PROS AND CONS
Each type of relay has its advantage and disadvantage
EMR - classic electromechanical
PROS: switches AC and DC voltage, no need for cooling, no leakage currents, etc
CONS: contact life, el. arc during disconnection of contacts, max. switching frequency, etc.
SSR - semiconductor relay
PROS: not subject to environmental influences, high service life, low EMI interference
CONS: for heavy loads it is necessary cooling, leakage current, higher price, etc..
In the case of the Railduino module, there are two possibilities of the relay
classic EMR - max. current 7A at 230V, not an inrush relay
semiconductor SSR - max. current 2A at 230V, inrush 30A
Relay outputs 3-6 and 9-12 of the Railduino module are designed for lower current loads - max. 5A output. Max. current 7A can only be used at outputs 1,2,7,8.
In the first place, it is necessary to clarify what type of load (resistive, capacitance or inductive) will be switched on/off and how much currents can be expected in the load.
In case of switching capacitance / inductive loads it is necessary to use either an external inrush relay with properly dimensioned contacts or SSR relays that can withstand high inrush currents or NTC limiting element which limits the starting current.