I have recently bought one of your (very nicely built) relay shields.
I’m not new to mech relays but I am to Solid State Relays (SSR) and I have a question, are they actually SSRs?
Your website says they are solid photo-coupled relays http://www.omega.com/temperature/z/pdf/z124-127.pdf (some rather dull info on SSR types) but the somewhat hard to find data-sheet on the relays you have used on your shield suggests they are mechanical http://img.smparts.com/pdf/relay_tianbo_hjr4102.pdf .
UPDATE: I see there is a Photocoupler on-board but this is merely for switching the mech relays. I think the product description “Seeed Relay Shield is a smart module with 4 solid relays” is rather misleading.