روش پنجه کلاغی برای کاهش مقاومت زمین به 2Ω

متن کامل
یکی از روش های کاهش مقاومت زمین به کمتراز 2 اهم، روش پنجه کلاغی است، که شکل اجرای این روش، در ادامه ارائه می شود. در این آرایش می‌توان هادی‌های از جنس و سطح مقطع هادی حفاظتی انتخاب نموده و آن‌ها را به صورت افقی و به شکل سه شاخه (پنجه کلاغی) در عمق حداقل ۷۰ سانتی متری از سطح زمین اجرا نمود و از یک سر به یکدیگر اتصال داد.

 



روش پنجه کلاغی برای کاهش مقاومت زمین به کمتر از 2 اهم


مثال: در منطقه‌ای با مقاومت ویژه ۵۰ اهم متر با اجرای ۵۰ متر سیم با مقطع ۵۰ میلیمتر مربع متصل به ۳ میله ارت مغز فولاد روکش مس با طول ۱۵۰ سانتی متر و قطر ۱۶ میلیمتر، نتیجه مقاومت سیستم زمین زیر ۲ اهم حاصل می‌گردد.

مطابق فرمول‌های مندرج در استاندارد BS۷۴۳۰، مقاومت حاصل از سیم مسی مثال فوق برابر ۱.۹۸ و مقاومت حاصل از میله‌های ارت برابر ۱۰.۸۲ خواهد شد که ایجاد اتصال مناسب بین آن‌ها و موازی در نظر گرفتن مقاومت‌های حاصله، مقاومت سیستم زمین برابر ۱.۶۶ اهم خواهد شد؛ بنابراین با محاسبات ساده به مقاومت زیر ۲ اهم خواهیم رسید. هر چند در خیلی از استاندارد‌ها رعایت اصول هم پتانسیل سازی ارجحیت تا عدد مقاومت سیستم زمین دارد.
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