It is clearly pointed out that the frequently used series system formula of average failure rate involves single and all multiple faults. For radial and “closed-loop design

opened-loop operation” medium voltage distribution systems

the multi-faults related to the reliability indices of a load point are divided into series and parallel ones. For series multi-faults

the precise formula of annual outage duration for series system has been derived with single and multiple faults being considered

which shows that commonly used formula contains only the first-order term of the precise formula; the k-order items of the precise formula cannot correspond to the annual outage duration caused by k-order faults. In addition

the effects of parallel multi-faults on the annual outage duration of a load point are analyzed

and some related formulae are obtained. Examples using typical distribution networks and reliability parameters have verified that the error caused by commonly used annual outage duration formula of series system is small and parallel multi-faults generally have little impact on the annual outage duration of a load point.