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ALSTEF
Member

Join Date: Oct 2007
Posts: 5

Breaker Classification

12/17/2007 1:15 AM

Dear All,

I thank you all for enlightening me about the BUSBAR CALCULATIONS. I would like to ask you one more question. This time its regarding MCB's.We use Merlin Gerin MCB's. I saw that there are three classification based on the curves B,C,D or K.

For example B curve MCB's can be used for loads that can tolerate(without tripping) currents momentarily upto 3 to 5 In.

C Curve (which we use ) MCB's will not trip for currents from 5 to 10In momentarily.

D Curve- for 10 to 15 In momentarily.

Iam really confused, are they talking about the S.C.fault current or starting current for the loads?Please enlighten me on this. Further i would also like to know the variuos parameters used in the breaker selection and their selection.

Thanking you all in advance
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Horrible Old Bat
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Join Date: Nov 2007
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Good Answers: 4
#1

Re: BREAKER CLASSIFICATION

12/17/2007 6:03 AM

It's about the time-current characteristics of the breaker. Each type of breaker has a characteristic curve that indicates when it will trip for the value of the over-current.

For resistive loads, B is usually adequate. For inductive loads you may need to go to a C breaker characteristic or even a D.

You need to check out various design codes for selecting the correct type of braker and the cables downstream of it.

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lordravindran
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Join Date: Oct 2007
Posts: 43
#2

Re: Breaker Classification

12/18/2007 3:29 AM

1. Select the class of the circuit breaker (B,C,D etc ) depending on load (See Merlin Gerin documents).

2. Decide if you need a differential circuit breaker.

3. Calculate the nominal current of the load that will be connected to the circuit of this breaker.

Select a circuit breaker with a current rating (called Calibre) which is within the range of 1.25 to 1.75 times the nominal current of the load. This statement is based on my own experience only.

4. Calculate the short circuit current just after the breaker. This is the supply voltage divided by the sum of the impedances from the supply point to the point of short circuit.

5. Now the breaker selected in (3) must have a breaking capacity (pouvoir de coupure Pdc) of at least 1.25 times the short circuit current calculated in (4).

I have successfully employed this procedure of selection of circuit breakers in many control panels for the industry as well as residential buildings for the last many years without any problem from the point of view of design.

MCB=circuit breaker=breaker, Breaking capacity(in kilo amperes)= pouvoir de coupure=Pdc(see Merlin Gerin catalogue)
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T Man
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Join Date: May 2006
Location: Pittsburgh PA
Posts: 121
#3

Re: Breaker Classification

12/18/2007 8:23 AM

You want to be able to withstand In Rush current for so long. Say you have a motor, when it starts, it going to pull around 6 X FLA. This is the inrush. You don't want the breaker to trip.This would be classified as thermal. Your Instantanoeus trip is for Short Circuit. Generally around 10 X FLA.

On the curve, when the line goes straight upo and down, not parabolic, thats is the INST Trip line, anything greater than that line should trip right away.

Choose breaker and or trip unit for your load

I hope this helps you

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oldeng
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Join Date: Aug 2007
Posts: 86
Good Answers: 2
#4

Re: Breaker Classification

12/18/2007 4:26 PM

Hi Alstef,

B characteristic MCBs 3-5 means that a current between 3 and 5 times the rating will trip the breaker in less than 100msec.Current above 3-5 times the rating will of course cause tripping in less than 100msec.

The 3-5 is the permissible spread in tripping current permitted by the spec.so design/manufacture must achieve this.

The thermal trip operates with characteristics just like a fuse and will not trip at a current about 1.4 times the rating.Currents above 1.4 times will trip the breaker eventually,with a time delay you can read off the characteristic chart.

The different characteristics,C,D,K etc. refer only to the 'will trip in under 100msec' part of the operation.

All breakers have the same thermal trip characteristics.

Additionally breakers have a short circuit rating say 6kVA ,10kVA,which refers to their capacity to break a short circuit.They should not be exposed to greater prospective short circuit currents than these values,but be supplied via fuses or current limiting circuit breakers to limit the current to the short circuit rating.

Unfortunately 'rating' tends to be used both for the normal current capacity and short circuit capabilities causing some confusion.

Can I recommend that you get a single phase MCB, (they are amazingly cheap), drill off the rivet head,pull off the side,and have a look at the guts,they are self explanatory.You will find a wonderful bit of compact elec eng,arc splitters,backward curved contact horns to facilitate arc movement etc.How the h they make them for a few euros??

Please continue to ask questions,its a pleasure for some of us to be able to help with basic practical information.

Sorry I cant quite follow what you are asking starting with 'Further........Please elaborate.
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ALSTEF
Member

Join Date: Oct 2007
Posts: 5
#5
In reply to #4

Re: Breaker Classification

12/19/2007 10:26 AM

oldeng,

Thank you very much for your clarification. Referring to the part that you are not able to comprehend, here is wat i wanted to ask you:

When i look upto the data sheet of any breaker, i see there are a lot of parameters that are being considered.like Icu, ISC, In,Ith etc.... could you pls provide me some link or pdf document that could make me undestand all this.

Thanks in advance.
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oldeng
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Join Date: Aug 2007
Posts: 86
Good Answers: 2
#6
In reply to #5

Re: Breaker Classification

12/19/2007 2:52 PM

Hi Alstef, www.beamainstallation.org.uk/assets/pdfs/CircuitBreaker.pdf has all the info for IEC/EN ,that is European MCBs. I know little about NEMA standards.

Note D types are defined as as 10-20 times rating, but the range can extend to 10-50 under IEC 898

Does anyone know whether the MCB is fit for further service after interrupting a fault Icu (Rated ultimate breaking capacity), assuming that you can find all the bits.
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lordravindran
Associate

Join Date: Oct 2007
Posts: 43
#7
In reply to #6

Re: Breaker Classification

12/22/2007 8:49 AM

ref: Does anyone know whether the MCB is fit for further service after interrupting a fault Icu (Rated ultimate breaking capacity), assuming that you can find all the bits.

Please refer to standard EN60898-1 July 2003 Norme Europeene Disjoncteurs.

Tables 18,19,20,21,22,23

For the sequence of closing/opening see paragraph 9.12.11.1

Circuit breaker must be in good working condition without any repair after closing and opening for well specified times as per European standard at rated Short circuit current and rated breaking capacity.

It would need at least 2 hours to read and fully understand the test sequence and currents involved. Standard gives definitive relations between short circuit current and breaking capacity. You are free to ask for clarifications, if any.
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solaaderinwale
Active Contributor

Join Date: Sep 2007
Posts: 21
Good Answers: 1
#8
In reply to #4

Re: Breaker Classification

12/26/2007 10:18 AM

Thanks for your explanation. I think it should suffice. However, I observe a small error in your comment which I believe is an oversight. Breaking capacity of MCBs is not expresswed in kVA as in the examples you gave. It is always in amperes, in multiples of thousands to be precise, say 5kA, 7kA etc. It is the maximum current the breaker can interrupt without getting damaged.

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oldeng
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Join Date: Aug 2007
Posts: 86
Good Answers: 2
#9
In reply to #8

Re: Breaker Classification

12/31/2007 5:04 PM

Glad you spotted the deliberate mistake,proves you're paying attention.

I got carried away with my HV reminiscences on another thread.You are quite correct,the rating is in k amps.

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Users who posted comments:

ALSTEF (1); Horrible Old Bat (1); lordravindran (2); oldeng (3); solaaderinwale (1); T Man (1)

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