Electronic switching for heaters and similar loads

IEC 60947-4-3 covers semiconductor controllers and contactors for non-motor AC loads. Resistance heaters, compatible lighting loads and transformer applications are practical examples. When reviewing solid-state relays and power controllers, check the load type for which the device has been evaluated.

The IEC publication record identifies IEC 60947-4-3:2020 as the third edition. The rated circuit voltage within its scope does not exceed 1,000 V AC. It addresses switching and control that reduces RMS AC voltage. This scope limit does not establish that every product can operate at 1,000 V.

A semiconductor contactor and a power controller do different jobs

A semiconductor contactor switches the load electronically. A power controller can regulate the power delivered using its selected control method. AC designs may use thyristors or triacs. Not every on/off SSR is a controller that provides proportional power adjustment from an analog command.

A temperature controller can use a PID algorithm to generate heating demand; the SSR or power controller applies that demand to the load circuit. A PLC can also issue the command. Check the control input's voltage and current requirements, output type and wiring arrangement.

Zero-cross switching, full-cycle control and phase-angle control

With zero-cross switching, the output begins conducting near a supply voltage zero crossing after receiving the turn-on command. This helps reduce switching noise with suitable resistive loads. Full-cycle control passes selected complete mains cycles and blocks others, changing average power through the proportion of cycles delivered.

Phase-angle control changes the point at which conduction begins during each half-cycle, altering the load waveform and RMS voltage. It can generate more electrical noise, so load compatibility and EMC measures must be considered together. Ordinary on/off commands to a zero-cross SSR do not automatically provide phase-angle control.

What does a 40 percent command mean for a 3 kW heater?

Consider a single-phase heater consuming 3 kW at 230 V, with resistance assumed constant. Using the relationship between voltage and current, its full-conduction current is approximately 3,000 / 230 = 13 A. With full-cycle control, a 40 percent duty ratio gives approximately 1.2 kW average electrical power. During the conducting cycles, however, the heater still receives 230 V; its voltage has not been reduced to 92 V.

This calculation explains the operating method; it is not sufficient by itself to select the SSR current rating. Also, 40 percent power demand does not mean 40 percent of the firing angle. Power must not be assumed to vary linearly with firing angle.

Load behaviour and cooling matter as much as the current label

Heater cold resistance, lamp inrush and transformer magnetizing current differ. Equal steady-state current does not mean equal switching stress. For transformer loads in particular, select a suitable firing method from the manufacturer's documentation; do not extend a resistive-load preference for zero-cross switching to every load.

The semiconductor output generates heat while conducting. Check heat-sink thermal resistance, panel temperature, airflow and current derating together. Also assess cable current-carrying capacity, terminal connections and the mounting surface. The labelled current is not the permissible current without cooling or at every cabinet temperature.

An off command does not provide electrical isolation

A small leakage current can flow through an SSR output when it is off, and a semiconductor failure can leave the load continuously energized. This leakage is not the same concept as the current monitored by earth-leakage protection. Provide appropriate isolation for maintenance and, where needed, an independent contactor or breaker to remove power under conditions such as excessive temperature.

Fuse and circuit-breaker selection involves more than load current. Review semiconductor short-circuit protection and the manufacturer's recommended protective combination. Being able to switch off electronically does not mean the device provides short-circuit protection or is suitable as an isolator.

For final selection, verify the exact model, load category, operating voltage, input type, cooling conditions and conformity documentation together. A standard reference alone does not guarantee precise temperature control or safe operation in every application.