Level 1, Unit 4, Section 1 – 1.4 Relays, Switches, Contactors & Transformers

1.4.1 — Switches

A switch is any device that opens or closes a circuit to control current flow. In HVAC/R, switches range from simple manual toggles to sophisticated automatic sensing devices that respond to pressure, temperature, or fluid flow. Every switch is described by two key characteristics: how it is actuated (what makes it change state) and what its normal position is (normally-open or normally-closed when the actuator is at rest).

Normally-Open (NO) vs Normally-Closed (NC)

Normally-Open (NO)

Contacts are open (circuit broken) at rest
Close (complete the circuit) when the actuator is activated
Examples: low-pressure switch on a pump-down circuit; thermostat cooling contact
A failed-open NO switch keeps the load off — safer for most control applications

Normally-Closed (NC)

Contacts are closed (circuit complete) at rest
Open (break the circuit) when the actuator is activated
Examples: high-pressure safety switch; low-pressure safety switch; thermal overload
A failed-open NC safety switch shuts down the equipment — the fail-safe design

Switch Types in HVAC/R

Switch Type	Actuated By	Normal Position	Typical Application
Toggle / selector switch	Manual (hand)	Latched in last position	System on/off; fan speed selection; heat/cool/auto mode selector
Disconnect switch	Manual (hand)	Operator-set	Isolates equipment from power for servicing; required within sight of equipment (CEC)
High-pressure switch (HPS)	Refrigerant discharge pressure	NC — opens on high pressure	Safety: stops compressor before discharge pressure reaches dangerous levels
Low-pressure switch (LPS)	Refrigerant suction pressure	NC (safety) or NO (operating)	Safety: stops compressor on low suction (loss of charge). Operating: controls pump-down sequence
Thermostat	Temperature (bimetal, electronic sensor)	Varies by stage (heating NO / cooling NC or vice versa)	Space temperature control; freeze stat; coil temperature limit; anti-short-cycle timer
Flow switch	Air or fluid velocity / differential pressure	NO — closes when flow is established	Chilled-water system interlock (compressor cannot run without water flow); evaporator fan interlock
Float switch	Liquid level	NO or NC depending on design	Condensate pan overflow safety; refrigerant liquid level in surge drum

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Worked Example — Diagnosing a High-Pressure Switch Trip

A split-system condensing unit shuts down and the high-pressure switch (HPS) is found open. The technician’s diagnostic sequence:

Measure discharge pressure with gauges — confirm it exceeded the HPS set point (e.g., 400 psig for R-410A).
Identify the cause of elevated head pressure: dirty condenser coil, failed condenser fan motor, or blocked airflow.
Correct the cause before resetting the HPS. On a manual-reset HPS, depress the reset button only after discharge pressure drops below the cut-in set point.
Run the system and confirm head pressure returns to normal operating range (350–390 psig at 95 °F ambient).
If the HPS trips again immediately after reset, the root cause has not been corrected — do not repeatedly reset.

1.4.2 — Relays

A relay is an electrically operated switch. A small current through the coil creates a magnetic field that moves an armature, changing the state of one or more sets of contacts. This allows a low-power signal (e.g., a 24 V thermostat output) to switch a higher-power load indirectly — without any electrical connection between the control circuit and the load circuit.

Contact Arrangements

Designation	Full Name	Contacts	Function
SPST-NO	Single Pole, Single Throw — Normally Open	1 set, open at rest	Simple on/off switching of one circuit; closes when coil energises
SPST-NC	Single Pole, Single Throw — Normally Closed	1 set, closed at rest	Simple on/off where energising the coil breaks (opens) the circuit
SPDT	Single Pole, Double Throw	1 common + 1 NO + 1 NC	Switches one circuit between two destinations; used for reversing, alternating, or changeover logic
DPDT	Double Pole, Double Throw	2 commons + 2 NO + 2 NC	Two independent SPDT switching actions simultaneously; common in reversing valve control and staging applications

Key Relay Ratings

Coil voltage: Must match the control circuit voltage exactly — common values are 24 VAC, 120 VAC, 240 VAC, and 24 VDC. Applying the wrong voltage destroys the coil.
Contact current rating: Maximum continuous current the contacts can switch. Exceeding this rating causes arcing, pitting, and welded contacts.
Contact voltage rating: Maximum voltage across the open contacts. Must exceed the circuit voltage being switched.
Coil resistance: Used to verify coil integrity. Measure with a multimeter — an open coil reads infinite resistance; a shorted coil reads near zero and the relay draws excessive current.

Common Relay Applications in HVAC/R

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Defrost Relay

Shifts the system from cooling to defrost mode on a time or temperature signal. Typically DPDT: one set of contacts energises the defrost heater; the other interrupts the compressor or reversing valve.

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Staging Relay

Sequences multiple compressors, heater stages, or fan speeds to match load. Each relay acts as a lockout or enable for the next stage, preventing all loads from energising simultaneously.

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Alarm Relay

Latches when a fault is detected (pressure trip, temperature limit) and powers an alarm indicator or BAS point. Requires a manual reset at the panel to clear, ensuring the fault is acknowledged before restart.

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Time-Delay Relay

Introduces a timed delay before contacts change state (on-delay or off-delay). Used for anti-short-cycle protection (prevents compressor restart for 5 minutes after shutdown) and fan-delay logic.

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Testing a relay in the field

With power off: measure coil resistance (should match specification, typically 100–1 000 Ω for 24 VAC coils). Measure contact continuity: NC contacts should read 0 Ω, NO contacts should read open (O.L. on a digital meter). With power on: apply rated voltage to the coil and verify contacts change state. A relay that hums but does not pull in has insufficient coil voltage or a seized armature.

1.4.3 — Contactors

A contactor is a heavy-duty relay designed specifically for switching high-current loads: compressor motors, condenser fan motors, and large resistance heaters. While the operating principle is identical to a relay, the construction is substantially more robust to handle the arc energy and mechanical wear generated by high-current switching.

Relay vs Contactor — Key Differences

Feature	Control Relay	Contactor
Contact current rating	Typically 5–30 A	Typically 25–400 A (matched to motor FLA)
Contact type	Small silver alloy contacts; light-duty arcing	Heavy silver cadmium oxide or silver alloy; arc-suppression chambers
Mechanical life	1–10 million operations	1–5 million operations (lower due to higher energy per switching event)
Auxiliary contacts	All contacts typically the same rating	Separate power contacts (high current) and auxiliary contacts (low current) for interlocking and indication
Coil voltage	24 VAC, 24 VDC, 120 VAC common	Almost always 24 VAC in HVAC/R; coil driven by thermostat control circuit
Replacement indicator	Replace when contacts pit or coil fails	Replace when contact pitting exceeds 1/32 in. depth or contacts weld; inspect at each PM visit

Contactor Construction

Main power contacts: Carry the full motor current. Arranged as 2-pole (single-phase) or 3-pole (three-phase). Contacts must be clean and flat — pitted or burned contacts cause voltage drop and motor overheating.
Auxiliary contacts: Low-current contacts used for control interlocking (e.g., a normally-open auxiliary contact that confirms the contactor has pulled in before allowing the next stage to energise).
Coil and armature: The 24 VAC coil pulls the movable armature against the stationary core, bridging the contacts. A coil that hums loudly has a loose or dirty armature face — clean or replace.
Arc suppression: Metal arc chutes or magnetic blowout coils extinguish the arc that forms when contacts break under load, preventing contact erosion and radio-frequency interference.

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Worked Example — Contactor Voltage Drop Test

A compressor is running at reduced capacity. The technician suspects the contactor contacts are pitted. With the system running at full load:

Set the voltmeter to AC voltage. Place one probe on the line (supply) side of the contactor and the other probe on the load side of the same phase.
Read the voltage drop across the closed contacts. Acceptable: <2 V. This reading = 1.8 V (acceptable).
Repeat for each pole. Phase B reads 8.5 V — well above the 2 V limit.
Shut down and lock out the unit. Inspect Phase B contacts: significant pitting and carbon deposits confirmed.
Replace the contactor. After replacement, voltage drop on all three phases: <0.5 V. Compressor current returns to nameplate FLA.

An 8.5 V drop at 18 A represents a power loss of 8.5 × 18 = 153 W dissipated as heat in the contactor — enough to discolour the contact housing and eventually destroy the contactor and wiring insulation nearby.

1.4.4 — Transformers

A transformer transfers electrical energy between two circuits using electromagnetic induction, with no direct electrical connection between primary and secondary. In HVAC/R, the most common transformer steps line voltage (120–600 V) down to 24 VAC to power thermostats, contactor coils, relay coils, and electronic control boards.

Transformer Types

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Control Transformer

The most common type in HVAC/R. Steps 120 V, 208 V, or 240 V down to 24 VAC for thermostats and control circuits. Housed inside the equipment or in a separate enclosure. VA-rated to match the total coil burden of the circuit it feeds.

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Isolation Transformer

Primary and secondary are wound to the same voltage, but the secondary is completely isolated from earth ground. Used for safety (eliminates shock path through earth) and to reduce electrical noise interference in sensitive electronic equipment.

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Autotransformer

Uses a single tapped winding rather than separate primary and secondary windings. More efficient and compact than a two-winding transformer, but provides no galvanic isolation. Used in some motor-starting applications and voltage adjustment.

Operating Principle & Turns Ratio

An alternating current in the primary winding creates a changing magnetic flux in the core. This flux induces a voltage in the secondary winding proportional to the ratio of turns:

Turns Ratio

V_primary / V_secondary = N_primary / N_secondary

A 240 V primary / 24 V secondary transformer has a turns ratio of 10:1 — the primary has 10 times as many turns as the secondary.

By the law of conservation of energy (ignoring losses), the power in equals the power out, so if voltage steps down, current steps up proportionally:

Current Relationship

I_primary / I_secondary = N_secondary / N_primary

A step-down transformer draws less current on the primary (high-voltage) side than it delivers on the secondary (low-voltage) side. Example: 40 VA at 240 V primary = 0.17 A; 40 VA at 24 V secondary = 1.67 A.

VA Rating & Burden Calculation

The VA (volt-ampere) rating of a transformer is the maximum load it can supply continuously. In a control circuit, the burden is the total VA drawn by all loads connected to the secondary: contactor coils, relay coils, and any electronic accessories.

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Worked Example — Control Transformer VA Sizing

A rooftop unit’s 24 VAC control circuit must power the following loads:

Load	Quantity	VA Each	Total VA
Compressor contactor coil	1	12 VA	12 VA
Condenser fan contactor coil	1	8 VA	8 VA
Control relay coils	3	4 VA	12 VA
Electronic thermostat / controller	1	10 VA	10 VA
Total burden			42 VA

A standard 40 VA control transformer would be undersized for this circuit. The correct selection is a 50 VA transformer (next standard size above 42 VA), providing an 18 % margin above the calculated burden.

Symptom of an undersized transformer: the control transformer runs hot, the 24 V output sags below 22 V under full load, contactors chatter or fail to hold in, and the transformer secondary fuse (if fitted) blows repeatedly. Always measure 24 V at the contactor coil terminals — not just at the transformer secondary — to detect excessive voltage drop.

Transformer Protection

Secondary fuse or breaker: Many control transformers include a secondary fuse (typically 3 A for a 40–75 VA / 24 V transformer). This fuse protects the transformer windings from a short circuit on the 24 V circuit — for example, a shorted thermostat wire touching the equipment chassis.
Primary protection: The transformer primary is protected by the branch circuit fuse or breaker on the line-voltage side.
Thermal protection: Class 2 control transformers include internal thermal protection that opens the circuit if the core overheats from an overloaded or shorted secondary.
Replacement: Replace a blown secondary fuse only after identifying and correcting the short circuit; a second fuse will blow immediately if the fault remains. Use only the rated fuse type and amperage.

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Common installation error: sharing one transformer across multiple systems

Field installers sometimes wire two roof-top units to a single control transformer to save cost. If both systems call simultaneously, the combined burden may exceed the transformer VA rating, causing the secondary voltage to collapse and both systems to shut down on low-voltage. Always provide a dedicated control transformer for each system unless the manufacturer explicitly allows a shared supply and the VA budget has been calculated.