IEC61850 specifies the communication architecture for the SAS (substation automation system). It brings the interoperation to the IEDs from different vendors. The impact of IEC61850 on substation is not only the communication but also substation design. It brings challenges to the secondary circuit and network design. The traditional description method of secondary circuit of SAS cannot meet the new requirements. A new method based the virtual contact is provided in this paper. A more detail considerations about network design are given in this paper.