Three-Phase Solid State Relays for Switching Resistive Loads HT Series


Heating elements, including tube heating elements, along with other types of loads, can be connected to a three-phase network using three main connection schemes:


"Star with Neutral"


To control a three-phase load when using a single solid state relay (SSR), it is recommended to use star-to-neutral load connection schemes (for cases with a rated operating voltage of a load of 220V) and a delta connection (for cases of a rated operating voltage of a load of 380V). The “Star without neutral” connection scheme is not recommended for use in conjunction with a three-phase SSR, since it does not provide uniform load distribution over the phases both in operating and emergency modes.

Three-phase general industrial SSR series HT-xx44.ZD3 (DC control) and HT-xx44.ZA2 (AC control) are designed for switching three-phase or three single-phase resistive load supply circuits. Relays provide simultaneous switching on each of the three phases.

Wiring diagram for three single-phase loads

Reliable operation of these SSRs in a given current range is ensured by the following technical solutions:

  • The copper base provides the most efficient heat dissipation from the output power element.
  • The use of various types of output power elements (depending on the modification) guarantees high reliability of the SSR.
  • Built-in shunt RC-chain increases the reliability of the SSR in terms of impulse noise.

HT solid state relays can have (depending on modification) as an output key:

  • triac output (TRIAC) - for relays with current up to 80A.
  • SCR (triode thyristor, or simply thyristor) output - for relays with a current of 100A and above.

The SCR output can significantly reduce the thermal resistance of the relay substrate and improve the heat sink performance. Relays of this type are designed to work in difficult operating conditions in the presence of fast transients in the power supply network: operation in a network with a high level of noise, operation on an inductive load, operation in conditions of high load current surges.

When operating solid state relays, it must be taken into account that the recommended current values will always be lower than the reference ones (indicated in the SSR designation).

Reference table of recommended load currents

AC control DC control Maximum allowable load current (for reference) Recommended resistive load current (per phase) I²t
HT-1044.ZA2 HT-1044.ZD3 10A 8A 85A²s
HT-2544.ZA2 HT-2544.ZD3 25A 19A 450A²s
HT-4044.ZA2 HT-4044.ZD3 40A 30A 840A²s
HT-6044.ZA2 HT-6044.ZD3 60A 45A 1800A²s
HT-8044.ZA2 HT-8044.ZD3 80A 60A 3200A²s
HT-10044.ZA2 HT-10044.ZD3 100A 75A 5000A²s
HT-12044.ZA2 HT-12044.ZD3 120A 90A 7200A²s

In addition, when the load is disconnected, the SSRs do not provide a complete opening of the electrical circuit, and the output terminals are energized. To completely disconnect the load (for example, during the period of equipment maintenance), it is necessary to take additional measures to disconnect the load power circuit, using contactors, knife switches, and load switches for this. When controlling an inductive load, a varistor must be installed in parallel with the load circuit.

SSR connection diagrams

Overall and installation dimensions