Control and prevention of isolation in electric units

Very important protective measure from the porazheyoniya current, is isolation of parts of electric units. The quality of isolation has to correspond to the environment and operating conditions.

Схема соединения электроустановки с заземлителем

Scheme of connection of the electric unit with the grounding conductor.

Only in this case isolation can carry out the basic purpose — to protect installation from the raised leakage currents, and sleyodovatelno, to protect from danger of defeat toyoky and from the fires.

High resistance of isolation creates bezopasyony operating conditions and the vozmozhyonost of the fires warns. Low resistance of isolation worsens operating conditions in networks with isolated ней­тралью, and in networks with the deadly earthed neutral privoyodit to breaks in power supply. The great value therefore gets control of the soyostoyaniye of isolation of networks.

Check of isolation of networks is proyoizvodit: a) at acceptance of installation after mounting or the reyomont, b) periodically in use at least 1 time a year, c) it is constant at operation of installation with по­мощью special devices of control of isolation. The norms ordered by the existing elekyotrotekhnichesky rules and standards have to be the basis for assessment of the condition of isolation. Insulation resistance of network of the lighting ustanovyoka on the site between 2 adjacent predokhranitelyayom, behind the last safety locks between any proyovod and the earth and also any 2 wires has to be not less than 0,5 megohms.

Измерение сопротивления изоляции мегомметром

Figure 1. Measurement of insulation resistance megger.

The specified norm does not belong to air provoyoda of outside devices and to the installations which are in crude rooms as resistance in them is extremely changeable and depends on air humidity. The lack of norms for such installations obliges to the bigger care at the choice of electric equipment, to more careful mounting and to more vnimayotelny control. The specified norms of isolation do not extend to electrical machines, transformers and accumulators therefore at measurements of isolation they are disconnected from network. Does not raise doubts that insulation resistance ма­шин and transformers has the same value in sense of safety, as well as insulation resistance of network. Again mounted electrical unit or installation on which repair is finished can be accepted in operation only after the careful proveryoka of its isolation concerning the earth and between phases. The same check is periodically made also on operable units as eventually under the influence of moisture, dust, caustic vapors and the temperatuyora isolation can become useless them.

In normalyyony production rooms необ­ходимо to make this check at least once a year, and in осо­бо crude — at least 2 — 4 times a year. Isolation review periods for the rooms constituting povyyoshenny danger concerning explosion or the fire are established in coordination with fire protection, depending on responsibility and character of the proizyovodstvo. According to the existing rules, it is necessary to perform measurement of the soproyotivleniye of isolation the operating voltage or tension in any case not less than 500 Century. Insulation test of the cable line of 6 — 10 kV and also determination of integrity of cable cores and check of compliance them on phases the megger are proizyovodit by not less than 2 people from whom 1 has to have qualification not below group IV, and the 2nd — is not lower than group III.

Схема постоянного контроля изоляции в установках с изолированной нейтралью

Figure 2. The scheme of constant control of isolation in installations with the isolated neutral.

Before insulation test of the cable line and also after it it is necessary to discharge the cable on the earth and to be convinced of total absence on it is mute the capacity zayoryad. Cables of 6 — 10 kV in the course of the ekspluatayotion subject within 5 min. to preventive isyopytaniye direct current voltage equal to the 5-fold tension of the rated linear napryayozheniye. Cables up to 1 kV test, as a rule, the megger 500 — 1000 V (fig. 1). At insulation test of electrical units all lamps, electric motors and other receivers of current and also transformers have to be disconnected from wires, and all fittings, on the contrary, are attached, all fusible inserts of safety locks are inserted, and vyklyuyochatel are closed. It allows to check isolation not only wires, but also all ustayonovochny armature connected to them.

Before measurement it is necessary to be convinced of absence of people near the part of the electric unit attached to the megger and to forbid being about it to touch current carrying parts in order to avoid neyoschastny cases. Performing measurement has to so расположить­ся with the megger that even the sluyochayny touch as the most working, and device proyovod to the parts of installations which are energized was impossible. The conductors serving for connection of the device to current carrying parts have to have the rubber izolyayotion. Systematic control of the condition of isolation gives the chance to timely find the damages which are inevitably arising in use which were not found at routine tests.

The easiest way constant конт­роля isolation, for example, in installations with the isolated neutral, is based on use of voltmeters or lamps (fig. 2). If isolation of all phases concerning the earth has identical resistance, each of voltmeters shows phase tension. If resistance of one of phases goes down, then the voltmeter connected to this phase will give the reduced evidence. On the contrary, indications of 2 other voltmeters will increase.

At short circuit of one of phases on the earth the voltmeter, podyoklyuchenny to it, will show 0, and 2 druyogy — linear stress. The lamps and voltmeters used to control of the izoyolyation have to possess rather big soprotivyoleniye that at connection between wires and the earth not to serve as the reason of deterioration in isolation, for example neon lamps (without filament), voltmeters static and electronic. Automatic control of isolation of network on the signal or on shutdown can be carried also out by means of special relays of leaks (for example, the RUV relay for the explosive environment) and RUNES relay (for the normalyyony environment).

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