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CIRCUIT BREAKER TYPES - How they work and inrush currents

CIRCUIT BREAKER TYPES - How they work and inrush currents
many times at learn electrics we are asked to explain the reasons for different

types

of mcbs why do we have type b type c and type d and what are those funny graphs in the back of the wiring regulations book in this video we will look briefly at what an mcb is what

types

there are what formats there are and those all important in rush current and response graphs what is the primary function of an mcb then it is to protect the cables downstream of the device and to protect persons livestock and property from the dangers of electricity an mcb is not there primarily to protect the appliances proper selection of the rating and type of an mcb is essential as incorrect choices may give rise to incorrect function during a fault what format do mcbs come in what kinds can you get we should all be familiar with the single pole mcb or miniature

circuit

breaker

these devices switch the phase or line conductor only and are used on single phase

circuit

s all mcbs will give three

types

of protection overload protection short

circuit

protection and earth fault protection we can move on to double pole mcbs these have a protection for the phase and the neutral in a single phase

circuit

as an example

they

are installed in caravans and are used in all domestic

circuit

s in many european countries we can see the symbols for both poles being protected on this close-up of a double pole mcb look at these two similar symbols on the left both poles have an x in line of one or three indicating that both...
circuit breaker types   how they work and inrush currents
are protected an over current in either the phase or the neutral will cause the device to operate on the right the protection symbol is only in line 1. line three therefore is switching only with no protection the two switches are linked so

they

both turn off at the same time which means that with a properly built

circuit

if line one the face side opens then the neutral side line two also opens then we have three pole or triple pole mcbs for use in three phase

circuit

s all three poles are linked so if one pole detects a problem then all three turn off together moving on we can have three pole plus neutral using the abbreviations tpn or 3p plus n with this type of mcb all four contacts are linked and operate at the same time but there is no sensing no protection in the neutral side this is just a basic switch and we can then progress to a 4-pole mcb now there is protection built into all three phases and into the neutral these are used where there is a likelihood of high neutral

currents

such as instances where there are excessive unbalanced loads how then does an mcb sense a problem what causes it to trip an mcb is basically a thermo magnetic switch both functions are built into the same mcb we have thermal sensing for slowly rising overloads and we also have electromagnetic sensing for short

circuit

s and earth faults the rating and the type designation will determine what the actual tripping current and tripping times of the device actually are the thermal sensing part is...
circuit breaker types   how they work and inrush currents
basically a bi-metallic strip that slowly heats up if it is overloaded in an mcb this will usually be overloads of current of long's duration in other words several minutes the bi-metallic strip will slowly distort during an overload until it reaches a point where it opens the contacts and disconnects the supply how quickly it distorts depends on the

breaker

rating and the overload current a good example of a biometallic strip in operation is a switch in your kettle as the water heats up the biometallic strip in the switch starts to bend and when the water boils it distorts enough to open the contacts and turn the power off let's have a little example a 32 amp type b

breaker

protects a socket ring

circuit

in an office in the middle of winter it's cold in the office and we end up with four 13 amp heaters all plugged into the same

circuit

at the same time giving a total load of 52 amps on the 32 m

breaker

this overload condition will cause the biometallic strip to start heating up and bending after about 15 minutes the biometallic strip reaches a point where it will open the contacts and disconnect to the supply note with overloads that if the overload is removed before the device trips the bi-metallic strip will gradually cool down and return to a normal state the electromagnetic part of the mcb will operate on excessive and rapidly rising

currents

that are typical of short

circuit

s and earth faults a sudden rise of hundreds of amps a sensing coil will monitor the...
circuit breaker types   how they work and inrush currents
current flowing through the mcb in normal conditions the coil has a magnetic armature in the center and this is connected to a hammer in no fault conditions the armature and the hammer are retracted when a fault occurs a massive current flows very quickly this causes the sensing coil to generate a magnetic field that shoots the armature through the coil the hammer is fired out and strikes the moveable contact that opens and breaks the

circuit

when the

circuit

breaks the supply is disconnected all mcbs will latch into the off position the disconnected position when

they

sense overloads or faults

they

will not auto reset we must physically reset them ourselves when we've cleared the fault we can look now at the response curves for mcbs different ratings and different

types

of mcb will respond in their own preset way to different overloads and fault

currents

looking at page 370 of the one regulations we find the response curves for type b mcbs and rcbos each type of mcb and even a fuse will have its own page in the book and each page will show the curves for different

breaker

s and fuse ratings with the type b shown here the range is from 6 amps to 125 amps time in seconds is shown vertically on the left hand side and the current is shown flowing along the horizontal axis note that the numbers are all shown as logarithmic scales logarithmic just means the numbers go up in blocks to the power of ten the first block is the numbers one to ten so one two three four five etc the...
second block covers the range ten to a hundred and is 10 20 30 40 and so on and then 100 to 1000 which is 100 200 300 you get the idea any response graph can be broken down into distinct areas the white area on this graph is the no trip area if the current is in the white area then that is a normal situation and the device will not trip notice the shape of the blue line and the so-called knee halfway along an overload current in the green area above the knee will cause the mcb to trip and default current in the yellow area will also cause the device to operate with bs en 608 980

breaker

s we have b type c type and d type mcbs the response graph for each is slightly different as shown here and can also be seen on pages 370 371 and 372 of the regulations book the distance along the bottom axis the amps is different for each type as is the position of the knee or instantaneous tripping point the top of the curve however is the same for all

types

of a particular rating let's look at what happens when we turn the power on current starter rush into the

circuit

we will always get a current search large or small let's switch on in this case the

inrush

current reaches a peak but does not cross the blue line and then it begins to settle down and return to normal operating levels because it always stayed within the boundaries of the response curve the mcb did not trip and this is a nicely balanced

circuit

but what would happen if the

inrush

current was excessive let's go back...
to our busy office the whole ceiling is full of fluorescent lights as far as the eye can see a type b

breaker

is installed and we can see on the response curve that we have a problem the current crosses the blue line at switch on this is often caused by the transformers that are in all the lighting units but the device does not know if this is just

inrush

current or a real fault it just knows that the blue line has been crossed and so it trips it disconnects the supply if and that is if the customer can get the lights to come on

they

are fine for the rest of the day because the current settles down into a steady state but the office manager could do without the hassle every morning installing a type c

breaker

will move the curve to the right the

inrush

current will stay the same but now it starts to settle down before it crosses the line that looks good but we must consider some other things when changing from a b type to a c type making this change will halve the maximum permitted zs for that

circuit

we must be aware of this and the new zs must be tested personally i always check the

circuit

zs before making the change we have a learn electrics video on what to do if the zs is above the permitted maximum and we will leave a link for this in the description to this video some people ask why not fit type c or d for everything and it is a question of balancing function against safety the device the mcb must operate fast enough to protect persons and livestock during a fault but...
not so quickly that the

circuit

is prone to nuisance tripping we must have the right size

breaker

so that any high

inrush

current do not cause nuisance tripping but the device should still operate correctly before a short

circuit

can damage the

circuit

cables what we want to do is to balance the over current and fault protection so that we get the best performance for the attached appliances and equipment whilst still maintaining a safe and protected

circuit

what are some of the differences between

types

b c and d

breaker

s this chart shows the three

types

of the popular bs en 60898

breaker

a b-type

breaker

will have almost instantaneous operation if the fault current or in-rush current exceeds five times the

breaker

rating so a type b 10 amp

breaker

will operate when 5 x 10 or 50 amps of current is detected a c type operates instantaneously at 10 times its normal rating and the d type at 20 times the chart also shows how the maximum permitted zs reduces as the

breaker

type becomes more robust to

inrush

currents

on this slide we have listed some common usages of the different

breaker

types

b c and d we should always aim to have the correct size and type of

breaker

installed stable enough that we do not get nuisance tripping but quick enough that the customer and the cables are safe from the dangers of electricity and of course always check that the zs is correct too as that can affect the response of the mcb as well well that's it we hope that you have enjoyed this video...
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