Cenvat credit and pla

This query is : Resolved

rahul (Querist)
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16 June 2012 Dear Sir,
Kindly see my following example:

There is start of plant & there is 'No' opening Cenvat Credit.

Excise duty of inputs purchased- 3,00,000
Excise duty of Sale during month-5,00,000

Is I am saying right that , now we have Cenvat credit of Rs. 2,00,000.

Now, Suppose every month , the equation is such that , excise duty of sales is never greater than (excise duty of purchase + opening credit).
Will one have to pay any excise duty during whole year. as we have cenvat credit for whole year.
If, Yes . why. as we have credit
If,No. why. as I have seen that some pay duty in PLA although they have credit.they do like following, let's Suppose :
opening 50,000
ed of purchases 3,00,000
------------
3,50,000
ed of sales 3,00,000
-----------
50,000

The ed of sales entered above is by substracting ed on sales of whole month say rs. 500000-excise duty paid for month through PLA RS. 2,00,000.
I want to know why they paid rs. 200000 through PLA . how the figure is arrived at?
I think it should be like this.
opening 50,000 + ed purchase 3,00,000 - ed on sales rs. 5,00,000= Rs. 1,50,000 (Net payable)
How this Pla came into and there is still rs. 50,000 credit in my former example.
Kindly give the reply as soon as possible as these concepts are very urgent for me.