Textbook lesson 48

Relays, Contactors and Overloads

Coils, contacts, auxiliary contacts, overloads and common control faults.

Training safety note: This is study material, not permission to perform electrical work. Practical activities must be done on approved training equipment or under licensed supervision with the current rules and workplace procedures.

In this lesson

Learning outcomes
Core theory
Trade application
Worked example
Workshop task
Fault-finding notes
Revision questions and answers

Learning outcomes

Explain the purpose of this topic in everyday electrical work.
Identify the circuit conditions that must be checked before relying on a reading.
Apply the relevant calculation, test or drawing interpretation in a supervised training scenario.
Recognise common apprentice mistakes and unsafe assumptions.
Record evidence in a form that another tradesperson can understand.

Core theory

Control circuits separate decision-making from power switching. A push button, sensor, timer or relay contact may control a contactor coil, while the contactor switches the load current. This makes larger loads safer and easier to automate, but it introduces new fault paths: coil supply, control neutral, interlocks, auxiliary contacts and overload contacts.

Ladder diagrams are read from supply rail to return rail, rung by rung. Apprentices should trace the conditions required for a coil to energise, then trace what the coil changes when it operates. Holding circuits, stop circuits and overload contacts are common because they create predictable behaviour during start, stop and fault conditions.

Modern electrical work often meets data, security, CCTV, EV charging, solar and automation. The electrician does not need to become every specialist at once, but should understand power requirements, segregation, earthing, surge protection, cable pathways and how to coordinate with standards and manufacturers.

Ladder logic

L+ ──[ STOP NC ]──[ START NO ]──[ OVERLOAD NC ]──( CONTACTOR COIL )── 0V │ │ └────[ AUX HOLD NO ]──────┘

The stop button is normally closed so a broken stop circuit prevents operation. The start button is normally open and energises the coil. An auxiliary contact maintains the coil after start is released. The overload contact opens the control circuit during overload.

Textbook depth: separating power and control

A contactor allows a small control circuit to switch a larger load circuit. The coil is not the load; it is the magnetic actuator that closes or opens contacts. Auxiliary contacts report the contactor state or create holding and interlock functions.

Start/stop circuits are designed so stop and overload controls fail safe. A normally closed stop button means a broken stop wire prevents operation. A normally closed overload contact opens when the overload trips. A normally open start button energises the coil only when pressed, and an auxiliary holding contact maintains the circuit after release.

Symptom	Likely control fault	Check
Coil never energises	No control supply, open stop/overload, failed start	Trace control voltage rung by rung
Coil energises but load off	Main contacts, overload, supply phase missing	Check power side under safe procedure
Contactor chatters	Low coil voltage or unstable control supply	Measure coil voltage while energised

Trade application

On site, this topic is rarely isolated. It connects to safety, drawings, protection, cable selection, terminations, testing and documentation. A good apprentice does not ask only “does it work?” They ask whether it is correctly supplied, correctly protected, correctly controlled, mechanically sound, suitable for the environment, and verifiable by inspection and test.

When using this material, build a notebook of standard methods. For each topic, write the normal value, the likely fault value, the test points, the instrument setting, and the action to take if the result is abnormal. This becomes a practical diagnostic map rather than a collection of memorised definitions.

Workshop practical

Wire a start/stop contactor circuit with overload contact and indicator lamps using extra-low-voltage training equipment. Draw the ladder diagram first, then compare the wiring to the drawing. Introduce one fault at a time and record symptoms.

Evidence to collect: labelled sketch, predicted readings, actual readings, explanation of differences, supervisor feedback and one improvement to your method.

Fault-finding notes

Confirm the complaint or task requirement in plain language.
Compare the installation against the drawing, label or expected circuit arrangement.
Prove whether supply is present at the correct point and under the correct condition.
Divide the circuit into smaller sections instead of testing random points.
After repair, test the protective measure, not just the load operation.

Common apprentice mistakes

Mistake	Why it matters	Better habit
Measuring voltage without a reference plan	The reading may be real, induced, back-fed or meaningless without a return path.	State the exact two points being measured and the expected value first.
Assuming a device is faulty because it is not operating	The fault may be supply, control, protection, return path, settings or mechanical load.	Prove each section of the circuit in sequence.
Recording only pass/fail	Future workers cannot see whether results were strong, marginal or abnormal.	Record actual values, conditions and instrument details.

Assessment standard

The assessor is looking for correct interpretation of control drawings, safe separation of control and power circuits, correct coil voltage, correct overload integration and structured fault finding.

Revision questions

What should be proven before this task is attempted on real equipment?
Which measurement would best confirm the main idea of this lesson?
What reading or symptom would make you stop and ask for supervision?
How could a poor termination change the behaviour of this circuit?
What information should be recorded for handover or assessment evidence?