Заземление и зануление электрооборудования

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Защитное заземление — преднамеренное электрическое соединение с землей металлических нетоковедущих частей, которые могут оказаться под напряжением вследствие замыкания на корпус (случайного электрического соединения токоведущей части с металлическими нетоковедущими частями электроустановки) и по другим причинам (например, разряд молнии).

Назначение защитного заземления — устранение опасности поражения током в случае прикосновения к корпусу электрооборудования и другим токоведущим металлическим частям, оказавшиеся под напряжением вследствие замыкания на корпус и по другим причинам.

Защитное заземление следует отличать от рабочего, предназначенного для обеспечения надлежащей работы электроустановок (заземляются нейтральные токи обмоток генераторов, силовых трансформаторов и т. д.), а также заземления молниезащиты.

В качестве заземлителей следует в первую очередь использовать так называемые естественные заземлители: водопроводные и другие металлические трубопроводы без антикоррозионного покрытия, за исключением трубопроводов с горючими жидкостями или горючими и взрывчатыми газами, свинцовые оболочки кабелей, металлические конструкции и арматуру железобетонных зданий и сооружений, имеющих соединение с землей, металлические шпунты, обсадные трубы и т. д. Алюминиевые оболочки кабелей и голые алюминиевые проводники в качестве заземлителей использовать не разрешается.

В тех случаях, когда естественные заземлители отсутствуют или их сопротивление превышает требуемое значение, применяют искусственные заземлители, состоящие из отрезков угловой стали (обычно размерами 50x50x5 мм) длиной 2,5—3 м, стальных труб диаметром 50 мм той же длины, отрезков круглой стали диаметром 12—14 мм длиной до 5 м. Указанные отрезки (электроды) погружаются в грунт на расстоянии друг от друга 3—5 м и соединяются между собой стальной полосой сечением не менее 40×4 мм.

Верхние концы электродов должны быть на глубине 0,7 м от поверхности. Соединительная полоса прокладывается в траншее глубиной 0,7 м. Все соединения осуществляются сваркой.

Количество электродов зависит от их размеров, удельного сопротивления грунта, глубины промерзания и некоторых других факторов и определяется на основании специального расчета.

В качестве заземляющих и нулевых защитных проводников в электроустановках напряжением до 1000 В могут быть использованы металлические конструкции зданий (если обеспечивается надежное соединение звеньев всей цепи), стальные трубы электропроводок, алюминиевые оболочки кабелей, металлические трубопроводы (кроме трубопроводов с горючими и взрывоопасными смесями, канализации, центрального отопления), металлические кожухи шинопроводов, металлические короба и лотки. Кроме того, используются нулевые рабочие провода электрической сети. Нельзя использовать в качестве заземляющих и нулевых защитных проводников тонкие металлические оболочки трубчатых проводов и свинцовые оболочки кабелей (АТПРФ, АСРГ и т. д.).

В электроустановках напряжением до 1000 В с глухим заземлением нейтрали нулевые защитные проводники должны быть выбраны таким образом, чтобы при однофазном замыкании на корпус или нулевой проводник происходило быстрое отключение защитой дефектного участка. ПУЭ требуют, чтобы при этом ток однофазного короткого замыкания в наиболее удаленной точке цепи превышал не менее чем в три раза значение номинального тока плавкой вставки ближайшего предохранителя или номинального тока комбинированного или теплового расцепителя автоматического выключателя.

В воздушных сетях переменного тока зануление осуществляется с помощью нулевого провода, проложенного на тех же опорах линии, что и фазные провода. На концах воздушных линий (или ответвлений) длиной более 200 м, а также на вводах в здания, электроустановки которых под­лежат занулению, должны выполняться повторные зазем­ления нулевого провода общим сопротивлением в сетях 380/220 В не более 10 Ом. При этом сопротивление каждо­го из повторных заземлений должно быть не более 30 Ом.

Для повторного заземления следует в первую очередь использовать естественные заземлители. Повторные заземления повышают условия безопасности, особенно при обрывах нулевого провода. В воздушных линиях длиной до 200 м и кабельных сетях любой длины повторные заземления не нужны, так как в них обрыв нулевой жилы маловероятен. В жилых домах с электроплитами в квартирах повторные заземления выполняются и при кабельных вводах.

Зануление не всегда обеспечивает необходимые условия безопасности людей, соприкасающихся с электроустановками, так как даже при токе однофазного короткого замыкания, превышающем в три раза значение номинального тока плавкой вставки предохранителя или расцепителя автомата, срабатывание защиты происходит с некоторой выдержкой времени (иногда несколько минут). В этих случаях целесообразно, особенно в помещениях с повышенной опасностью, дополнительно к занулению применять защитное отключение.

Нулевые защитные провода линий запрещается использовать для зануления электрооборудования, питающегося по другим линиям, так как они могут быть отключены для ремонта.

Аппараты защитного отключения представляют собой автоматические выключатели, снабженные устройствами, реагирующими на ток утечки.

Функции устройств защитного отключения следующие: а) защита от глухого замыкания на землю; б) защита от неполного замыкания на землю; в) постоянный автоматический контроль состояния изоляции сети и цепей зануления; г) самоконтроль. Наилучшие условия безопасности обеспечивает совмещение зануления и защитного отключения в одной установке. Наиболее простые аппараты защитного отключения срабатывают при глухом замыкании на землю с временем отключения 0,1—0,2 с и обеспечивают безопасность при прикосновении к заземленным нетоковедущим частям, оказавшимся под напряжением.

Применение устройств защитного отключения в квартирных электрических сетях позволит резко повысить электробезопасность.

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