The "usage limit exceeded" message, see below, is received when a safety measure is triggered in order to prevent excessive downloading that may be in violation of our licenses for various databases. The proxy server will block the user and automatically expire after 1 hour.
There is a known issue with ProQuest Congressional: downloading a volume more or less exhausts all of a user's download “ability”. So users will generally have to wait 1 hour to download another volume.
In Ebook Central, one may receive a "Download limit reached" message after downloading the permitted number of pages that day. The page count will usually, but not always, reset after 24 hours. These limits on downloads are in place as part of digital rights management (DRM) and are only enforced by ProQuest, not determined by ProQuest.