Technical description
7 Application
7.1 Trip circuit supervision
VM55.EN005
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7.1.2.
Trip circuit supervision with two digital inputs
The benefits of this scheme is that no external resistor is
needed.
The drawbacks are, that two digital inputs from two separate
groups are needed and two extra wires from the relay to the CB
compartment is needed. Additionally the minimum allowed
auxiliary voltage is 48 Vdc, which is more than twice the
threshold voltage of the dry digital input, because when the CB
is in open position, the two digital inputs are in series.
The first digital input is connected parallel with the
auxiliary contact of the open coil of the circuit breaker.
Another auxiliary contact is connected in series with the
circuitry of the first digital input. This makes it possible to
supervise also the auxiliary contact in the trip circuit.
The second digital input is connected in parallel with the
trip contacts.
Both inputs are configured as normal closed (NC).
The user‟s programmable logic is used to combine the digital
input signals with an AND port. The delay is configured
longer than maximum fault time to inhibit any superfluous
trip circuit fault alarm when the trip contact is closed.
The output from the logic is connected to a relay in the
output matrix giving out any trip circuit alarm.
The trip relay should be configured as non-latched.
Otherwise, a superfluous trip circuit fault alarm will follow
after the trip contact operates, and the relay remains closed
because of latching.
Both digital inputs must have their own common potential.
