5
TGH1266E - 05/
04
The Insulation
Resistance
General
One determining factor for the availability of an electrical system is the insulation resistance. It
appears at the head of the list of protection goals for electrical safety. This applies as a fundamental
principle, regardless of the form of the system.
A planning decision to construct a safe and reliable power supply with the goal of achieving
maximum availability means:
•
constructing the power supply as an IT system. By doing so, greater operational safety
and reliability, greater fire safety, greater safety from accidents and a higher reliable
resistance to earth are achieved than in the case of a system form where the active
conductors are connected to earth.
•
use of the correctly selected insulation monitoring device. The advantages cited in the
foregoing paragraph stand or fall with the selection of the A-ISOMETER which is
appropriate to the respective application. Unless there is functional monitoring, it is
impossible to utilise the advantages of the IT system. The desired high level of
insulation resistance cannot be maintained in the long term without continuous
monitoring.
•
reduction of maintenance costs and outage times thanks to the use of an insulation fault
location device. Rapid location of an insulation fault immediately after it has occurred
is a long-cherished wish on the part of the maintenance staff. The fault is reported and
located without interruption of operations, and without the need for possible night and
weekend work. The actual repair can then be carried out at a suitable point in time.
The system operator makes the decision as to whether the insulation fault location
device to be used should be a fixed installation or a portable device.
•
An alternative to power supply in the form of an IT system may be offered by a TT or
TN system which is equipped with residual current monitors and residual current
detection devices. While this does not make it possible to avoid switching off when a
first low-impedance fault occurs, it nevertheless guarantees a time advantage as far as
information is concerned. As a result, deteriorations in the insulation and creeping
insulation faults are detected at an early stage and a considerable number of shutdowns
are prevented in this way.
Ensuring that the insulation is well-maintained is a decisive factor as far as the reliability of the
power supply is concerned. This is only possible with the use of suitable monitoring devices. These
devices cannot prevent the insulation fault or the deterioration of the insulation. However, early
and rapid detection with information about the location of the fault will simplify maintenance to
an extraordinary degree.
Without suitable tools, fault location was (and continues to be) time-consuming and troublesome.
Week-end work was frequently necessary in order to locate the fault or faults while the system was
switched off.
Modern monitoring devices such as the EDS3065, EDS470 or RCMS470 solve these problems.
Thanks to the automatic location of the insulation fault, the necessary fault elimination is made
very simple and the required high level of insulation is maintained. Localising takes place during
continuous operation and it is not necessary to shut the system down.
