Typically, battery ground voltage (without fault) should sit midway between battery positive and negative. The Vigilant checks the ground potential relative to the battery mid-point and will detect an imbalance if the ground potential is not mid-battery voltage. The closer the ground fault to battery positive (or negative) the signal will have a higher ground leakage current. When measuring for a ground fault the Vigilant is looking for a nominal difference of 9% (not adjustable on present system). This to say, on a 120V system with the battery mid-point at 60V, if the ground voltage is below 54.6V or above 65.4V then a ground fault warning will be raised. Be aware that some systems have one battery terminal intentionally grounded (usually positive on telecommunication systems). If this is the case then the small green wire loop in the monitor should be removed, or, do not ground the monitor chassis. This will prevent the monitor recording a ground fault.