Choosing MCB for cable overload protection

[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.

 

[Darkwood]

Cable rating have a buffer applied before been printed in tabled form in the BS7671 you can check this by ringing any manufacturer and discuss their CCC of the cable and compare them to the BS7671...

Other regulations also cover the fact that a circuit should thus be designed so that a small overload cannot occur for the long duration.. this clearly covers the grey area of devices not tripping in time.

Mcb's may also need a grouping factor applying to them if they are next to other units be a mcb's contactors etc even in distribution boards, these factors can be sourced from the manufacturer and also can depend on loading of the mcb... themselves.

All this will negate the need for a allowance to be factored on your cable size if you comply with reg 433.1.2 where compliance with 433.1.1. (i) and (ii) will automatically mean you comply with (iii).

You can debate this till the cows come home but if you are in a situation where you are loading the cable over the mcb rating but less than 1.45 x the mcb rating for a long duration then you haven't complied with 433.1 this is why you don't need to compensate for it because following 433.1 means it should not happen.

 

[Mids]

Cable rating have a buffer applied before been printed in tabled form in the BS7671 you can check this by ringing any manufacturer and discuss their CCC of the cable and compare them to the BS7671...

Sorry, I have a language barrier. Do you mean that manufacturer CCC and CCC in BS7671 differ?

Other regulations also cover the fact that a circuit should thus be designed so that a small overload cannot occur for the long duration.. this clearly covers the grey area of devices not tripping in time.

Mcb's may also need a grouping factor applying to them if they are next to other units be a mcb's contactors etc even in distribution boards, these factors can be sourced from the manufacturer and also can depend on loading of the mcb... themselves.

All this will negate the need for a allowance to be factored on your cable size if you comply with reg 433.1.2 where compliance with 433.1.1. (i) and (ii) will automatically mean you comply with (iii).

You can debate this till the cows come home but if you are in a situation where you are loading the cable over the mcb rating but less than 1.45 x the mcb rating for a long duration then you haven't complied with 433.1 this is why you don't need to compensate for it because following 433.1 means it should not happen.

I agree with you at all.

Discussion stretched because I didn't see note 2.

 

[Engineer54]

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.

Your original question had been answered many posts ago, not my problem that you don't or can't understand what has been explained!! Hopefully Darkwood has put the lid on it for you, in his last 2 paragraphs of post #46....

 

[Darkwood]

Sorry, I have a language barrier. Do you mean that manufacturer CCC and CCC in BS7671.

In a nutshell yes for various reasons consider the differing thermal attributes of the insulation and outer environmental protective sheaths between different brands of identical cables this alone would give different CCC of a cable in the BS7671 ... i often design machine controls that require special cables with specific attributes and you would be amazed how small variations in composition between brands can lead to a noticeable CCC difference.

That in mind then the regs has to be tabled for the worse case scenerio in that cable manufacturers will have to make their cables to a BS standard but as long as the cables meets this there could be still a difference in CCC so the BS 7671 table their cable CCC with regards the minimum CCC that the cable manufacturers have to meet..anything over this can save a lot of money if you get your CCC from the manufacturer rather than the BS7671.

Large projects commonly use manufacturers details of cable CCC and disregard the tabled CCC of cables in the regs.

Even taking into account they will put the minimum tabled CCC that a cable of that type and size would have they will also apply a small buffer for error (i assume - don't quote me on that one)

 

[Chr!s]

Cable rating have a buffer applied before been printed in tabled form in the BS7671 you can check this by ringing any manufacturer and discuss their CCC of the cable and compare them to the BS7671...

Other regulations also cover the fact that a circuit should thus be designed so that a small overload cannot occur for the long duration.. this clearly covers the grey area of devices not tripping in time.

Mcb's may also need a grouping factor applying to them if they are next to other units be a mcb's contactors etc even in distribution boards, these factors can be sourced from the manufacturer and also can depend on loading of the mcb... themselves.

All this will negate the need for a allowance to be factored on your cable size if you comply with reg 433.1.2 where compliance with 433.1.1. (i) and (ii) will automatically mean you comply with (iii).

You can debate this till the cows come home but if you are in a situation where you are loading the cable over the mcb rating but less than 1.45 x the mcb rating for a long duration then you haven't complied with 433.1 this is why you don't need to compensate for it because following 433.1 means it should not happen.

Most Cable Manufacturers use ERA 69-30, and this assume Air ambient of 25C, Ground temp 15C, Soil Resistivity 1.2k.W/m and depth of lay 0.8m, if you apply the factors to those in BS7671 you will achieve pretty much same values as the Cable Manufacturers.

 

[Darkwood]

I use Lapp and Igus cabling a lot and thus this isn't spec'd within the BS7671 so end up seeking manufacturing tables... i agree most market bought cables from your typical wholesaler will be similar but not all cases some do have better thermal dissipation; for your relative small jobs be it commercial or industrial then the difference is usually insignificant but the larger installs can see a saving by choosing carefully who's cable you use.

 

[Mids]

Your original question had been answered many posts ago, not my problem that you don't or can't understand what has been explained!! Hopefully Darkwood has put the lid on it for you, in his last 2 paragraphs of post #46....

Please, show me the post with answer.
Maybe, #10 ?

There is also a clause in the regs that any circuit design should not allow for a small overload for a long duration, as this can't be guaranteed with a ring main they have to allow for a safety net so cables are not over-rated with regards to a small overload which could mean the overload protection could take many hours to trip and exceed the cables rating .... if this occurs it can't dissipate its generated heat fast enough and you get a runaway temp' rise without the overload protection tripping i time to stop cable damage or worse.

Sorry Tel but ill contradict your comment as insulation should be factored in to design if its likely to be added in say a loft at a later date whether by a known arrangement or simply the fact that its likely to occur with the energy efficiency schemes in the near future.

The 1.45% is not because of possible insulation its still applied after you calculate a cable running through insulation if the design requires you to allow for it; if you comply with regs 433.1.1 part (i) and (ii) and are using a BS 6088, 6047-2,61009-1 or BS 88-2.2 or a BS86-6 this will result in compliance automatically with 433.1.1 (iii) thus you don't need to apply it separately.

I have to admit, I understood it not well or translated not well. I decided that darkwood talks about ring main, that we don't use in Russia as said.
I didn't understand:

this can't be guaranteed with a ring main they have to allow for a safety net so cables are not over-rated with regards to a small overload which could mean the overload protection could take many hours to trip and exceed the cables rating

and

The 1.45% is not because of possible insulation its still applied after you calculate a cable running through insulation if the design requires you to allow for it

Ring main can have small overload for a long duration because user can connect any load? OK
In other cases (not ring main) cables are over-rated? Why?

I understood nothing about 1.45% and insulation.

Where I can see that answer?

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

 

[Darkwood]

My comments apply to any circuit not just ring mains ... cables are not over-rated if the install is designed correctly assuming all derating factors have been applied for installation methods and external influences accounted for too.

Without reading through the whole thread can i ask if you have allowed for all other derating factors when calculating cable size if not the a cable may seem larger then required because your not aware it goes through insulation for example... i don't understand your comment that cables are oversized ?

 

[Mids]

Even taking into account they will put the minimum tabled CCC that a cable of that type and size would have they will also apply a small buffer for error (i assume - don't quote me on that one)

Russian analog of BS7671 and cable standard allow overload 16% for a time.

 

[Mids]

Seems to me telectrix and Boydy were mistaken saying that my english is good. :icon10:

I don't understand everything.

Darkwood, don't get me wrong. Can I ask you put some dots and commas in your posts?

For exapmle, I couldn't understand long time your post. I read it at first like this:

Without reading through the whole thread can i ask if you have allowed for all other derating factors when calculating cable size, if not the a cable, may seem larger then required because your not aware it goes through insulation for example... i don't understand your comment that cables are oversized ?

Right version:

Without reading through the whole thread can i ask if you have allowed for all other derating factors when calculating cable size.
If not, a cable may seem larger then required because you not aware it goes through insulation for example.
I don't understand your comment that cables are oversized ?

Could you rewrite in other words this message?

There is also a clause in the regs that any circuit design should not allow for a small overload for a long duration, as this can't be guaranteed with a ring main they have to allow for a safety net so cables are not over-rated with regards to a small overload which could mean the overload protection could take many hours to trip and exceed the cables rating

Now I understand this:

The 1.45% is not because of possible insulation. Its still applied after you calculate a cable running through insulation if the design requires you to allow for it

I know what derating factors means.
Btw, this equation looks bad for understanding:
It >= In/(Cg x Ca x Ci x Cc)
What do we derate - protective device or cable CCC? Ofcourse, CCC.

Looks better for understanding:
Cg x Ca x Ci x Cc x It >= In

 

[Mids]

------

 

[Darkwood]

There is also a clause in the regs that any circuit design should not allow for a small overload for a long duration, as this can't be guaranteed with a ring main they have to allow for a safety net so cables are not over-rated with regards to a small overload which could mean the overload protection could take many hours to trip and exceed the cables rating.


The regulations BS7671 state that if you design a circuit you need to ensure that in your design a circuit cannot have a small overload for a long period of time. If you adhere to this regulation then cables do not have to be larger to account for overloads > mcb rating but less than 1.45 times .

If you find you have a circuit in such a situation where it is been overloaded but <1.45 times the mcb rating then you have not designed the circuit correctly.

As for my punctuation i apologise, its more my crap keyboard missing key strokes rather than my poor English.