Description
2.8 Electronic trip unit ETU
3VA27 molded case circuit breakers & 3WL10 air circuit breakers
60
Manual, 10/2018, L1V30499596002-01
Ground-fault protection (GF)
The GF release detects residual currents between the conductors and grounded, electrically
conductive parts of the installation. The ground-fault protection function responds if the
ground fault current exceeds the set tripping current I
g
for the set delay time t
g
.
Ground-fault protection can be implemented as a definite-time (t
g
= constant) and as an
inverse-time function (I
2
t).
On 4-pole and 3-pole circuit breakers with an external current transformer for the neutral
conductor, the GF release calculates the vector sum of the currents of the three phases and
the neutral conductor. The difference between this sum and zero describes the ground fault
and tripping occurs, defined by the setting current I
g
and the delay time t
g
. The possible
responses are to disable the protection or just to output alarms.
Ground fault residual current protection (Rc)
Residual currents from 3 A to 30 A can be measured directly with an external summation
current transformer and evaluated by the ETU660 (LSIG), which is additionally equipped with
a rating plug Rc for residual current protection and the metering function MF Advanced.
Direct measurement of ground faults with a summation current transformer is an alternative
to the calculated ground-fault protection method of the GF protection function.
Ground fault return (G
ret
)
Currents in the grounding conductor of the transformer star point can be directly measured
with high accuracy using an external toroidal-core transformer and evaluated with the
ETU660 (LSIG). Direct measurement of ground faults with a toroidal-core transformer is an
alternative to the vector sum calculation of the GF protection function. The ETU requires an
external power supply for the function (e.g. Breaker Connect module or MF Advanced
metering module)
Monitoring of the switch-on phase (start-up inrush adaptation)
Using this function it is possible to individually adapt the operating value of the protection
functions ST, INST, and GF toward higher thresholds over a certain time and thus to make
them less sensitive. During the switch-on and starting phase, for example, of motors with
high inrush currents or during heavy starting, and also for transformers or lamps, this can
prevent undesired false tripping of the circuit breaker.
The duration in which this monitoring and the alternative, individually set values for ST,
INST, and GF are active can vary between 100 ms and 30 s. After this, the parameters are
reset to their default protection settings defined for normal operation.
In a self-powered ETU, monitoring of the switch-on phase and activation of the alternative
protection parameters are started every time the circuit breaker closes.
In an externally powered ETU, you define an activation threshold between 0.1 and 10 x I
n
.
Startup is possible again whenever the peak value of the phase with the largest current falls
below this threshold. If this threshold is then crossed as the current rises, the new monitoring
is automatically started again.
