The voltage inputs (including the current measurement
input if the current transducer is of current–to–voltage type)
must be of high impedance in order not to affect the
measurement. This depends on the output impedance and the
impedance of the leads that can be long in a HVDC
application, where the transducers may be placed in a large
switchyard while the meter is placed in a nearby building.
Some voltage transducers might have medium output
impedance rather than low, therefore it is preferable to have an
input impedance in the range of GΩ rather than in the range of
MΩ that is usual for AC meters. An instrumentation amplifier
type of input is therefore preferable.
F. Channel isolation
For AC meters, the current and voltage channels are most
often isolated by means of an input transformer on the current
input. This is not possible for the case of DC meters. The
output of the current transducer will be isolated from the high
voltage circuit and only locally earthed in the meter area. For
the voltage divider there is a more complex situation, where
one extreme is that the earth of the low voltage circuit is
situated in the switchyard and the other is that the divider low
voltage arm is removed to be placed in the control room. The
latter alternative is by far the more attractive one for the
metering application and this analysis is based on it. The input
circuit of the electricity meter can therefore be designed with
differential inputs.