Limit of the voltage and current of my electronic product for CE certification
When it comes to CE marking it all depends on which directives your product falls under. 

The Low Voltage Directive 2014/35/EU applies to all products which operate between 50 - 1000 VAC and 75 - 1500 VDC.
In your case, as your device is only operating from 30V you shouldn't have to conform to the LVD.

The EMC Directive 2014/30/EU applies to pretty much all electronic devices today and therefore you will have to make sure your device complies.
"The Directive applies to products liable to generate electromagnetic disturbance, or the performance of which is liable to be affected by such disturbance, and to fixed installations."

The RoHS Directive 2011/65/EU affects all electrical and electronic devices and is to limit the hazardous materials used in manufacture as to prevent environmental hazards upon disposal. As long as all your components are RoHS compliant and the PCB manufacturer makes the boards to RoHS compliance then you should be fine. Just remember to get RoHS certificates for every device and process and to keep them stored in case you need them. 

The WEEE Directive 2012/19/EU is not strictly a CE directive, and I'm not sure of the legal implications but I'd recommend looking into it as it concerns how your customers can safely dispose of your device and minimise damage to the environment.

I've just listed the directives which I usually deal with and is true for most electronic devices, however you can find all the CE directives here. Depending on what your device is some more may apply to you. Only once you have gone through the appropriate directives and have sufficient evidence can you then CE mark your product.