10
TGH 1360E
The insulation resistances RF+ and RF- can be calculated according to the
following equations:
•
for symmetrical coupling, i.e R1=R2 and Rm1=Rm2
Principle of operation
(U
N
x Rm1 - (U1+U2) x R2)
R
E+
=
U2
•
for asymmetrical coupling, i.e. relation AK+ / AK- = 2:1
(U
N
x Rm2 - (U1+U2) x R1)
R
E-
=
U1
R
E+
=
(U
N
x Rm1 x (U
N
x Rm2 - R2 x (U1 + U2)))
(U2 x (U
N
x Rm1 - U1 x R2))
R
E-
=
(U
N
x Rm1 x (U
N
x Rm2 - R2 x (U1 + U2)))
(U1 x (U
N
x Rm2 - U2 x R2))
Condition:
R1 = 2 x R2
Rm1 = 2 x Rm2
These calculations of the insulation resistances are done by the integrated
microcontroller. The results are indicated on the display.
Symmetrical and
asymmetrical
insulation faults
The measuring values are determined by the device with a passive measuring
principle. This entails some specialities regarding the measured insulation values.
If the relation between the single insulation values RF+ and RF- is high
(asymmetrical faults) the accuracy of the higher value is less than the accuracy of
the lower value, because the measuring voltage drop for an insulation fault is
gathered at the opposite side of the coupling network. That means that the
main measuring voltage for the higher value decreases with the dropping of the
lower insulation value. In the reverse case the measuring voltage of the lower
value increases with the dropping of this value. Because of this, the lower value
is measured with the highest accuracy.
In practice, this effect may entail a little change of the higher value in display
when the lower value increases or decreases.
The accuracy of both values is identical if the insulation values are symmetrical.
