Choosing MCB for cable overload protection

[Mids]

As I said russian analog of BS is older - 2000y last edition.
Circuit-breaker IEC 60898-1 (International Standard) released in 2003.
So i thought, that BS, edited in 2008, could have more relevant requirements for overload protection.
Russian Electrical Regulations have only 2 requirements Ib <= In, In <= Iz.

I don't really compare standards. I try to find reason of requirement 1.45Iz >= I2 in BS.

Physics one for all.

 

[Chr!s]

Appendix 4, BS7671:2008


My mistake. I read it:
"1.45Iz >= I2 for BS 88, BS 1361,BS EN 60898, BS EB 60947-2;
In <= Iz for BS EN 61009-1 (RCBO) only"

ОК. Now i get it. We don't need to check 1.45Iz >= I2 for these devices.
But fuse BS 88 has I2=1.6In, if I don't mistake. Look at fig 3.3A, 3.3B in BS7671.

I still see only one explanation that cable in my example doesn't overload - because cables being derated 50% as telectrix said.

Btw, there is only radial circuits in Russia, as I know.

Okay take your example

Example.
Ib=9A
In=10A, I2=1.45In (MCB to BS EN 60898)
Iz=11A

1. Ib < In success (just for circuit design)
2. In < Iz success
3. 1.45Iz > I2 success

One day current in circuit (I) increases to 14A. CB will not operate because I < I2 or 14A < 14,5A. Cable will be overloaded because I > Iz or 14A > 11A.




You select a Cable with an Iz greater/= In, and a In greater/= IbMCB 10 Amp (Inf)Non fusing current 1.13In
(I2) Fusing Current 1.45In

Obviously you want your 10 amp MCB to be able to carry the Full 10 AMPS (Ignoring any derating requirements)

So BS7671 states small overloads of long duration need to be avoided avoided.

So your MCB must not trip within 1 hour at a rating of 11.3 amps and must trip within 1 hour at 14.5 amps, so you 14 amp overload will trip the device, it just may not be in the conventional time. Though designed correctly this shouldn't happen.

The 1.45 comes from testing and the calculated withstand of the cable. Providing the Protective device trips within the conventional times the no deterioration will occur to the cable, no limit as such is placed on the number of overloads the cable can be exposed to before ageing of the cable will be effected, obviously these instances need to be kept to a minimum.

 

[Mids]

so you 14 amp overload will trip the device, it just may not be in the conventional time. Though designed correctly this shouldn't happen.

Look at fig 3.5 in BS7671:2008. MCB with In=10A will no operate at 14A.
Or you can look at ABB made MCB time-current curve here (page 4): http://www05.abb.com/global/scot/scot209.nsf/veritydisplay/44c5b5ac208f3f25c1257ad7004ec86a/$file/2CDC002157D0202_view.pdf
Even BS EN 60898 says: I<1.13In operating is not guaranteed, I>1.45In operating is guaranteed.
I think we shouldn't hope for 1.4In operating.

PS I changed link to english version of document.

 

[Chr!s]

Look at fig 3.5 in BS7671:2008. MCB with In=10A will no operate at 14A.

But you are only taking into account the Leading edge of the device, Type B trailing X3 In and Leading X5 In.

You also need to take into account the +/1 20% tolerance(60898).

Lets not forget also as the Temp of the MCB at 14 Amp is now increased, this heating effect will decrease the 1.45 to a lower value.


Or you can look at ABB made MCB time-current curve here (page 349): http://www05.abb.com/global/scot/scot209.nsf/veritydisplay/d0161b51fd055159c1257a3e001ee675/$file/SYSTEM_PRO_2012.pdf
Even BS EN 60898 says: I<1.13In operating is not guaranteed,

It states it must not trip within 1 Hour, and that depends upon MCB rating!

I>1.45In operating is guaranteed.

I = 1.45 must trip within 1 hour

I think we shouldn't hope for 1.4In operating.

I will have a read through 60898

 

[Mids]

But you are only taking into account the Leading edge of the device, Type B trailing X3 In and Leading X5 In.

Sorry, I don't understand you. 3In and 5In are characteristics of short circuit protection.

You also need to take into account the +/1 20% tolerance(60898).

Where I can find information about this?

Lets not forget also as the Temp of the MCB at 14 Amp is now increased, this heating effect will decrease the 1.45 to a lower value.

I can't agree with you. Operation of thermal trip unit is based on heating effect of continuous current. You cant heating effect "multiply" on heating effect. Thermal trip time-current characteristics are represented on the graph. In other words, 1.13In heating effect is not enough to trip bimetallic strip, 1.4In is not enough too and 1.45In is already enough.

Ofcourse, operation of MCB depends upon an ambient temperature.

It states it must not trip within 1 Hour, and that depends upon MCB rating!
I = 1.45 must trip within 1 hour

Your remark is true. The graph confirms it.

 

[Chr!s]

Sorry, I don't understand you. 3In and 5In are characteristics of short circuit protection.

Okay, when you look at the curves for a given MCB you will see two lines, the trailing and the leading. From the bottom you have the magnetic part of the MCB as you move up yo then move into the thermal area. So if you draw your 10 x 1.13 you will see its sits next to the trailing edge, draw your 10 x 1.45 and you will see its sits on your leading edge, now draw out your 14 amp and you will see it sits to the left of the leading edge, so as i said it will trip the device just in a unspecified time.



Where I can find information about this?



I can't agree with you. Operation of thermal trip unit is based on heating effect of continuous current. You cant heating effect "multiply" on heating effect. Thermal trip time-current characteristics are represented on the graph. In other words, 1.13In heating effect is not enough to trip bimetallic strip, 1.4In is not enough too and 1.45In is already enough.

Well i think you will find it does have an effect to a degree, same with fuses, one of the reasons why some fuses even though they have a factor of 1.6 are deemed to meet the 1.45.

Ofcourse, operation of MCB depends upon an ambient temperature.

And don't forget Mutual heating, as i said above there are watt losses in the MCB the more current the more losses the lower the fusing current becomes.


Your remark is true. The graph confirms it.

P.s my error on the 20% this apply's to the magnetic part not the thermal.

 

[Mids]

Thank you all for discuss. I wish you happiness, smiles and health in New Year! Protect you wires! Don't drink too much, leave something to me. :icon10:

Chr!s, I will reply in NY.

 

[Mids]

Okay, when you look at the curves for a given MCB you will see two lines, the trailing and the leading. From the bottom you have the magnetic part of the MCB as you move up yo then move into the thermal area. So if you draw your 10 x 1.13 you will see its sits next to the trailing edge, draw your 10 x 1.45 and you will see its sits on your leading edge, now draw out your 14 amp and you will see it sits to the left of the leading edge, so as i said it will trip the device just in a unspecified time.

I have a problem - language barrier. I don't understand what is the trailing line and what is the leading line. The trailing line is bottom? The leading line is top? So 1.4In doesn't cross the leading line.

I'll try to describe how I read the graph. Look at my picture. Red area - thermal unit, blue area - magnetic unit. The bottom curve, started in point I1, means non-tripping of thermal unit. The top curve, started in point I2, means tripping of thermal unit. The area placed between the top and bottom curves means that unit may to trip or may not to trip.
For example, I=3In. So thermal unit doesn't trip in 3 sec (its guaranteed). Thermal unit trips in about 1 min 20 sec (its guaranteed). Between 3 sec and 1 min 20 sec unit may to trip or not.
I saw MCB testing video. Usually MCB tripped in time between these curves.
If we look at vertical line I=1.4In we can see that it cross only bottom curve. It means that unit may trip in time between 20 sec and infinity. Middle is infinity. So there is no guarantee that unit will trip and cable will be protected.

And don't forget Mutual heating, as i said above there are watt losses in the MCB the more current the more losses the lower the fusing current becomes.

You can use mutual heating rate when you have several MCBs side by side.

Operation of thermal trip unit is based on heating effect of continuous current. It's represented on the graph. MCB characteristics doesn't change cause of self heating because it based on self heating (bimetallic strip heating).

Original image edited by me.

 

[Engineer54]

Why are you asking a question, and then telling those that answer you, No they are wrong??

If you know how each element of the thermal/magnetic protection curve operates, then why ask in the first place, cause it seems you know more than anyone that's replied in this thread so far!! lol!!

 

[Mids]

I want to find a reason of requirement I2<=1.45Iz. I asked this question, because somebody maybe knows the answer or can help to find it if he is interested. It's normal discussion - assumptions, refutation. And I admitt my mistakes. Look above.

In fact I'm not agree with Chr!s only in some questions. Maybe I'm not right.

Do I behave not correctly? Sorry.

 

[Chr!s]

I want to find a reason of requirement I2<=1.45Iz. I asked this question, because somebody maybe knows the answer or can help to find it if he is interested. It's normal discussion - assumptions, refutation. And I admitt my mistakes. Look above.

In fact I'm not agree with Chr!s only in some questions. Maybe I'm not right.

Do I behave not correctly? Sorry.

As I said, its a comprise.

The MCB offers what we call close protection, but you wont find a MCB that trips the moment it exceeds In.

We have what we call the conventional time and this relates to the fusing current. In your example its 1.45 in and must trip within 1 hour (conventional time).

Through testing, experience and calculations relating to cable thermal withstand a figure of 1.45 has been derived. And what the IEC standard states is that if the protective device operates within the conventional time no damage will occur to the cable, so won't effect its overall life span. Thats not to say repeated overloads wont eventually efect the life span, hence the warning in BS7671 with respect to small overloads.

 

[Mids]

So we turn back what telectrix said - cables being derated 50%?

 

[Chr!s]

So we turn back what telectrix said - cables being derated 50%?

No, theres no need to derate the cable unless the fusing current exceeds 1.45. So 3036 fuse has a fusing curent of 2, so 1.45/2 = 0.725 which then is applied to the cable calculations.

 

[Mids]

I mean that tabulated CCC already derated 50%.

 

[Chr!s]

I mean that tabulated CCC already derated 50%.

The CCC of the cables sre based on 70 C, if we take you example the cable would reach a Temp of 114 C under overload.