Siemens SENTRON - Page 49

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Types AC, A and F residual current protective devices are unable to offer protection
in these cases. The device does not trip because the DC residual current does
not cause any change over time in the transformer induction of the RCCB which
operates according to the induction principle. A smooth DC residual current (or
leakage current) due to a creeping insulation fault leads to pre-magnetization of
the transformer material of Type AC, A and F residual current protective devices.
Figure 20 shows the difference between a transformer signal without and with DC
residual current superimposition. Without DC residual current (I
ΔDC
), an AC residual
current (I
ΔDC
) causes modulation on the eld strength axis H of magnitude I. In
accordance with characteristic magnetization curve M of the transformer a voltage
of magnitude II is induced. A DC residual current (I
ΔDC
) owing via the residual
current protective device shifts the working point of the transformer on the H axis.
An AC residual current (I
ΔDC
) with the same value as in the case without DC residual
current causes modulation of the same magnitude on the eld strength axis H
of magnitude III. Although the change III has the same value as I, a signicantly
smaller voltage of magnitude IV is induced in the transformer. If it is assumed that
a signal of size II is necessary for tripping, it becomes clear that the considerably
smaller signal IV is not sufcient for this.
Only much higher AC residual currents would lead to attainment of the necessary
signal level. This shows that Type AC, A or F residual current protective devices are
no longer able to trip if a purely sinusoidal residual current, which could otherwise
be tripped without any problems, occurs simultaneously. The desired protective
function of the residual current protective device is therefore no longer guaranteed.
B
H
IV
II
I
III
M
Figure 20: Pre-magnetization due to DC residual current
I
Δ
= I
ΔAC
I
Δ
= I
ΔDC
+ I
ΔAC
I Modulation with AC residual current I
ΔAC
II Tripping signal generated by I
ΔAC
III Superimposition of DC residual current
I
ΔDC
with AC residual current I
ΔAC
IV Tripping signal on superimposition of
DC residual current I
ΔDC
with AC residual
current I
ΔAC
I
Δ
total residual current
B Inductance
H Field strength
M Magnetization characteristic of the
transformer