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FAQ - Frequently Asked Questions

1) How to calculate a relay for a three phase motor load ? exemple:

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                   = 26,7 A
400 x 1,732 x 0,9 x 0,9

The current rating of the relay must be 4 x I of the motor in order to withstand starting currents:
>>> 106,8 A >>> it's necessary to take a 125 A relay.

2) Advantages of Solid State Relays in comparison to a electro-mechanical relays ?

Because of their semiconductor technology-based design, SSRs provide a higher degree of reliability, longer life and reduced electromagnetic interference (EMI), as well as faster response times and vibration and shock resistance, when compared to the EMR counterparts. This is due to the fact that SSRs have no mechanical contacts to wear out or arc, which is the primary cause for EMR failure.

3) Protection of the Solid State Relays from short-circuit of the load Link with:


4) How to choose the right heatsink ?


5) Mounting without heatsink or mounting on back plate of cabinet

A curve without heatsink is given for the SO/SC or SG type of solid state relays.
When mounted on the back plate of a cabinet, the relay/plate contact must be correct.
An aluminium back plate of 150mm x 150mm x 3 mm corresponds to approximately 4°/W
An aluminium back plate of 300mm x 300mm x 3 mm corresponds to approximately 2°/W
In all cases, it is recommended to perform a test and measure the heat dissipation.
A steel plate will have greater thermal resistance.

6) Working problems with SWTA:

Problems from the mains:

  • The SWTA should have its own power wires of the biggest possible cross section
  • It should be placed as close as possible to the power source to reduce cable length hence the line impedance
  • Inverters (if any), should be equipped with filters
  • If several SWTA are used, they should have their 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 at the same moment, whatever the mains voltage value.

9) In what application would I use a zero voltage turn-on v. 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 however the load is energised

  • Leakage current of the relay is too high as regards the holding current of the load.
  • Short-circuit of the relay.
  • The relay input is on because of the leakage current from the device controlling the relay (PLC, temperature controller…)
  • Output voltage of the relay has been under-specified.

11) Control is ON but load is off

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

12) You can not start your SMCV/SMCW or SVTA/SWTA

Check if there is a connection between terminals 5 & 6

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

This is not possible because of the technology used. In fact current will always go through one SSR only

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

This is possible but you should respect some conditions:

  • This must be done for thermal reasons only (in fact each SSR should be specified to switch alone the load current)
  • This is better to use DC SSRs with low turn-on time: SOM range is more suitable

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

It's reasonable to use 2 or 3 single phase relays. Our SGT/SVT 75, 95, and 125A are used for loads with non permanent  peak current (as lamps, motors...), or to have a better protection.

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

Altitude affects insulation and heat dissipation, both get lower as altitude grows.

 Heat dissipation will get worse, so it might be necessary to increase the size of the cooling element (heat sink) used with the SSR. Tests must be done at nominal current and ambient temperature with T°C measurement on the heat sink close to the relay. Temperature should not get over 100 °C.

 For more detailed information.

17) How to ensure grounding/earthing of SSR assemblies ?

There are several ways to ensure grounding of SSR assemblies:

- Solid State Relays have a protective earth connection ; or
- Their construction ensures a good connection to its fixing mean (Din rail, plate, panel,...) having themself a protective earth connection.

For more detailed information.