What exactly is the difference between the guard and the shield connection for insulation measurement devices?
Regarding: METRISO PRIME (+), METRISO C, METRISO 5000A, METRISO 5000D-PI The shield is a shield for the sensitive COM input of the meter to shield capacitive interference that may result in an unstable display. The higher the impedance, the more the shield will work. Some insulation measuring instruments have permanently connected, shielded cables (METRISO 5000A) and therefore do not require a shield connection. The METRISO C has pluggable cables and therefore a shield connection. There is the extra shielded cable with the double plug.
With the guard parasitic surface currents, which are caused by the measuring voltage, are sucked off, so that these currents, which represent the surface resistance, do not enter into the result, if one only wants to measure the contact resistance. Only the current that flows through the DUT reaches the COM input, the surface currents are blocked by suitable ring electrodes around the + connection of the Gard connection. The guard connection has the same potential as the COM connection, but is metrologically separated from the COM input.