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Fluke 6500 – Appliance test in an IT system
Why the Fluke 6500 appliance tester cannot be used for measurements in an IT system?
An appliance tester is suitable for tests and measurements on electrical equipment in a TN or also a TT system,
as only in these systems is the protective conductor used for protection against electric shock, refer also to Wikipedia.
https://en.wikipedia.org/wiki/Earthing_system#IT_network
In an IT system which is existing in hospitals, chemical industry or mining, appliance testers can only be used with restrictions.
What is the difference between TN and IT system?
In an IT system, there is NO electrical connection between the protective earth conductor and the live conductors,
so no leakage current can flow between the live conductors and the protective earth conductor even at an earth fault.
In practice, however, there are coupling capacitances due to the wiring, which leads to the display of “ghost voltages” when measuring with high-impedance equipment (e.g. multimeters).
The insulation status of an IT system is continuously monitored, reported and in some installations even switched off by insulation monitoring devices (IMD).
Which tests need to be carried out?
The following measurements are required for the safety of electrical equipment according to the relevant standards for appliance testing (EN 50678, EN 50699):
A.) Measurements without mains voltage:
Protective conductor resistance
Insulation resistance
Substitute leakage current (alternative method)
B.) Measurements with mains voltage
Protective conductor current
Touch current
Functional test (load current measurement)
Measurements without mains voltage can theoretically also be carried out in an IT system, but this is not possible with the Fluke 6500.
For measurements with mains voltage, the protective conductor is absolutely necessary,
as the current in the protective conductor or the touch current against the protective conductor is measured.
Why does the insulation monitoring device indicate an error?
To detect a connected PE conductor, the appliance tester carries out a test when it is powered on; if no PE is detected, an error message is displayed.
For Fluke 6500, error 77 is displayed, for Fluke 6500-2 "The mains upply Earth connection is missing" see picture.
The insulation monitoring device (IMD) of the IT system detects this monitoring circuit at the input of the appliance tester and interprets this as an error.
How can appliance tests be carried out?
In order to carry out unit tests in an IT system, a local TN system must be set up.
For this purpose, a common (single-phase) insulated variac is connected to the IT system on the primary side.
At the output, one of the live conductors is connected to the protective conductor and thus to earth.
The appliance tester will be connected to this "artificial TN system".