Well, neither of these devices directly monitor current to
earth.
The old "Voltage Operated Earth Leakage Circuit Breakers"
monitor the voltage between a dedicated ground rod, and
the earth conductors of the installation. They are required
to trip before this reaches 50V (on a 240V supply), although
the tripping voltage was normally significantly lower than
this. The reason for such a voltage appearing would be that
a current is flowing to earth which is generating a significant
voltage drop across the installation's ground rod resistance
(due either to excessive current flow or a poor ground rod).
We refer to as a high earth fault loop impedance (other
faults can give rise to high earth fault loop impedance too).
The "Voltage Operated Earth Leakage Circuit Breaker" (VOELCB)
is designed to detect this condition and disconnect the supply,
as the overcurrent protective device may not be seeing enough
current to trip it quickly enough, or even at all. Note that
there do not need to be ground connections in a VOELCB
installation other than the dedicated VOELCB ground connection,
in which case it will trip at quite low leakage currents as
the earth rod resistance is effectively increased by the
resistance of the the VOELCB's sense coil. On the other hand,
you can have as many additional ground connections as you like,
and if you have enough to generate a low total earth rod resistance,
then the installation's earthing conductors may never reach 50V
above real ground, and the VOELCB has no need to trip as the
overcurrent device will do so instead. These devices are now
much misunderstood nowadays, as electricians haven't been
taught about them for some time, but they are still in service,
although never newly fitted anymore.
This is the reason why the voltage operated, earth leakage unit is no longer
recommended in the UK, since it requires a functional earth to be present,
an RCD (generic term) doesn't.
Also, the VOELCB is not designed to protect against
electrocution, only too high a ground rod impedance which
might prevent the overcurrent protective device tripping on
a short circuit to earth. The RCD does this, but in addition
it provides protection against electrocution. It is also
much easier to install, without having to install a dedicated
ground rod and ensuring it doesn't have an overlapping earth
resistance area with any other ground connection, including
things like water/gas service pipes.