1.4. Electrical Insulation checking procedure
Definition : 1.4.1. Definition of a fault of insulation
The fault of isolation arises when the impedance between one of the phases and the electrical protection (EP) (or a phase and the metal frame of the receiver) becomes very low. That is when a phase contacts a EP wire or a metal mass.
1.4.2. Introduction
To ensure the smooth running and a perfect safety of devices and electrical installations, all the conductors are isolated: sheath for wires, varnished for the winding.
When the quality of these insulations decreases, leakage currents can circulate from a conductor to another one and, according to the importance of the faults of isolation (the worst fault being the short-circuit), causes damages more or less serious.
An equipment presenting a fault of isolation can break down, burn or causes a fault on the installation itself and consequently, activates protection devices, that is the cut of all the installation...
To prevent and be able to protect itself against risks linked to an insufficient isolation or to a deterioration of the level of the isolation, proceedings must be made. It concern as well the electrical equipment as the installations on which they are connected.
Those proceedings are realized during the commissioning, on new or renovated elements, then periodically to see their evolution during the time.
1.4.3. Electrical insulation
An electrical installation, an equipment or both have to have a good isolation to ensure the protection of the people. The insurance of a good level of isolation allows to guarantee a better continuity of service. |  |
 | The isolation results in measurements of resistance between every active conductor and the metal frames, and between active conductors (installation or equipment switched off). |
1.4.4. Principle of the measurement
We apply, thanks to the controller of isolation or megohmmeter, a direct voltage test and we collect the resistance value of isolation. The test must be realized under a voltage at least equal to the voltage of the network feeding the part to be checked. Test voltages in LV are generally of 500V.
Note : Remark:
./ The ohmmeter does not allow to realize controls of isolation because the voltage of measurement is too low and does not give a significant value.
./ The megohmmeter delivers a voltage of 500V to 1000V and can destroy a part of the devices, in particular the electronic systems. In that case, we have to disconnect these devices and test them separately.
./ During the measurement of an isolation with regard to the earth, it is recommended to place the positive pole of the test voltage on the earth, to avoid problems of polarization of the earth when we proceed to multiple tests.
Reminder : MINIMAL VALUE OF ISOLATION RESISTANCE
The minimum values of isolation resistances are defined according to the rated voltage of the tested circuit and correspond to the requirements of the standard.
1.4.5. Measurement of isolation on electrical installations
Before the commissioning, receivers disconnected, between every active conductor (conductors of phase and the neutral) to verify that none of them underwent mechanical damage during the installation.
Before the commissioning, interconnected active conductors, connected receivers, to verify the isolation of all the conductors with regard to the earth.
1.4.6. Measurement of isolation of a rotating machine
We can check the quality of the isolation of the windings with regard to the earth or the windings between them.
We can also check the isolation of the motor connected on the installation, with regard to the earth.
