In a TN earthing system the cornerstone of protection against electric shock is that the protective device ‘circuit breaker’ shall not exceed it’s maximum disconnection time.<\/p>\n
For a 230 v\u00a0to earth\u00a0TN system (covers both single phase\u00a0and\u00a03-phase)\u00a0that is 0.4 seconds\u00a0(final circuits less than or equal to 32A) see T41.1\u00a0\u00a0and 5 seconds (distribution circuits and >32A) see 411.3.2.3<\/p>\n
In order to meet those disconnection times for fault protection in a TN system a circuit breaker needs ‘X’ amount of current (Ia) …<\/p>\n
A Type B circuit breaker is 5 x their rating ( Ia\u00a0 = \u00a05 x In )
\nA Type C circuit breaker is 10 x their rating ( Ia = 10 x In )
\nA Type D circuit breaker is 20 x their rating for a 0.4 sec disconnection ( Ia = 20 x In)
\nA Type D circuit breaker is 10 x their rating for a\u00a05 sec disconnection ( Ia = 10 x In )<\/p>\n
So by using this information and\u00a0Ohms Law (I = V\/R) we can see that a 10 A Type B circuit breaker needs at least 50 A and we know our voltage is 230 v\u00a0so the maximum resistance (impedance) would be 230 \/ 50 = 4.6 ohms<\/p>\n
However because the voltage is never a constant 230v we need to make allowances for the fluctuation. Hence the new Cmin values.<\/p>\n
The Electricity Safety, Quality and Continuity Regulations provide the value for Cmin as 0.95<\/p>\n
So the new calculation is…<\/p>\n
(230 x 0.95) \/ 50 = 4.37 ohms which is the maximum Zs for a 10 A Type B circuit breaker<\/p>\n
Summary: The formula for the maximum Zs<\/p>\n
Zs = (U0 x Cmin) \/ Ia<\/p>\n
<\/p>\n","protected":false},"excerpt":{"rendered":"
In a TN earthing system the cornerstone of protection against electric shock is that the protective device ‘circuit breaker’ shall not exceed it’s maximum disconnection time. For a 230 v\u00a0to earth\u00a0TN system (covers both single phase\u00a0and\u00a03-phase)\u00a0that is 0.4 seconds\u00a0(final circuits less than or equal to 32A) see T41.1\u00a0\u00a0and 5 seconds (distribution circuits and >32A) see…<\/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":[10],"tags":[],"class_list":["post-215","post","type-post","status-publish","format-standard","hentry","category-amendment-3-bs76712015","th-blog blog-single has-post-thumbnail"],"yoast_head":"\n