27
TGH1266E -
05/04
Points to be Considered before Use
A Sensible Approach
to Fault Location
The sensitivity of the EDS3065 is 5 mA. Hence the maximum insulation resistance which can be
detected is dependent on the form of the voltage, the level of the voltage, and the system leakage
capacitance that is present.
In order to start the search for insulation faults in a sensible manner, it is advisable to consult the
following characteristic curves beforehand. These show:
•
the insulation fault which can be found, in relation to the system voltage (characteristic
curves 1a and 1b);
•
the maximum permissible system leakage capacitance, in relation to the system voltage
(characteristic curves 2a and 2b);
•
the reduction in response sensitivity when system leakage capacitances are higher
(characteristic curve 3);
•
the maximum test current with the insulation fault present (characteristiccurve 4).
An example:
In a 400 V 3AC IT system, the insulation monitoring device which is already present shows an insulation
fault of 100k
Ω
. The system leakage capacitances of the system are less than 1
µ
F and are therefore
negligible.
A look at characteristic curve 1b shows that an insulation fault of about 50 k
Ω
can be found in a 3 AC
system with 400 V. Therefore it makes no sense in this case to start searching for the fault with the
EDS3065.
Another example:
In a 110 V DC IT system, the insulation monitoring device which is already present shows an insulation
fault of 10 k
Ω
. Characteristic curve 1a shows that in a 110 V DC system, an insulation fault can be
found from about 20 k
Ω
upwards. Therefore in this case, it makes sense to start the search for the
insulation fault, and the chances of finding the fault are very good.
As a basic rule, it is necessary to take account of the possibility that the total insulation resistance
of a system is made up from the parallel connection of several insulation faults. It is not known which
individual faults contribute to this. If a fault is not found with the EDS3065, even though this ought
to be the case according to the characteristic curves, the cause may be the sum of a number of
individual faults. In this instance, none of the individual faults is of sufficiently low impedance that
it can be detected by the EDS3065.
Another reason why insulation faults are not found may be an excessively high system leakage
capacitance (on this point, see characteristic curves 2 and 3). When considering the system leakage
capacitances, a point to note is that the division of the capacitances upstream and downstream of
the clamp-on probe is not arbitrary. The pre-capacitance of the entire system must account for at
least 50% of the total capacitance. Otherwise a reduction in the response sensitivity must be
expected.
