Electromechanical relays can be expensive, large, take longer to work than modern semiconductors and have a finite contact life, particularly for large power contactor relays on big ship electrical systems. Solid state relays, which use thyristors, also known as Silicon Controlled Rectifiers (SCR), Triodes for Alternating Current (TRIAC) or transistor outputs are used instead.
The output device (SCR, TRIAC, or transistor) is optically-coupled to an LED* light source Inside the relay.
Being a solid state there are no moving parts to wear out, no sparking on contact, and no contact corrosion and they can be switched on and off quicker than mechanical relays for massive ship electrical system sound great. However, they are expensive to build for very high current ratings where electromechanical contactors dominate.
Solid state relays will only open the AC circuit at a point of zero current as their inherent hysteresis (memory) maintains current continuity after the LED is de-energised, until It falls below the holding current’. Known as ‘zero-crossover switching’, this is the advantage of uninterrupted operation during the sine wave when using AC power and avoids large voltage spikes.
A problem associated with SCRs is that they are more likely to fail shorted (or closed) rather than open, as is the case of EM relays. As a ‘fail-open’ condition is deemed to be safer, EM relays are often preferred for certain applications.
