Controlling electric heaters is one of the most common applications of relays in industrial automation. In such systems, where the load is a resistive heating element, solid state relays (SSR) are ideal.

Unlike classic relays, SSRs have no moving mechanical parts, which allows them to operate silently and with virtually no wear. This is crucial in systems where heaters are switched on and off very frequently.

What do you need to select an SSR for a heater?

Selecting the right SSR relay for a heater requires consideration of several basic parameters: supply voltage, operating current, switching frequency and type of mounting.

Heating loads (resistance heaters) are characterised by stable current consumption, which is why they work well with solid state relays equipped with a zero crossing function, which significantly reduces current surges and electromagnetic interference in the network, resulting in a longer service life for the entire system.

Selection examples – which Carlo Gavazzi model for which heater?

Typical example: a 9 kW heater powered by 400 VAC (3-phase) draws approximately 13A per phase. In this case, a three-phase SSR with a minimum load capacity of 15 A should be used, e.g. Carlo Gavazzi RZ3A60D55, which is ideal for this type of application.

For single-phase systems, e.g. a 3 kW, 230 VAC heater, a good choice would be the RM1A48D50, which supports up to 50 A and is very often used in furnaces, dryers and laboratory equipment.

Installation, cooling, operating conditions

It is also worth paying attention to the installation method. If the relay is to be installed in a switchboard, a DIN rail mountable version, e.g. RGS1A60D75KKE, will be a more convenient solution. The RM1A series models are designed for panel mounting and require good cooling – it is often necessary to use an additional heat sink, especially at higher currents, to effectively dissipate the excess heat generated by the operating solid state relay.

When selecting an SSR solid state relay, environmental conditions such as ambient temperature, humidity and air flow must also be taken into account. Carlo Gavazzi SSR relays can operate in a range from -40 to +80°C, making them suitable for most industrial applications.