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 situation should a type D circuit breaker be used?” I knew the answer was in Part 4 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?
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.
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.
In the good old days prior to Amendment 3 the amount of current needed to achieve the maximum disconnection times would have been…
A Type B circuit breaker is 5 x their rating ( Ia = 5 x In )
A Type C circuit breaker is 10 x their rating ( Ia = 10 x In )
A Type D circuit breaker is 20 x their rating ( Ia = 20 x In)
So 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
However, Amendment 3 gave us a new set of disconnection times for Type D
A Type D circuit breaker is 20 x their rating for a 0.4 sec disconnection ( Ia = 20 x In)
A Type D circuit breaker is 10 x their rating for a 5 sec disconnection ( Ia = 10 x In )
And 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.
230 x 0.95/(5 x In) for Type B
230 x 0.95/(10 x In) for Type C
230 x 0.95/(20 x In) for Type D with a 0.4 sec maximum disconnection time.
230 x 0.95/(10 x In) for Type D with a 5 sec maximum disconnection time.
So the reasons for the different types is…
Lets’ say we are using a 20 A circuit breaker
A Type B would take up to 100A before tripping ‘instantaneously’
A Type C would take up to 200A before tripping ‘instantaneously’
A Type D would take up to 400A before tripping ‘0.4 seconds’
A Type D would take up to 200A before tripping ‘5 seconds’
The circuit we are protecting has a motor connected which when starting has an inrush current of 120A for 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.
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.
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.
Sorry for the long winded reply but I think this will make a good post for my blog so I went on a bit.
Regards
Take care
Paul