Protection for Ungrounded System

Maybe I hasn't understand your meaning totally.

Firstly, which potentail grade are you using now?

If it is low as 380V ,we still use thermal relay to deal overload and short circuit, residual current device to ground fault.

If it is higher voltage , now we don't use in protector in break, but use computer protect system, which it to collect signals of current and potential then calculation,to check whether there are fault in circuit.

You can add overcurrent protection to your scheme as an earth fault condition is a form of overcurrent as well because it contains positive and negative sequence components with the zero sequence current that will flow. The fault current will therefore be I1+I2+I0 which will be considerably more than normal load current. With carefull calculation and with the right type of relay you will be able to distinguish between phase and earth faults.

Try reading the Wikipedia article on earthing systems.

It sounds to me AGAIN as a basic misunderstanding. Since the original info is so sketchy, let assume delta connected motors. Well, they are protected according to the code. And the only grounding is in the main power panel, again according to the code.

And the code is updated every decade or so. If you are grandfathered, it may or may not be the wisest thing to stick with it..

On the other hand, safety ground from the housing of the tools has to be totally separate, local, and is not expected to carry much current. IF, safety device is in this circuit, it has to trigger at 10 - 20Milliamps depending on local code.

If the system is completely ungrounded, there will not be any ground fault current as there is no path for the same. In this case ground impedance will be infinity. Recall that ground fault currents are minimized by having neutral impedance. A ungrounded system is equivalent to having an infinite neutral impedance.

Earth fault protection will not be necessary , unless there is a possiblity of two simultaneous earth faults to form a closed loop, through the charged circuits.

protection against phase faults, and unbalances will be needed for circuits. Thermal protection for the motors will be needed measuring phase currents and negative sequence currents.

i find your answer is very related to the question i posted..lastly, do you have any recommendation on the best device to protect our system...

I have not worked with sensitive earth fault protection before. May be the following links will be useful to you

http://www.startco.ca/library/papers/Ground-Fault%20Paper.pdf

What protection are you talking about? Over current or short circuit or earth fault?