Ideal dielectrics with a resistance equal to infinity, do not exist. Even the high-quality insulator possesses some leak. For measurement of insulation resistance use special devices - megohm meters, but sometimes it is possible to manage also simpler means.
1. If the device has the metal casing, hit on the last phase tension can constitute danger, especially if the person at the same time concerns the grounded subject. For detection of such leak use the screw-driver with the neon indicator intended for search of a phase wire. Without holding the case of the device and a sting of the screw-driver, touch the sensor, and press a sting to the site of the metal casing of the working device which is not coated. If the bulb is lit even very poorly (much more weakly, than by search of a phase), leak is available. Carry out such inspection at both polyarnost of connection of the device to network. To eliminate leak, ground the device. Use for this purpose only the special earth bar, but not pipes of a water supply system, heating, gas supply, a zero wire, a braid of a television cable, etc. at all.
2. If it is impossible to arrange grounding for one reason or another, in the beginning make sure that leak has the capacitive, but not resistive nature. For this purpose use the multimeter working in the ohmmeter mode on a limit of 20 megohms. Pull out a fork of the checked device from the socket, but put the network switch in the included situation. Connect one probe of a multimeter to the case of the device, and another - to one of contacts of a network fork. Do not touch at the same time by hands either that, or another not to bring an error in measurement. The multimeter has to show infinity still. Connect the probe to other pin of a fork - the result has to remain invariable. Then repeat both measurements, having changed polarity of connection of probes for opposite.
3. If even the slightest leak on a direct current is found, immediately stop use of the device and send him to repair. If that is not found, so the reason of hit of tension on the building consists only available parasitic tanks. When using of such device do not concern at the same time its case and any grounded objects and also other electric devices having metal casings at all. In case it is necessary that several such devices stood nearby, disconnect them everything from network, connect among themselves their cases wires then again connect to network. If it is about video equipment, grounding formally not demanding (for example, DVD players, TVs), it is not obligatory to carry out such connection, the main thing is to watch that all of them were connected by cables with each other and nearby there were no two groups of devices between which connection is absent. For example, if one DVD player is connected to one TV, and another - to the second, and among themselves players are not connected, the simultaneous touch to cases of both players can cause notable electroblow. Its danger is eliminated if to connect cases of devices with each other.
4. Use the megohm meter only in case are sure that the high voltage developed by it will not damage electronic components of the device which isolation you are going to check. Connect device probes to points which have to be isolated from each other then begin to twist the handle, or, depending on device type, turn on tension converter. At the same time do not concern probes at all. Make sure that the measured resistance is more than minimum admissible. Then stop to rotate the handle or switch off the converter then repeat measurement at the return polarity of test tension.