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Topic Title: MCB Curves Help
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Created On: 06 August 2013 05:04 PM
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 06 August 2013 05:04 PM
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pcrichton

Posts: 3
Joined: 06 August 2011

I am looking at some MCB protection curves and am looking for some help with them.

There seems to be two curves, one above the other, and then there is space in between the curve. One starts from 1.13 x In, and the other starts from 1.45 x In. I am aware that these values are related in some way to the testing within BS EN 60898-1 and have read about the conventional non tripping current and conventional tripping current which are described very ambiguously.

Is it that you are only guaranteed to trip at the top curve and only guaranteed to not trip at the bottom curve and anything in between is not guaranteed?
 07 August 2013 06:04 PM
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gkenyon

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I think you are talking about the "acceptable range" that defines whether an mcb meets the standard; a real mcb will operate somewhete in the region between the two curves, but we don't know where without testing it. I.e. for a given current, any mcb complying with the standard will operate more quickly than the 1.45 x In line but more slowly than the 1.13 x In line.

Another way of looking at this, is that one Type B mcb may trip in 0.1 seconds with a current of 3 x In, whereas another (at the other end of the acceptable range) may require 5 x In. When we are looking at the properties for safety, we assume the Type B mcb we have is a "5 x In" one , but if we are looking at the effects of "inrush" currents that may occur at switch-on or after a power dip, we need to consider that the Type B mcb may well be a 3 x In one.

A manufacturer is likely to make their design somewhere in the middle of this (e.g. a 4 x In design for Type B), and in reality there will be a "standard distribution" around this for all qualifying Type B mcb's (i.e. statistically, many more mcb's in a batch in this example will be around the 3.7 to 4.3 x In range, than would be found nearer 3 x In or 5 x In).

Another thing which often confuses people is the "discontinuity" in the curves. The mcb has two components that can trigger a trip: the Magnetic element, and the Thermal element. The Thermal element takes more time to operate, and is generally there to protect against "overcurrent". The magnetic element operates from the large magnetic fields that occur with large currents, e.g. in a fault. The "discontinuity" in the curve, is where the magnetic element "takes over" from the thermal element.

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Eur Ing Graham Kenyon CEng MIET TechIOSH
 07 August 2013 07:22 PM
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OMS

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There seems to be two curves, one above the other, and then there is space in between the curve. One starts from 1.13 x In, and the other starts from 1.45 x In. I am aware that these values are related in some way to the testing within BS EN 60898-1 and have read about the conventional non tripping current and conventional tripping current which are described very ambiguously.


There are two curves - but what you are looking at (as Graham describes above) is the tolerance within the breaker standard

a Type B breaker can trip between 3 - 5 x In
a Type C breaker can trip between 5 -10 xIn
a Type D is potntially out of scope but can trip between 10 - 20 x In

You'll note the convergance of the curves as you get into the thermal trip region - it's here that the breaker must not trip (non tripping current)when exposed to 1.13 x In and must trip within conventional time at 1.45 x In (conventional tripping current) (Conventional time being one hour for MCB's up to 63A)

Regards

OMS

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Failure is always an option
 07 August 2013 10:16 PM
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cookers

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Such things make discrimination an interesting and often misunderstood topic. I suggest you only enquire deeper if you really are having trouble sleeping!
 11 August 2013 06:30 PM
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sreeraasi94

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There is a difference in the response levels of the instantaneous trip current levels between the three types of MCBs (i.e.) For a Type 'B' MCB, the instantaneous trip level is between 3 to 5 In, where In is the rated current of the MCB; for a Type 'C' MCB, it is between 5 to 10In and for a Type 'D' MCB, it is between 10 to 20In.

Please note that the difference is only in its instantaneous pick-up current ranges. The thermal curve is IDENTICAL for all the three types. The thermal curve of all the three types of MCBs is such that for a current of 1.13In, tripping must not happen in under one hour; for a current of 1.45 In, tripping should happen within one hour for MCBs with rated currents below or equal to 63A and within 2 hours for MCBs rated above 63A; And, for a current of 2.55In, it must trip within 1 to 60 seconds for MCBs with a rated current below 32A and between 1 to 120 seconds for MCBs with rated currents above 32A.

Hope it is clear.
 16 August 2013 08:37 AM
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pcrichton

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I understand the properties that all 3 breakers (Type B, C, D) must exhibit to be within the testing standards in BS EN 60898-1, as has been mentioned by many of you above. I did think however a manufacturer would provide a curve unique to its own MCB, as opposed to the 2 curves defined in the standards that provide a time window when it may trip. I suppose this is probably down to the temperature of the contacts when the fault is present at the breaker. I did think however that the manufacturer would be able to provide a more accurate tripping curve than the limitation that the standard states. Any thoughts on this?
 16 August 2013 05:45 PM
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OMS

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Manufacturing tolerances would almost certainly result in an upper and lower bound anyway.

Generall the manufactureres can provide you with specific test data, but you usually have to ask

What benefit would it give you if you had specific data anyway ?

Regards

OMS

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Failure is always an option
 19 August 2013 11:06 PM
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gkenyon

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Originally posted by: pcrichton

I understand the properties that all 3 breakers (Type B, C, D) must exhibit to be within the testing standards in BS EN 60898-1, as has been mentioned by many of you above. I did think however a manufacturer would provide a curve unique to its own MCB, as opposed to the 2 curves defined in the standards that provide a time window when it may trip. I suppose this is probably down to the temperature of the contacts when the fault is present at the breaker. I did think however that the manufacturer would be able to provide a more accurate tripping curve than the limitation that the standard states. Any thoughts on this?
Part of my earlier response:

A manufacturer is likely to make their design somewhere in the middle of this (e.g. a 4 x In design for Type B), and in reality there will be a "standard distribution" around this for all qualifying Type B mcb's (i.e. statistically, many more mcb's in a batch in this example will be around the 3.7 to 4.3 x In range, than would be found nearer 3 x In or 5 x In).


Manufacturers' data often reflects this, and their "improved performance over the standard" as it were, expecially in the performance of the magnetic element.

But as OMS says, what is the benefit of this: your circuit should be designed to cope with foreseeable use and maintenance, which would include the possibility that the manufacturer's replacement compatible mcb may be from a newer range of products, which may well have a different (but still EN60898-conformant) range of operation? In other words, EN60898 is still your "reference standard"?

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Eur Ing Graham Kenyon CEng MIET TechIOSH
IET » Energy » MCB Curves Help

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