1) How do you calculate a relay for a three-phase motor load? For example:

U: 400V
P: 15kW
Efficiency: 0.8 for a motor < 5kW
Efficiency: 0.9 for a motor > 5kW
Power factor: 0.8 for a motor < 5kW
Power factor: 0.9 for a motor > 5kW
V3 = 1.732

Nominal current absorbed by the motor:

I =               1500/                   = 26.7A
       (400 x 1.732 x 0.9 x 0.9)

The relay’s current rating must be 4 x I of the motor in order to withstand starting currents:
>>> 106.8A >>> a 125A relay must be used.

2) Advantages of Solid State Relays vs. Electromechanical Relays

With their semiconductors and a technology-based design, SSRs are more reliable, have a longer service life and less electromagnetic interference (EMI) than EMRs. They also have faster response times and are vibration and shock resistant, when compared to the EMR equivalent. This is because SSRs have no mechanical contacts to wear out or an arc, which is the primary cause for EMR failure.

3) Protecting Solid State Relays from load short circuits, click on this link:


4) How to choose the right heatsink


5) Installation without a heatsink or installation on the back plate of cabinet

A curve without a heatsink is associated with SO/SC or SG types of solid state relays.
When installed on the back plate of a cabinet, it is essential that the relay/plate contact is correct.
A 150 mm x 150 mm x 3 mm aluminium back plate corresponds to approximately 4°/W.
A 300 mm x 300 mm x 3 mm aluminium back plate corresponds to approximately 2°/W.
In any case, we recommend testing and measuring the heat dissipation.
A steel plate has greater thermal resistance.

6) Problems relating to SWTAs:

The mains:

  •          An SWTA must have its own power cables with the biggest possible cross section.
  •          It should be placed as close as possible to the power source to minimise cable length, hence line impedance.
  •          Inverters (where applicable), should be equipped with filters.
  •          If several SWTAs are used, each one must have its own line to the power source (do not bridge them)

7) What does “zero voltage turn on” mean? (or synchronous or zero cross)

The relay will only turn on when the mains (output) voltage is near zero.

8) What does instantaneous or random turn on mean?

When the control voltage is on, the relay will switch on simultaneously, irrespective of the mains voltage value.

9) In what application can I use a zero voltage turn on vs. a random turn on relay?

Zero cross relays are used with resistive loads while random turn on relays are used with inductive loads (motors, transformers, coils, etc.).

10) Control is off but the load is still energised

  •          Relay leakage current is too high in relation to the load holding current.

  •          Relay short circuit.
  •          The relay input is on because of the leakage current from the device controlling the relay (PLC, temperature controller, etc.)
  •          The relay’s output voltage has been underspecified.

11) Control is ON but load is off

This mainly happens with zero cross SSRs with DC control: the input signal is rectified but not filtered; at zero cross, the input signal is too low to make a valid control signal.
Solution: install a capacitor across the input.

12) You cannot start your SMCV/SMCW or SVTA/SWTA

Check whether there is a connection between terminals 5 & 6

13) Is it possible to place AC SSRs in parallel?

This is not possible because of the existing technology being used. In fact, current will always flow through one SSR only.

14) Is it possible to use DC relays in parallel?

This is possible, but it must comply with the following conditions:

  •          This can only be performed for thermal reasons (in fact, each individual SSR should be specified to switch the load current).
  •          We advise using DC SSRs with low turn on time: SOM range is more appropriate.

15) For an AC-51 3-phase load > 50A, can I use an SGT or SVT product, or are these relays limited to 50A because of the connections?

2 or 3 single-phase relays can be used. Our 75, 95, and 125A SGTs/SVTs are used for loads with non-permanent peak currents (in lamps, motors, etc.), or to provide better protection.

16) Under what conditions is it possible to operate celduc's solid state relays & contactors installed at altitudes of 2000 m to 5000 m above sea level?

Altitude has an impact on insulation and heat dissipation, both decrease as altitude increases.

 Heat dissipation is also less, so it might be necessary to increase the size of the cooling element (heatsink) used with the SSR. Tests must be carried out using a nominal current, in an ambient temperature with a T°C measurement on the heatsink near the relay The temperature should not exceed 100°C.

 For more detailed information.


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