Maximum current / voltage to the ground.

This might has been asked for millions of times but i couldnt find it.

I know that the maximum voltage to the IO pin is 5,5 V, maximum current is 40 mA, does this apply to the ground too? Is There A way to extend this limit?

The way to extend the limit is to use a switching transistor / opto isolator / relay etc. to switch larger loads.

There are many ways depending on the load type and switching speeds so probably your best bet is google.

So 5V is the max?

Yup unless you use something I mentioned above.

There are many ways of extending it and its up to you to choose the correct one for the type of load you want to switch. Most people start of with a simple relay or switching transistor as they are easy to obtain and simple to wire up. Ebay is full of devices for the Arduino to switch higher loads at very good prices.

Ok. Switching tranzistors.
SO is this kind of setup is safe to use ?? :o :o :o
http://bildr.org/blog/wp-content/uploads/2012/03/rfp30n06le-arduino-motor.png

i gues not. If i suply e.g. 20V and start switching (pwm) with tranzistor so it would "drop" down for e.g. 16V. Its still too much for a ground pin.

Without knowing exactly what you want to do I am just poking around in the dark.
I don't mind giving my time to anyone but I can see this is going to be an uphill struggle.

Take a look at the many examples you can find via google and pick out the one that is best suited for your particular needs (whatever they may be ????)

If its a motor you want to drive then google "Arduino motor" and dig around till you find a project with similar characteristics. Chances are if its Arduino driven its already been done.

Disclaimer: not quite a hardware person anymore.

laisut:
Ok. Switching tranzistors.
SO is this kind of setup is safe to use ?? :o :o :o
http://bildr.org/blog/wp-content/uploads/2012/03/rfp30n06le-arduino-motor.png

i gues not. If i suply e.g. 20V and start switching (pwm) with tranzistor so it would "drop" down for e.g. 16V. Its still too much for a ground pin.

I think the schematic is missing a gate resistor to the Arduino pin. Further it is OK as far as I know. See e.g. the schematic attached to this post

There is a maximum current of 200mA for the ground and Vcc pins; there is also a maximum current per port (8 pins; consult the datasheet).

Current flows back to the source. So the current from the Arduino pin flows back to the Arduino power supply and the current through the motor flows back to the motor's power supply.

The sensible limit is GND-0.3V, Vcc+0.3V, ie use schottky protection diodes if you have
to clamp the voltage.

If less than a fraction of a milliamp is all you have to deal with the built-in protection diodes
can take that. (ie a 10k series resistor can be a very simple precaution)

If you push large currents through the internal protection diodes you risk damaging them
(obvious), but also you risk latching-up the entire chip (bad bad news). Google "CMOS latch-up"

laisut:
So 5V is the max?

NO. It's 0.5volt above VCC, and 0.5volt below ground.

If the Arduino is OFF, max voltage on a pin is 0.5volt.

That's why 5volt zener diodes never fully protect.
And schottky diodes only protect if you limit current.
An 1N5819 has a Vf of 0.6volt at 1Amp.
Dumping over-voltage on VCC could also raise VCC above the safe limits of the whole board.
Leo..

This might has been asked for millions of times but i couldnt find it.

Not suppressing you can't find it, this is a none question, it is based on a fundamental misunderstanding of how electronics works. So to address your problem we have to find out what you are not understanding. The best way to do this is to describe what you want to do exactly and how you propose to do it.