The protection relay: types and use in residential installations

What a relay is — the basic principle

An electric relay is an electromagnetically controlled switch: a small current (the control current) energises a coil that generates a magnetic field, which in turn moves a metal armature that opens or closes the contacts of a power circuit (high current).

The fundamental value of the relay is the galvanic isolation between the control circuit and the power circuit — a sensor at 12V DC can switch a 230V AC circuit with no direct electrical connection at all.

Modern relays come in many types: electromechanical (coil + physical armature), solid-state (with semiconductors) and numeric/digital (with a microprocessor). In residential applications, the most common are electromechanical relays and solid-state relays with a comparator.

1. The voltage relay (UV/OV relay)

The voltage relay monitors the mains voltage and disconnects the load when the voltage leaves a configurable band (typically ±10% of the 230V nominal, i.e. 207–253V per EN 50160).

What exactly it protects against:

• Undervoltage (brownout) — electric motors (pumps, AC compressors) overheat when running at reduced voltage: the current rises as 1/U, and the heat as I²
• Overvoltage — electronic equipment and LEDs are sensitive; a 10% overvoltage can dramatically shorten their lifespan or destroy the equipment
• Voltage recovery after an outage — the relay with a time delay (typically 3–15 seconds) protects AC compressors from premature restart before the pressure has equalised

2. The phase monitoring relay (three-phase)

For residential three-phase installations (pumps, large air-conditioning units, workshops), the phase monitoring relay simultaneously checks all three phases and trips at any anomaly that could damage the motors.

The most common failures it prevents:

• Phase loss — the most dangerous scenario: the three-phase motor tries to run on 2 phases, the current rises to 1.7–2× normal, the winding burns out within minutes
• Wrong sequence (phase reversal) — motors with a fixed direction (pumps, lifts, centrifugal fans) can break mechanically or flood if they run in reverse
• Voltage asymmetry — a 3–5% imbalance in voltages produces a 15–25% imbalance in currents (because of the motor’s low impedance), which overheats the winding

3. The thermal relay (motor overload)

The thermal relay is dedicated to protecting electric motors against prolonged overloads. Unlike the MCB, the thermal relay is adjustable (potentiometer) and has a time-current characteristic closer to the thermal behaviour of the protected motor.

It works on the same principle as the bimetal in the MCB, but sized for high currents and with fine adjustment. In residential settings it appears on:

• Pool or irrigation pumps (motors 0.5–2.2 kW)
• Motors for automatic gates and garage automation
• Industrial fans in workshops or greenhouses
• Air compressors for household use

Relay vs. MCB — the essential difference

The MCB protects the conductors and the circuit — it trips when the current exceeds the thermal capacity of the cable. The protection relay protects the connected equipment — it trips when the equipment’s operating parameters (voltage, phase balance, temperature) leave the normal operating band.

A motor can run for years with a current below the MCB limit but with a slightly unbalanced voltage — and it will burn out gradually without the MCB sensing any problem. The protection relay does sense it.

The numeric/digital relay

Modern protection relays are in fact small microprocessors that measure the electrical parameters in real time and apply configurable algorithms. Compared with the electromechanical ones:

• Digitally configurable thresholds (not a potentiometer)
• They store the last N trips with timestamp and cause
• RS485/Modbus communication with automation and home automation systems
• Multiple functions in the same body (voltage + phases + frequency + power)

The multifunction relay — one module, all the protections

In residential settings, the dominant form today is no longer the single-function relay, but the multifunction protection and monitoring relay (Fig. 0 and Fig. 4): a 1–2 DIN-unit module that integrates all the functions described above into the same body.

Purpose and role in the installation: it continuously monitors the voltage, the current and (on three-phase models) the phase balance, compares the measured values with adjustable thresholds and, when exceeded, commands disconnection of the load through an output contact (typically up to 40–63 A) — directly or through a contactor. In practice it replaces a row of separate relays (voltage + phases + thermal) with a single device that:

• Measures U, I, P and the power factor continuously — and displays them on the front LED
• Protects against under/overvoltage (UVP/OVP), thermal overload (OTP), phase loss and wrong sequence, minimum/maximum power and leakage current (with an external toroid)
• Delays reconnection after the return to parameters (it protects compressors and pumps from premature restarts)
• Logs the events and, on models with USB/Modbus, reports them to the automation system

The cost of a numeric relay with Modbus communication (50–150 EUR) is justified for houses with photovoltaic systems, backup generators or complex technical installations.

Normative reference

In I7-2011, the relay does not appear as a standalone „protective device" (as the MCB/RCCB do), but as a functional control (switching) device, dealt with in subchapter 5.3.4.5 — „Isolation (sectioning), switching and control devices". There, Art. 5.3.4.5.3.2 b) explicitly names it among the means of functional switching:

„[…] functional switching may be achieved by means of switches, semiconductor devices, contactors, relays, plugs and socket-outlets of rated current not exceeding 16 A."— I7-2011, Art. 5.3.4.5.3.2 b)

The role of these devices is defined in Art. 5.3.4.5.0.1: the measures of „isolation (sectioning), switching and non-automatic control […] are used for the purpose of preventing or avoiding the dangers of personal injury or damage (destruction) to the equipment in electrical installations" — exactly the function of the protection relay: it does not protect the cable, but the equipment.

The way the relay actually commands disconnection is described in Art. 5.3.4.5.2.4:

„Automatic circuit breakers, contactors etc. are opened remotely through the button (switch) that acts in the trip circuit of the coil."— I7-2011, Art. 5.3.4.5.2.4

In other words, the relay does not itself interrupt the power circuit, but commands the coil of a contactor — the scheme in the box above. The product standards for protection relays are from the IEC 60255 series, and voltage relays for generators and photovoltaic installations additionally comply with the grid-connection requirements (EN 50549).