
Woodward
Manual MRI3 GB
8
DOK-TD-MRI3 Rev.A
Figure 3.3: Phase current measuring and earth-current detection by means of Holmgreen-circuit.
This connection can be used with three existing phase current transformers when combined phase
and earth-current measuring is required.
Disadvantage of holmgreen-circuit:
At saturation of one or more C.Ts the relay detects seeming an earth current.
* This arrow shows the current flow in forward direction, for this LED
→←
lights up green
Voltage measuring for the directional detection:
Figure 3.4: Measuring of the phase voltages for the directional detection at overcurrent, short-circuit or earth-fault
protection (I>, I>>, IE> and IE>>).
For details on the connection of ER/XR-unit type c.t.s, see para 4.5.