MorganS:
Relays are stressed when they are switched. The worst case is switching OFF a DC load with high inductance. The inductor wants to keep the current flowing and it will cause a spark to jump across the relay contacts while they are opening.That's why relays rated for 250V AC can only switch 32V DC. The AC voltage will go to zero in a 50th or 60th of a second, extinguishing the spark.
Leaving the relay on all day is best for it. An SSR in the same duty cycle will stay hot all day and that's why it needs a heatsink.
The relays on the cheap boards are probably OK for 6-7A. However the PCBs may not be designed with that in mind and they are almost invariably not designed for mains safety. Look for something that has a very clear high-voltage area delineated on the PCB and nothing from the LV side of the board crosses into that area. Also look for strong connections that will withstand mistakes like you dropping the object and grabbing the power cord to stop it hitting the ground. (You should also have good strain relief at the cord-entry point on the box.)
Hello MorganS,thank you for your replay.I was fearing the same...the terminals will melt,the copper tracers on the PCB to small etc etc.
What about this 12V relay?it still looks a bit flimsy to me..if i look closely inside..the hole 1500 Watts is passed inside the relay via that small thin flimsy white wire..of course you can connect the 1500 watts line across the other 2 terminals but i'm still not sure is suitable for passing 1500 watts 20 hours/day.
Sabin