MC → motor. Click the EOCR body while holding to fake an overload trip — the moment 10-4 opens, you'll watch the motor stop on its own.

EOCR acts on current. Next comes the sibling that acts on time — the timer (T), whose contacts flip only after a configured delay following coil energization.

Try it yourself

Open the practical project list →

Check your understanding

Question: Why can the motor stop even though the EOCR current paths remain connected?

Answer: Its trip contact interrupts the MC coil path, opening the MC main contacts. The EOCR itself is not a main-circuit breaker.

Practice path and references

The practical project list may require sign-in. Create a project, choose a task and place the relevant components, then compare the contact/wiring paths explained here. The link does not preload a specific completed circuit.

Reference: Q-Net public-problem notice, 2025-08-04 edition, problem 14 pages 11-1 to 11-4 (work/wiring instructions) and 11-9 (component diagrams). Read the original for each task and any amendments. Site examples are distinct from general installation design requirements.

Separate trip, contactor and power faults

If MC drops when EOCR trips, its control-stop path responds. If MC remains energized, trace whether the coil supply bypasses 10-4. If MC drops but the motor still appears supplied, inspect the main power path separately; an auxiliary indicator alone cannot prove isolation. Resetting the simulated trip only restores the protection contact: a correctly stopped holding circuit still needs a new start command.