One of the biggest tech support questions and problems we have seen with all relay controllers (not just USB) is the unusual expectation that relays make a 0 Ohm connection between the Common and Normally Open or Normally Closed contact.  This is a huge misconception, which accounts for about 10 to 15 of our returns per year.  The real measurement between Common and Normally Open connections has been measured as high as 150 Ohms on a high-quality mechanical relay!

The reason for the unusually high resistance is the simple fact that high-quality power relays, when new, include a anti-corrosive coating on their contacts.  This coating burns off after a few on/off cycles of high current flow through the contacts.  After this coating is burned off, the relay will drop to a 0 Ohm reading on your meter.

It should be stated this would never be a problem, except high-power relays are often chosen because of their low cost, and not for their specialty, which is high power switching.  Power relays will frequently corrupt low-power signals, such as audio, video, or other types of high-speed logic.  The contacts of a power relay are significantly different than the contacts of a signal relay.  Signal relays should be chosen for low-power signals.  Power relays should be chosen for power switching applications.  High-current relays should be chosen for high-power loads.  Choosing the correct relay is critical to the reliability of the controller and the integrity of the switched signal.