1. Configure the settings for the Excel Services unattended service account.

The unattended service account is a low-permissions account that Excel Calculation Services can impersonate when establishing a data connection that uses single sign-on (SSO) credentials from an environment that is not Windows-based, or None, as the authentication method.

Note This procedure is a requirement only for servers that do not have Kerberos enabled. If you have Kerberos enabled, you can skip this section and proceed to Configure settings in Excel Services Trusted File Locations.

2. Configure the settings in Excel Services Trusted File Locations

Trusted file locations manage who can act as an author for workbooks that are available under Excel Services. The trusted locations can be designated as SharePoint locations, UNC paths, or HTTP paths. Excel Services loads workbooks only from directory paths that the administrator designates.

3. Configure the settings in Excel Services Trusted Data Connection Libraries

Trusted data connection libraries are SharePoint document libraries that contain Office data connection (.odc) files. The .odc files are used to centrally manage connections to external data sources.

This section gives instructions to configure so that Excel Services can be used as a data source for Excel Services…

These might point you in the right direction – if not drop me a line and we can chat about it.

Can u give some input on this scenario:

I have uploaded excel file with automatic refresh and its used trusted data connection placed on sharepoint. When I from excel services menu choose open to look at data in excel it says data connections is not allowed and the users have to allow these – why is this extra action necessary when everything on sharepoint is trusted ?

Its not an option to change to allow all data connections.