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Topic Title: Discrimination of MCBs???
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Created On: 10 February 2010 05:52 PM
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 24 February 2010 06:40 PM
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Chris123

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a little late

Mcbs discrimiation come in two forms overload and fault current, overload is straight forward, using tracing paper draw the to curves of the two devices if the line dont cross then discrimination is achheived, if the lines do croos then discrimiation is partial to that piont,

for fault current u use the minimum peak triiping currents and the peak cut off current at the given pfc similar to the pre arc and total arc for fuses, again trace the curves or use the charts total or patial fault discrimation may be achived.
 24 February 2010 09:07 PM
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CarlCosby

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Originally posted by: Chris123


for fault current u use the minimum peak triiping currents and the peak cut off current at the given pfc similar to the pre arc and total arc for fuses, again trace the curves or use the charts total or patial fault discrimation may be achived.


Im kind of there with you on this one, but not 100%. I will get the charts for the crabtree devices. Could you explain further so that I can understand it.

Not late as not started yet

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Carl
 24 February 2010 09:20 PM
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spinlondon

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Infinity, just another milestone to those who know no limits.
 24 February 2010 09:22 PM
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CarlCosby

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Originally posted by: spinlondon

Infinity, just another milestone to those who know no limits.


did you see that prog?

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Carl
 24 February 2010 09:26 PM
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spinlondon

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No, I might have a look later though.
I was watching the one about light, and then the one about the forces of earth affecting us.
 24 February 2010 09:28 PM
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CarlCosby

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You might be too late, its only on the iplayer for 1 week.

-------------------------
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Carl
 24 February 2010 09:37 PM
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Chris123

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Hi Carl

Downstream Type b 16amp minimum peak tripping current 68 amps, upstream say type c 63 amp
pfc at 16amp =300amp so using the peak current curve we have a peak current of about 400amps
the minimum peak tripping current for a type c 63amp is 446 amp so discrimination is acheived.

if the peak current was greater than the 400amps say 1000amps then its partial upto the 446 amps

hope this helps
 24 February 2010 09:40 PM
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CarlCosby

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Originally posted by: Chris123

a little late



Mcbs discrimiation come in two forms overload and fault current, overload is straight forward, using tracing paper draw the to curves of the two devices if the line dont cross then discrimination is achheived, if the lines do croos then discrimiation is partial to that piont,



for fault current u use the minimum peak triiping currents and the peak cut off current at the given pfc similar to the pre arc and total arc for fuses, again trace the curves or use the charts total or patial fault discrimation may be achived.


Hi Chris,

Im now looking at the charts in BS 7671,

60898 6a type c, 60a to trip for 0.1-5 sec discon time
60898 50a type c, 500a to trip for 0.1-5 sec discon time.

If 510a were to flow from a fault then both would trip right? Cant see how discrimination can be achieved. Ive decided to supply the new DB in the basement with the one im changing over on the ground floor, using a 50a 3PH MCB. It wont cause too much trouble, after all its a salon not a food production factory.

-------------------------
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Carl
 24 February 2010 09:41 PM
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CarlCosby

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Originally posted by: Chris123

Hi Carl



Downstream Type b 16amp minimum peak tripping current 68 amps, upstream say type c 63 amp

pfc at 16amp =300amp so using the peak current curve we have a peak current of about 400amps

the minimum peak tripping current for a type c 63amp is 446 amp so discrimination is acheived.



if the peak current was greater than the 400amps say 1000amps then its partial upto the 446 amps



hope this helps


Hi Chris, where do you get this info from? Tried looking up technical data for the MCBs but cant find any?

-------------------------
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Carl
 24 February 2010 09:43 PM
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rocknroll

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Originally posted by: CarlCosby

You might be too late, its only on the iplayer for 1 week.


Its available for download on Megaupload here;

http://www.filestube.com/6d7df...-AC3-MVGroup-org.html

Download all four parts and when you double click on part one all of the parts will unzip into one media file to watch.

To infinity and beyond...............................

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------
 24 February 2010 09:50 PM
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Chris123

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Hi Carl the charts in Bs7671 will only show mcbs where the minimum requirements are met, many mcbs have greater energy limitng properties than the minimum standard, im using the hager curves which are differnent than those in bs7671,
its all to do with let through energy, some mcbs limit this greatly compared to the min standard.
for fault current discrimination u need to use manufactures info.
Peak current is the let through of that device at a given pfc, if this is lower than the minimum tripping current of the upstream mcb then discrimination is acheived

hope this helps
 24 February 2010 09:59 PM
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CarlCosby

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Thanks Chris, it does help. Im still a little confused, only because this isnt something I do every day. I will call electrium tomorrow to get the crabtree tech data, unless of course somebody already has it and can email the PDF to me?

carlcosby@me.com.

Edit: Ive re-read and re-read your 2nd post and now fully understand. Im surprised OMS didnt highlight this?

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Carl

Edited: 24 February 2010 at 10:07 PM by CarlCosby
 25 February 2010 10:02 AM
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OMS

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Edit: Ive re-read and re-read your 2nd post and now fully understand. Im surprised OMS didnt highlight this?


Which bit of this didn't you read Carl :

I suggest you sketch the characteristics of say a 32A Type B MCB onto the characteristics of say a 63A or 80A Type C MCB and then draw a line vertically at say 1kA PSCC and report the impact on both breakers taking into account the instantaneous tripping characteristic

As a general rule, an MCB will not discriminate with an upstream MCB in most circumstances


or this

In very simple terms, to achieve total discrimination, the total energy let through of the downstream device must be less than the pre arcing I2t of the upstream device. If it isn't - nothing on earth is going to stop that device opening even if the relative total I2t values are matched.


Regards


OMS

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Failure is always an option
 25 February 2010 10:35 AM
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CarlCosby

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I did read what you said OMS, chris123 made what he had written look/sound credible. So is chris wrong? I'm calling electrium in a mo to get hold of PDFs tech data on their crabtree range. I need to look for myself to understand it better.

Thanks for the input OMS

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Carl
 25 February 2010 11:13 AM
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OMS

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Originally posted by: CarlCosby

I did read what you said OMS, chris123 made what he had written look/sound credible. So is chris wrong? I'm calling electrium in a mo to get hold of PDFs tech data on their crabtree range. I need to look for myself to understand it better.

Thanks for the input OMS


No Chris is not wrong - he in fact pointed out that if you used the 1kA value I mentioned then discriminatiuon is only partial ie up to 446A in the example.

In practical terms for the example you would need to ensure that in excess of 68A was flowing for a fault at the load end but in excess of 446A was flowing for a fault at the DB end - any ratio in between is a function of R1 and Rn or R1and R2 circuit resistances. Clearly you can see that the circuit both needs to drop significant fault levels along it's length (a function of conductor size and circuit length) and that somewhere along the circuit length that discrimination becomes partial (assuming it was achieved in the first place).

I suggest you will discover that the constraints of design load and volt drop will conspire to ensure that the fault levels along this example circuit would be such that total discrimination is not achieved.

As I said, MCB's on MCB's is a difficult scenario as the instantaneous tripping characteristic is broadly similar across a wide variety of ratings (or alternatively you only have very narrow grading margins into which the actual parameters of circuit design need to fit).

My advice would be to get some curves for fuses in the first instance - the charcteristics make discrimination studies simpler - get to grips with that first and then substitute the fuses for MCB's and see if you can still maintain discrimination.

Firstly though you really need to think abouit why you want to achieve discrimination in a system and what are the consequenses of partial or total failure - as I said, in many cases it's not really necessary

Regards

OMS

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Failure is always an option
 25 February 2010 11:31 AM
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CarlCosby

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Thanks OMS, bet your hand is aching after typing that!

Maybe I think too much into things, I just want my mates shop to be as good as it can be. To be honest it's a salon, if a fault in the basement part of the install causes it's 50a sub to trip then it's not the end of the world, because the remaining part of the salon still has power. After all it's only a salon not a food prodution factory as mentioned before.

I spoke to crabtree, he confirmed that discrimination can be achieved to a point. He also noted that they are bringing out a new range of domestic MCBs in a months time, something to do with a new supplier?

He will be emailing me PDFs for their range of MCBs tomorrow, if anybody wants em just holla.

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Carl
 25 February 2010 11:50 AM
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OMS

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I spoke to crabtree, he confirmed that discrimination can be achieved to a point.


That is exactly the point Carl - it is often impossible to engineer the circuit parameters with MCB's - essentially it's the same fault current that causes tripping anywhere betwen 0.1 seconds and 5 seconds

Regards

OMS

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Failure is always an option
 25 February 2010 01:53 PM
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AJJewsbury

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Mcbs discrimiation come in two forms overload and fault current, overload is straight forward, using tracing paper draw the to curves of the two devices if the line dont cross then discrimination is achheived, if the lines do croos then discrimiation is partial to that piont,

The overall device characteristics are really described by an area of the graph, rather than a line - BS 7671 charts only show the slowest edge.

Don't know if this is of any use to anyone, but it might provide some background: http://www.voltimum.co.uk/file...reaker_protection.pdf

- Andy.
IET » Wiring and the regulations » Discrimination of MCBs???

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