Choosing MCB for cable overload protection

[Mids]

Finally I found it!

Russian Standard 50571.5-94 links to P-IEC 60364-4-43 ed1.0 (withdrawn, publication date 1977-01-01).

Russian Standard 50571.5-94

Overload protection

1) Ib <= In <= Iz
2) I2 <= 1.45Iz

Note
Protection in accordance with this paragraph does not provide complete protection in some cases, such as long-term overcurrent smaller in value than I2, and do not always cost-effective solution.
It is assumed that the electric network is designed so that the small
overload with long duration will not occur often.

My mistake. Sorry. I read reg. 433.1 not well. I missed note 2.

BS7671

433.1
Every circuit shall be designed so that a small overload of long duration is unlikely to occur.

433.1.1
...........
NOTE2
Protection in accordance with this regulation may not ensure protection in all cases, for example, where sustained overcurrents less than I2 occur.

So. Cables are not derated 50%.

IMHO. Note2 applies to light circuit and outlet circuit. I2<=Iz is better requirement for overload protection in that case.

 

[Chr!s]

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.


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.
View attachment 22005

Okay, you have two lines, I1 and I2, the top half is thermal, the Middle a mixture and the bottom magnetic, the above is a C type I1 is the trailing edge and I2 is the leading, when looking at the magnetic section of the curve trailing I1 = ×5 and leading I2 = ×10. So as I said 1.13 sits next to the trailing edge, 1.45 sits on the leading and your 1.4 (14) sits to the left of the leading(I2) in the red. The device will trip in an unspecified time, so thermal protection not guaranteed.

I disagree 1.4 In will trip the device, when you look at the top section (thermal) in the middle of the red section there would be a line this would be the nominal seting, the area either side is the variations of the device. So your 1.4 In sits between the nominal setting and the leading edge I2.

Yes you use mutual heating when MCBS are grouped, though I think you will find that at 1.4 In the heat produced will lower 1.45.

 

[Mids]

Thank you, Chr!s. Now I understand your position. Difference in our positions about 1.4In: I'm saying - tripping is not guaranteed, you are saying - tripping in an unspecified time. Not big difference. Idea about middle curve seems logical, because MCB is mostly analog device that has a big tolerance as you said.

The main thing that we have achieved - reg 433.1.1 doesn't protect against sustained overcurrents.
I would not ask questions, if I saw note2.

Thanks, everyone. I think question is closed.

 

[Chr!s]

Thank you, Chr!s. Now I understand your position. Difference in our positions about 1.4In: I'm saying - tripping is not guaranteed, you are saying - tripping in an unspecified time. Not big difference. Idea about middle curve seems logical, because MCB is mostly analog device that has a big tolerance as you said.

The main thing that we have achieved - reg 433.1.1 doesn't protect against sustained overcurrents.
I would not ask questions, if I saw note2.

Thanks, everyone. I think question is closed.

Hi, yes not a big difference, basically thermal protection not guaranteed.

With regard to heating effect, if the mcb is tested to 30 C then at 40 C we derate. If we assume bi matalic strip 70 C x In and 114 C at 1.45 In, if temperature in bi matalic strip is proportional to the square of the current then you can see the need for derating.

At 1.4 In the bi mstalic strip would say be st 110C close to 114 C operational temperature.

At this sustained current would the ambient air around the bi metallic strip still be at 30 C

 

[Mids]

Thermal unit characteristic already based on heating effect and represented on the graph. You shouldn't take into account MCB self heating.

if temperature in bi matalic strip is proportional to the square of the current then you can see the need for derating.

Lets construct close dependences.
T - temperature
I - current
t - time to trip
T ≈ I^2
t ≈ 1/T ≈ 1/(I^2)
Build graph of that function t = 1/(I^2). It will be close to thermal unit characteristic.

Update
Btw.
70°C ≈ I^2
147,175°C ≈ (1.45I)^2
137,2°C ≈ (1.4I)^2

147,175-137,2 ≈ 10°C its more than 4°C :stuart:

 

[Chr!s]

[QQUOTE=Mids;882099]Thermal unit characteristic already based on heating effect and represented on the graph. You shouldn't take into account MCB self heating.


Lets construct close dependences.
T - temperature
I - current
t - time to trip
T ≈ I^2
t ≈ 1/T ≈ 1/(I^2)
Build graph of that function t = 1/(I^2). It will be close to thermal unit characteristic.

Update
Btw.
70°C ≈ I^2
147,175°C ≈ (1.45I)^2
137,2°C ≈ (1.4I)^2

147,175-137,2 ≈ 10°C its more than 4°C :stuart:[/QUOTE]

Lol

I didn't work it out, just trying to point out how the temp of the bi metallic strip would effect the ambient temperature around it so reducing the 1.45.

I think we got there in the end

 

[Mids]

Your assumption is not pointless if you have tradition to hide from the cold your MCBs deep into wall and cover it by insulation. :biggrin5:

Ofcourse, you're right in case such as MCB placed in small insulated space. It would be good to look it in regs.

 

[Chr!s]

Your assumption is not pointless if you have tradition to hide from the cold your MCBs deep into wall and cover it by insulation. :biggrin5:

Ofcourse, you're right in case such as MCB placed in small insulated space. It would be good to look it in regs.

Lol

Lets remember the 1.13 and 1.45 are from cold start conditions.

How do you explain the hot start time current graph compared to the cold start, there is a big shift in the thermal release lines I1 and I2?

 

[Mids]

I was waiting when you say it.

Where did you find hot start time current graph?

There are mismatches in ABB docs:
http://www05.abb.com/global/scot/sc...257ad7004ec86a/$file/2CDC002157D0202_view.pdf
On the graph represented tripping curves from cold state. But in the table "Tripping characteristics" I2 given for operating temperature. Where is true?

Lets look at the main source - IEC 60898-1 (BS EN 60898). There is from hot state.

 

[Marvo]

The tripping time will change with ambient temperature of the MCB but the current carrying capability of the wire or cable will also change in the same direction. Protection of cables by thermal MCB's is not particularly accurate but because of the safety margins in the regsand guidelines it also doesn't need to be.

 

[Chr!s]

I was waiting when you say it.

Where did you find hot start time current graph?

There are mismatches in ABB docs:
http://www05.abb.com/global/scot/sc...257ad7004ec86a/$file/2CDC002157D0202_view.pdf
On the graph represented tripping curves from cold state. But in the table "Tripping characteristics" I2 given for operating temperature. Where is true?

Lets look at the main source - IEC 60898-1 (BS EN 60898). There is from hot state.
View attachment 22061

So if i apply 1.45 x In with a cold start we attain a temperature lets say 114 C (thermal operation Temp) I2 = 1.45 in one hour.

If i apply 1.45 x In at hot start, 1.45 In = I1 in one hour(well very close too).

So if the Ambient Temperature was constant at 30 C, why the change in Thermal characteristics, the Bi metallic strip would attain the same temperature.

As i said in a earlier post, their are watt losses in the MCB the heat generated effects the thermal characteristics.

 

[Mids]

Chr!s, I dont understand you again.

Basic document, IEC 60898-1 (BS EN 60898), says that thermal unit will trip with current 1.45In less than one hour after hot start (operating with I=1.13In for one hour). We should use it at first.

ABB document says 2 different things:
1) less 1 hour from cold start; (graph)
2) less 1 hour from hot start. (table)
"< 1 hour" its not "= 1 hour" as you say. So case 1 and case 2 could have different time to trip.
Ofcourse, in second case time to trip will be less. In fact longer time in case 1 is not a problem, because cable has cold start too.

If MCB is not insulated, its characteristics don't change because of self heating. Because MCB heat doesn't change ambient temperature.
Ofcourse, MCB placed in small insulated space will trip faster because heat can't dissipate and ambient temperature increases and changes thermal unit characteristics. If you can calculate this effect and link to calculations in BS7671 or other document, its great. Otherwise, you should use worst case - operating at temperature 30C, because in that case time to trip is longer.

 

[Mids]

The tripping time will change with ambient temperature of the MCB but the current carrying capability of the wire or cable will also change in the same direction. Protection of cables by thermal MCB's is not particularly accurate but because of the safety margins in the regsand guidelines it also doesn't need to be.

I mostly agree with you. Its ideal case. But I have example - MCB in box, placed outside, protects cable insulated in wall. Cable has worst conditions to dissipate heat.

 

[Engineer54]

Chr!s, I dont understand you again.

Basic document, IEC 60898-1 (BS EN 60898), says that thermal unit will trip with current 1.45In less than one hour after hot start (operating with I=1.13In for one hour). We should use it at first.

ABB document says 2 different things:
1) less 1 hour from cold start; (graph)
2) less 1 hour from hot start. (table)
"< 1 hour" its not "= 1 hour" as you say. So case 1 and case 2 could have different time to trip.
Ofcourse, in second case time to trip will be less. In fact longer time in case 1 is not a problem, because cable has cold start too.

If MCB is not insulated, its characteristics don't change because of self heating. Because MCB heat doesn't change ambient temperature.
Ofcourse, MCB placed in small insulated space will trip faster because heat can't dissipate and ambient temperature increases and changes thermal unit characteristics. If you can calculate this effect and link to calculations in BS7671 or other document, its great. Otherwise, you should use worst case - operating at temperature 30C, because in that case time to trip is longer.

Why not put all your questions relating to ABB's MCB data to ABB's customers technical support dept?? I'm sure they will give you all the answers/reasoning you are seeking. But, ......will you be satisfied with those answers and reasoning, somehow i think not ...lol!!

 

[Mids]

Engineer54, if you read this thread, you wouldn't ask your question.

Question of this thread (why reg. 433.1 doesn't protect cable totally) is closed already - look posts #31 and #33. And I have no questions to ABB.

As I can see you are satisfied to discuss a person instead a question. It makes you laught. I'm happy for you.


Btw, thread can be closed if nobody doesn't mind and if Engineer54 doesn't want to talk about my person anymore.