Hi, daros
I think you are confused about Used Margin and Usable Margin.
The example in the Babypips School is based on a ridiculously large position — 80 standard lots. In other words, the position size is [B]8 million EUR.[/B] Also, the example ignores the Spread. And it assumes that Maintenance Margin = Initial Margin (more on that in a moment)
[B]Let’s use the same numbers as in the example, so as not to add more confusion at this point.[/B]
After the 80-standard-lot position was opened, Balance was $10,000, Equity was $10,000, Used Margin was $8,000, and Usable Margin was $2,000. Let’s see where those numbers came from.
[B]Balance[/B] refers to the funds which were in his account [B]before any of his open positions were opened.[/B] So, prior to this 80-lot trade, his Balance was $10,000. And, [B]until his position is closed,[/B] his Balance will remain at $10,000.
[B]Equity[/B] refers to what his Balance would be, if all his open positions were closed right now. Equity does not take Margin into account, and in this example the Spread is being ignored. Equity does go up or down, as open positions show profits or losses. But, in the example, there has been no profit or loss in the 80-lot position, yet. So, Equity is still $10,000.
Note that profit or loss on open positions is called Open P/L (or Floating P/L). When those positions are closed, Open P/L becomes simply P/L (or Booked P/L).
[B]Margin[/B] refers to a portion of his funds which his broker freezes (impounds, sets aside, or places in escrow) as a sort of performance bond. The trader will not be able to access that Margin amount for any purpose, for the duration of his trade. When his position is closed, that Margin amount will be un-frozen, and the trader may use it (or withdraw it) as he sees fit.
In the example, the 80-lot position required a Margin amount of $8,000. And, because the trader was using this amount of Margin, it is called [B]Used Margin.[/B] That sum is frozen, and he can’t draw on it to cover any losses which might occur in his position. Therefore, the only funds available for covering losses would be the $2,000 which is not frozen (not committed to Used Margin). This $2,000 portion of his account is referred to [B]Usable Margin.[/B]
When losses had consumed all of the $2,000 of Usable Margin, there was no more Usable Margin left — and the very next pip of loss triggered a [B]Margin Call.[/B]
[B]Just before the Margin Call,[/B] the numbers in this example were: Balance = $10,000, Equity = $8,000 (because $2,000 had been consumed by losses), Used Margin = $8,000 (this figure hasn’t changed), and Usable Margin = 0 (because it was all consumed by losses).
[B]Immediately after the Margin Call,[/B] the numbers would be: Balance = $8,000, Equity = $8,000, Used Margin = 0, and Usable Margin = $8,000.
Note: There is an error in the School example at this point.
The School example shows Usable Margin = 0, after the Margin Call, and this is incorrect.
The correct figure is $8,000, as above.
[B]Let’s distinguish between Initial Margin and Maintenance Margin.[/B]
In the example above, the trader was allowed to use all of his Usable Margin to cover his losses, but he was not allowed to use any of his Used Margin to cover losses. When all of his Usable Margin had been consumed, he got a Margin Call.
Some brokers allow a portion of Used Margin to be used “again” to cover losses. In your post, you referred to the Oanda policy: Maintenance Margin = 50% of Used Margin. Let’s see how this would affect the School example, above.
With 50% Maintenance Margin, the trader would have to commit $8,000 Used Margin in order to open his position, but he would not have received a Margin Call after $2,000 of losses. Instead, he would be allowed to hold his position for an additional $4,000 of losses (50% of the Initial Margin amount of $8,000).
If he did this, then the numbers would be as follows:
[B]Just before the Margin Call,[/B] the numbers would be: Balance = $10,000, Equity = $4,000 (which is his Balance minus $6,000 total losses), Used Margin = $4,000 (this is the Maintenance Margin amount), and Usable Margin = 0.
[B]Immediately after the Margin Call,[/B] the numbers would be: Balance = $4,000, Equity = $4,000, Used Margin = 0, and Usable Margin = $4,000.
I hope that helps to clear things up for you.