{"id":245,"date":"2016-03-15T15:38:29","date_gmt":"2016-03-15T15:38:29","guid":{"rendered":"http:\/\/www.the-regs.co.uk\/blog\/?p=245"},"modified":"2016-03-15T15:38:29","modified_gmt":"2016-03-15T15:38:29","slug":"maximum-zs-values-in-a-tt-earthing-system","status":"publish","type":"post","link":"https:\/\/the-regs.co.uk\/blog\/?p=245","title":{"rendered":"Maximum Zs values in a TT earthing system"},"content":{"rendered":"

The maximum disconnection times for a TT system is 0.2 seconds for final circuits up to and including 32A (Table 41.1) and for distribution circuits or circuits greater than 32A it’s 1 second (411.3.2.4).<\/p>\n

The problem is that with TT systems the external loop impedance value (Ze) will be too high to achieve these 0.2 and 1 second maximum disconnection times so as an alternative we must use RCDs for fault protection, as their maximum disconnection time of 40mS at 5 times the rating will meet the requirements of T41.1 and 411.3.2.4. <\/p>\n

The maximum Zs when using RCDs for fault protection in a TT system is based on 411.5.3 and Table 41.6 applies.<\/p>\n

If you look at NOTE 1: in 411.5.2 it does say that an appropriate overcurrent protective device (ie circuit breaker) may be used on a TT system for fault protection if the Zs value is permanently low.<\/p>\n

So if you were living next door to the substation and your Zs values were low enough then it is possible to use circuit breakers or fuses for fault protection and the maximum Zs values would be the same as TN (see 411.5.4) This is the same formula for overcurrent protection as in a TN system (411.4.5).<\/p>\n

Remember though this is very rare and and even if you could use circuit breakers for fault protection RCDs would still be needed for additional protection etc. <\/p>\n","protected":false},"excerpt":{"rendered":"

The maximum disconnection times for a TT system is 0.2 seconds for final circuits up to and including 32A (Table 41.1) and for distribution circuits or circuits greater than 32A it’s 1 second (411.3.2.4). The problem is that with TT systems the external loop impedance value (Ze) will be too high to achieve these 0.2…<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":""},"categories":[1],"tags":[],"class_list":["post-245","post","type-post","status-publish","format-standard","hentry","category-general","th-blog blog-single has-post-thumbnail"],"yoast_head":"\nMaximum Zs values in a TT earthing system - the-Regs : BS7671 18th Edition Online Training<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/the-regs.co.uk\/blog\/?p=245\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Maximum Zs values in a TT earthing system - the-Regs : BS7671 18th Edition Online Training\" \/>\n<meta property=\"og:description\" content=\"The maximum disconnection times for a TT system is 0.2 seconds for final circuits up to and including 32A (Table 41.1) and for distribution circuits or circuits greater than 32A it’s 1 second (411.3.2.4). The problem is that with TT systems the external loop impedance value (Ze) will be too high to achieve these 0.2...\" \/>\n<meta property=\"og:url\" content=\"https:\/\/the-regs.co.uk\/blog\/?p=245\" \/>\n<meta property=\"og:site_name\" content=\"the-Regs : BS7671 18th Edition Online Training\" \/>\n<meta property=\"article:published_time\" content=\"2016-03-15T15:38:29+00:00\" \/>\n<meta name=\"author\" content=\"the Regs Guy\" \/>\n<meta name=\"twitter:card\" content=\"summary_large_image\" \/>\n<meta name=\"twitter:creator\" content=\"@theregsguy\" \/>\n<meta name=\"twitter:site\" content=\"@theregsguy\" \/>\n<meta name=\"twitter:label1\" content=\"Written by\" \/>\n\t<meta name=\"twitter:data1\" content=\"the Regs Guy\" \/>\n\t<meta name=\"twitter:label2\" content=\"Estimated reading time\" \/>\n\t<meta name=\"twitter:data2\" content=\"1 minute\" \/>\n<script type=\"application\/ld+json\" class=\"yoast-schema-graph\">{\"@context\":\"https:\\\/\\\/schema.org\",\"@graph\":[{\"@type\":\"Article\",\"@id\":\"https:\\\/\\\/the-regs.co.uk\\\/blog\\\/?p=245#article\",\"isPartOf\":{\"@id\":\"https:\\\/\\\/the-regs.co.uk\\\/blog\\\/?p=245\"},\"author\":{\"name\":\"the Regs Guy\",\"@id\":\"https:\\\/\\\/the-regs.co.uk\\\/blog\\\/#\\\/schema\\\/person\\\/0d7feabc48e1ab05b0c346e599ea65cf\"},\"headline\":\"Maximum Zs values in a TT earthing system\",\"datePublished\":\"2016-03-15T15:38:29+00:00\",\"mainEntityOfPage\":{\"@id\":\"https:\\\/\\\/the-regs.co.uk\\\/blog\\\/?p=245\"},\"wordCount\":229,\"commentCount\":0,\"publisher\":{\"@id\":\"https:\\\/\\\/the-regs.co.uk\\\/blog\\\/#organization\"},\"articleSection\":[\"General\"],\"inLanguage\":\"en-GB\",\"potentialAction\":[{\"@type\":\"CommentAction\",\"name\":\"Comment\",\"target\":[\"https:\\\/\\\/the-regs.co.uk\\\/blog\\\/?p=245#respond\"]}]},{\"@type\":\"WebPage\",\"@id\":\"https:\\\/\\\/the-regs.co.uk\\\/blog\\\/?p=245\",\"url\":\"https:\\\/\\\/the-regs.co.uk\\\/blog\\\/?p=245\",\"name\":\"Maximum Zs values in a TT earthing system - 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