Hi, I recently received an email from a student who’d just passed his 17th Edition City & Guilds 2382-15 exam saying…<\/p>\n
There was only one question that I had absolutely no idea on, it was regarding Circuit breaker Types. I can’t remember the question exactly, but but it was along the lines of “In what situation should a type D circuit breaker be used?” I knew the answer was in Part\u00a04 but I could not find it anywhere. Just for peace of mind, could you point me to a section where I might find the answer?<\/p><\/blockquote>\n
The question looks more like an electrical general knowledge one which they have a habit of including now and again. There is no specific mention regarding the situations when you’d use a Type D circuit breaker within BS 7671.<\/p>\n
Chapter 41 table 41.3 gives the maximum Zs for different Types, B, C and D. The reason we use different Types is because of the amount of current needed to achieve their maximum disconnection times.<\/p>\n
In the good old days prior to Amendment 3 the amount of current needed to achieve the maximum disconnection times would have been…<\/p>\n
A Type B circuit breaker is 5 x their rating ( Ia\u00a0 =\u00a0 5 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 ( Ia = 20 x In)<\/p>\nSo to calculate the Max Zs for a circuit breaker it was simply 230\/(5 x In) for Type B, 230\/(10 x In) for Type C and 230\/(20 x In) for Type D<\/p>\n
However, Amendment 3 gave us a new set of disconnection times for Type D<\/p>\n
A 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 5 sec disconnection ( Ia = 10 x In )<\/p>\nAnd because the supply voltage is never at a constant 230v in Amendment 3 we also saw the introduction of the Cmin value to take account of this, giving us a new formula for calculating the maximum Zs values.<\/p>\n
230 x 0.95\/(5 x In) for Type B
\n230 x 0.95\/(10 x In) for Type C
\n230 x 0.95\/(20 x In) for Type D with a 0.4 sec maximum disconnection time.
\n230 x 0.95\/(10 x In) for Type D with a 5 sec maximum disconnection time.<\/p>\nSo the reasons for the different types is…<\/p>\n
Lets’ say we are using a 20 A circuit breaker<\/p>\n
A Type B would take up to 100A before tripping ‘instantaneously’
\nA Type C would take up to 200A before tripping ‘instantaneously’
\nA Type D would take up to 400A before tripping ‘0.4 seconds’
\nA Type D would take up to 200A before tripping ‘5 seconds’<\/p>\nThe circuit we are protecting has a motor connected which when starting has an inrush current of 120A for\u00a0 0.2 seconds this would trip a Type B circuit breaker but could take up 10 seconds (using the Type D graph on page 327 BS7671) to trip a Type D circuit breaker. As it’s only for 0.2 seconds the Type D will not trip.<\/p>\n
So Type D circuit breakers are used in instances where there may be a large ‘inrush current’ when equipment is turned on. i.e motors. Type C are often used on large fluorescent lighting circuits. Type B are general use most commonly used on domestic circuits.<\/p>\n
Remember the Maximum Zs value is higher on Type B but lower on Type C and D and this must always be taken into account if changing the circuit breaker Type.<\/p>\n
Sorry for the long winded reply but I think this will make a good post for my blog so I went on a bit.<\/p>\n
Regards<\/p>\n
Take care
\nPaul<\/p>\n","protected":false},"excerpt":{"rendered":"Hi, I recently received an email from a student who’d just passed his 17th Edition City & Guilds 2382-15 exam saying… There was only one question that I had absolutely no idea on, it was regarding Circuit breaker Types. I can’t remember the question exactly, but but it was along the lines of “In what…<\/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":[1,17],"tags":[],"class_list":["post-342","post","type-post","status-publish","format-standard","hentry","category-general","category-part-4-protection-for-safety","th-blog blog-single has-post-thumbnail"],"yoast_head":"\n
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