precautions for residual magnetism

How large of a generator are you dealing with? Arcing can be slightly suppressed by placing a capacitor across contacts, but be aware that the capacitor must have the voltage rating and sufficient capacity to do any good.

yes u r correct.. these generators supply over 1000volts....there are many important machines in ships which are to be functioning 24/7....any power failuer to these machines may lead to great trouble.....(ship's navigation,pumps which lubricate main engine, oily water seperator,etc...).so the precautions for these generators are very important....

"Yes, u r correct" means to me that the 1000+ volts generators are AC generators (otherwise known as "alternators,") as I assumed.

Residual magnetism in such machines should actually never occur due to the fact that, once started after a shut-down, the magnetic fields alternate from one polarity to the other in equal strengths. If the field windings are energized prior to the start of the machine, then the field can polarize the iron in the armature (field and armature are used merely as a point of reference even though the field may actually be the rotating part, but bear with me . . .), so that condition should be avoided. In the case where the field is made up with permanent magnets, such cannot be avoided, and it is common practice to maintain relative rotation between the field and armature. In cases where the rotating part needs to be worked on, maintaining relative rotation between the two is impossible, which is the only condition under which the residual magnetism may need to be eliminated.

The only time a transformer "cares" whether or not the generator's output is offset above or below zero is when the transformer is grounded to the alternator, otherwise, it doesn't even notice. However, connecting the alternator's and transformer's neutrals is common practice.

Nevertheless, placing a "dummy" load on the output of the alternator will soon reestablish a nonpolar magnetism (eliminate residual magnetism) in the iron, but the dummy load must cause enough alternating, magnetizing current to flow to overwhelm any residual magnetism caused by permanent magnets or by energizing the field before rotation begins.

There's a lot more to running primary power plants than what I have put in this post, but I really can't tell, by your posts or comments, how much you know about the subject. I await your reply.