Hello,

Currency amount can be consist of different currency codes with different currency rates. That is very possible that in one currency code you have positive value in currency and balance amount, the other currency code negative value in currency and balance amount. But sum of those 2 is positive in one column and negative in another column.

You can dig more reviewing balances split by currency code. Also consider currency revaluation for the account.

Hope this helps

In this case I have a specific currency code in one line like USD in Curr Bal column and in currency amount it is positive but in balance column it is negative and in average rate column I have irrational number because it is calculated based on this formula “Balance/currency amount”

In this situation when I execute Currency Revaluation I have a large amount of gain in this Cash Account code part misc that I have defined my banks in it.

Hi,
Well, if currency rate is fluctuating, it is possible. 
E.G. my example from Poland. In September within few days currency rate EUR/PLN has changed from 4.40 to 4.60. We have received e.g. 10 000 EUR from customer one day valuated in PLN as 44 000. 
Few days later we have paid 9800 EUR to our supplier, but we valuated it as PLN 45 800. At the end of the day we owned 200 EUR, valuated as debt of 1800 PLN. 

Currency revaluation process is dealing with such situations - valuates your closing position in foreign currencies at closing rate. 
You must dig into transaction details to identify the reason of such balance. It is still possible that you have transaction accounted at extraordinarily high or low rate manually entered, which is ruining your accounting balance position. But it might be normal as well.