Is it fruitful to work as maintenance engineer ( Electrical/Mechanical) for an Electrical Engineer ? will there any harm effect on career or not ?

On short terms basis there is no harm but on long terms basis yes it may harm an Electrical Engineer.
Maintenance is good field and every engineer, specially Mechanical Engineer should work in it as you get lot of experience regarding quality of works performed by Construction team and quality of materials used.
This gives more knowledge and experience to you as a Construction Engineer for later projects in your life.
Moreover, maintenance, if treated as a speciality is very rewarding if you embark upon your own business in this field.
Not many engineers touch upon this line and there is always a growing demand for renovation type of works in the third world.

of course it is fruitful specially in young age where you can learn a lot and have a wide experience specially in the equipments normally out of your scope , such experience will give you a great push in the future in mid level and management scopes.

Maintenance section of any industry is really fruitful.
You will be exposed to all electromechanical systems and be able to determine system faults, flaws, problems prior to happening.
Also, it can develop to top management, large facility management as well as operational managment.
The industry is stable and always needed not like construction or contracting..

Maintenance ,It's also an important part in the industry.If you want to move into Oil & Gas Sector you should have experience in the same field.You might have earned a very good experience in the maintenance field. Wherever you work you will have something new to learn.So experience makes one more professional and skilled,it won't harm your career.If you want to get an opening in oil & Gas sector ,apart from these experiences you have to do some certification course related with Oil & Gas sector which can boost your career goal.

I am very happy to see replies from you people and appreciated your point of view. i have moved now healthcare sector for broad experience after electrical and mechanical job and doing M.Phil in project management. please suggest that which is the best high salary industrial sector for project management after electrical engineering and4 year experience in maintenance and healthcare sector ?

Any areas of Electrical Engineering have some hazardrous effect but with strong adherence to safety the job is good and enjoyable.

When you love your work and you will give your BEST on it, regardless of maintenance or construction fields, you will grow your career on the top of it.