Hello, There is grounding issue i cant understand.
we connect in series the two seurces.
but why our ground has to be connected from one side to plus and the other ground is connected to the minus?
what will happen if i connect the ground of the left side to minus and the right ground to minus too?
What will be the equivalent in this case?
Thanks.

 

First of all, usually there is no need to connect to "Ground", unless an application requires it. And given that the application is not mentioned there is no reason at all to connect anything to "ground".
So unless an application requires a common side of the DC supply to be tied to the safety "green-wire Ground" there is usually no benefit at all.
And unless that power supply is much different from normal, there is no power delivered to the circuit connected on the left, because there is no connection to the positive side of that supply.

 

Because you cannot connect the ground to different voltages.

When two supplies are in series, the positive is of one is connected to the negative of the other. To do this, the two supplies must be isolated, i.e. have no common connection.

If you want a plus and minus supply, the common is be the junction of the two supplies. You may optionally connect this to ground if you know that ground is not going to be connected anywhere else in the circuit.

 

A power supply as the one shown has three isolated outputs. These do not share any common connection, and so if any portion of the external circuit being powered requires a connection to that "green wire safety ground" , which ever supply connections to be grounded can be connected to it. My usual choice is to not connect any of them, because no benefit is provided by making the connection.

 

Are the two red plugs in your picture opposite ends of the same wire? If not, what are those two wires connected to?

There is no need to connect the two earth grounds (the two green plugs) together, as these are hard tied within the supply and almost certainly hard tied to the "safety ground" of the power plug (the round third prong in the U.S.).

For many/most applications, you don't need to connect the earth ground to anything and can let your supplies float, though sometimes there are reasons why you want/need your circuit to be earth-referenced.

If you do earth-reference your system, you want to do it at one point and one point only. Otherwise you risk ground loops and/or shorting out one of your supplies.

To put your supplies in series, you connect the positive of one of them to the negative of the other.