My ryobie bench grinder keeps tripping the rcd within a second of turning it on?

Assuming your local tech confirms that your bench grinder tests ok. The problem is probably that the grinder draws too much current for the RCD and is therefore detected as an earth leakage. High amperage devices often do that. Your best bet would be to plug it in to a socket which is on a non-rcd circuit. Maybe you have an RCD on the whole house though? I would start with returning the grinder and trying a different brand with a lower amperage/wattage rating. If that doesnt work, I would call an electrician to discuss installing a different RCD or running a separate circuit for your power tools.