Relays, Contactors, Starter Circuits and Electrical Interlocks

SOPEEC 4th Class Power Engineer Part 4A — Free Study Guide 2026 Exam

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Relays, Contactors, Starter Circuits and Electrical Interlocks

Key Takeaways Explain This Topic AI

A control circuit commands switching; it is distinct from load-power isolation.

Normally open/closed refers to the device’s drawing reference state.

A three-wire holding circuit normally requires a new start after loss of control power.

Overload protection, short-circuit protection and safety functions have distinct duties.

Last updated: October 2026

Separate the power circuit and the control circuit

An electrical control system uses switches, relays, contactors, timers and protective devices to control equipment. The motor power circuit carries the main load current. The control circuit operates the contactor coil or another actuator, often at a different voltage. Opening a control switch may stop the motor but is not the same as opening and locking a rated isolating device. Stored energy and alternate supplies must still be assessed.

A relay uses a coil to move contacts. A contactor performs switching duty appropriate to a larger load, such as a motor. An overload relay responds to the specified overload condition and usually opens a control contact to de-energize the contactor. Short-circuit protection is a separate function provided by correctly coordinated fuses or circuit breakers. A contactor is not automatically rated to interrupt every possible fault current.

Read contact states correctly

Normally open and normally closed refer to the device’s defined normal, unactuated or de-energized reference condition, as shown by the drawing convention. They do not necessarily describe the condition during normal plant operation. A normally open contact on a running motor’s contactor may be closed because the coil is energized. A pressure switch’s reference state must be read from its specification and legend, rather than inferred from the word “normal”.

In a ladder diagram, rails represent the control supply and rungs represent functional paths. Series contacts implement an AND condition for continuity; parallel branches implement an OR condition. A coil symbol represents the controlled relay or contactor. Cross-references identify contacts operated by the same coil elsewhere in the drawing. Mechanical linkages, interlocks and field conditions may add constraints that are not obvious from one isolated rung.

Three-wire motor starting

A common three-wire starter has a normally closed STOP pushbutton and overload contact in series with the coil. A momentary normally open START pushbutton energizes the coil when the series path is healthy. A normally open auxiliary contact closes with the contactor and forms a holding path around START. The motor continues running when the operator releases START, provided STOP, overload and the required permissives remain closed.

Pressing STOP opens the coil circuit and drops out the contactor. Loss of control power also drops it out. When power returns, the holding contact is open, so a new start command is needed in this arrangement. This differs from a maintained two-wire demand, which can restart automatically when its conditions return. The intended restart behaviour is a safety and process design decision; do not assume all motors behave like the example.

Interlocks, timers and protection

An electrical interlock can use a normally closed auxiliary contact from one contactor to inhibit an incompatible contactor. Reversing starters also use the specified mechanical interlock so opposite directions cannot be energized together. A timer can delay an action after coil energization or de-energization. The diagram must identify which type is intended; changing the timer type can reverse the operating consequence even if its seconds setting matches.

A permissive enables starting when a prerequisite is satisfied. A running interlock can trip operation if a required condition is lost. Emergency-stop and safety functions need the specified architecture, monitoring and reset behaviour. Wiring every signal through one ordinary relay does not automatically make a fail-safe system. A welded contact, broken wire, lost supply or failed sensor must be considered according to the design.

A troubleshooting scenario

A motor stops when START is released. In the example starter, the holding path may not be completing: possibilities include an auxiliary contact, wiring or contactor mechanical fault. That symptom does not prove the main motor winding is defective. A motor that will not start at all requires checking the documented permissive and control indications through authorized diagnostics. Do not bridge STOP or overload contacts to see whether the motor can run.

Operators should understand the drawing and communicate symptoms, equipment state and trip information. Qualified electrical personnel perform testing and repair under the approved safe-work procedure. After work, verify the intended start, stop, interlock and restart behaviour, restore covers/guards and record results. A successful unloaded start alone does not prove every protective function or the safe condition of the driven machine.

Separate contact state from device health

A normally closed stop contact is drawn in its defined normal, unactuated condition. Pressing STOP opens that contact and breaks the described control path. A broken conductor can also interrupt the path, which explains one reason for this arrangement, but it does not make every possible fault safe. A welded contact, an unintended alternate feed or an incorrect jumper can defeat the intended action. Testing and fault finding therefore require the approved isolation and verification procedure, circuit identification and competent electrical personnel.

Comparison aid

• Element Described function Distinction
• START contact Momentarily completes the coil path It is not load-power isolation
• Holding contact Maintains the described run command It opens when the contactor drops out
• STOP contact Interrupts the control path Contact state does not prove all power absent
• Overload device Responds to the specified overload condition It differs from short-circuit protection
• Source notes

Official scope and technical references checked 2026-10-17: Reference 1 ; Reference 2 . Archived DOE handbooks are used for physical principles, not current Canadian legal requirements.