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. care must also be taken to ensure there are no unexpected paths by which leakage current may be encountered as this will defeat the system, and extra care must be taken in the design of the amplifier/buffer circuit to prevent oscillation as the guard, especially if it is used over a coaxial cable, may have a strong capacitive coupling to the input.