 Any situation in which the source to be measured has a very high output impedance is vulnerable to leakage current and if sufficient insulation is not practical then a driven shield will improve performance.Coaxial cable is well suited for use as a guard; if electromagnetic shielding is also required then triaxial cable should be used as depending on the type of buffer circuit any noise on the guard may be amplified in the output.The limiting factor for this method is the input impedance of the voltage buffer, the JFET or CMOS op-amps typically used may have input impedances of many teraohms which is sufficient for most applications.