30V 10A relay with 48V 6A

Can i use 48volt 6A power with a relay that have 30vDC and 10a?

This would be the maximum load the relay's contacts are designed to handle. Although the current is within spec, the voltage to switch is too high. Under these conditions, the contacts will typically weld closed and have exponentially reduced life expectancy. Physically, the contact size is OK but the gap/clearance and/or contact material is incorrect for the application.

relays have multiple ratings.

on the coil side, there is a voltage rating. Often these are rated for one voltage but they can be used with other voltages. after all, a coil is all that is being driven.

as for the contacts, there is a DC rating, an AC rating, resistive load rating, an inductive load rating as well as a hp rating.

best thing would be to post the data sheet and also list the device you want to control. Please indicate if the load is AC or DC.

http://www.ebay.com/itm/5V-4-Channel-Relay-Module-with-Optocoupler-for-Arduino-PIC-ARM-DSP-AVR-MSP430-/310823196671

This is the relay, i can find only this type already assembled for Arduino. I'll use it with a coil that have 48v and 6A.

If i use two relay with same power supply to half current on each relay with in same time activation? Should work? Or i will burn one relay and after the other?

Example: 14A 24v with 2 relay, 7A on each.

Is your load DC? Do you need the relay to open while the load is drawing full power? If the answer is "no" to either of the above, the relay is probably going to be OK, otherwise it's insufficient and will risk welding closed due to an inability to break the load arc (the contacts don't move far enough apart).

Your problem is supply voltage not load current, so putting them in parallel will not help. You need to put them in series; that will help a lot because you now have twice the contact spacing available. It is only a delaying tactic though because any slight timing error between the relays (e.g. one has a tiny bit stronger spring) means that one relay will take most of the arc energy. It may not be as prone to welding closed, but the contacts will erode quickly (which increases contact resistance...) and if one relay does weld closed, you probably won't notice because the other one will continue working for a little while.

If your load is inductive (e.g. a motor), the relay is in NO WAY sufficient. With inductive loads, you need to design a snubber circuit to go across the relay, compute the voltage peak that the snubber produces with the load in question, and then choose your relay ratings from there. Otherwise it will weld closed very quickly.