Choosing MCB for cable overload protection

[Mids]

Hello. I need advice about choosing MCB for cable overload protection.

I'm using BS7671:2008 Requirements for Electrical Installations.
Reg 433.1.1 has 3 requirements:
1. Ib <= In
2. In <= Iz
3. 1.45Iz >= I2
I don't understand reason of 1.45Iz >= I2.
And I think this requirements are not enough to cable overload protection.

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.

Sorry for my english.

 

[telectrix]

you would ideally select a cable size where Iz > 1.45 In. although cable rated at 11A will happily stand 14A due to the safety factor allowed in BS7671.

 

[Mids]

you would ideally select a cable size where Iz > 1.45 In

I agree with that.

although cable rated at 11A will happily stand 14A due to the safety factor allowed in BS7671.

I want to understand the reason of this safety factor.
Why are you shure that cable happily stands overload of almost 30%? Practice?

 

[telectrix]

well, the regs. allow for a 145% overload to trip the MCB in 1 hour, also, personal experience. i have had a 16mm T/E cable running at 100A for 30 minutes and measured it's temperature at no more than 40deg. where things go ---- up is when the muppet loft insulators bury everything in a foot of fibreglass.

 

[Mids]

I think you example is not enought :biggrin5:
Because russian (I'm from Russia) cable standart, based on IEC60502-1:2004, says that PVC copper 16mm cable in air stands 89A and 89x1.16=103.24A in overload (probably overload means temporary overload). So 100A is not problem for this cable. I don't know what is T/E cable, but i think its not a big diffirence.

Why I ask about all this you - englishman? Because last russian standard, like BS7671m is older than BS (2000y). It says In < Iz. It hasn't requirement I2<=1,45Iz. But for MCB (EN 60898) I2=1,45In, so 1,45In<=1,45Iz and In <= Iz. Same.

I'm not pretty good know english language. At first i didn't understand, what mean "go ---- up" :biggrin5:
I know what "----" means. But ---- and cables together.. Now i get it - "stop functioning". What does mean "the muppet loft insulators"?

 

[telectrix]

over here, there is a government scheme for free loft ( attic) insulation. the idiots that install it just blanket over cables and then stick up notices to say that it's not safe to enter the attic. never mind cables being derated 50%. sorry about the "---- up". it's an expression we use when not allowed to use the F word.

edit: and your english is very good.

 

[guest119]

Agreed, well done your english is good.
t/e is twin and earth. Flat twin cable with an earth core. Sometimes known as pvc/pvc or twin and skin.
Available here up to 16mm2 with a 10mm2 earth. Personally havent seen any bigger.

Boydy

 

[Mids]

cables being derated 50%

I prefer docs for proof. But if english, russian rules and you say the same so.. I should to give up. Maybe.. :biggrin5:

sorry about the "---- up". it's an expression we use when not allowed to use the F word.

Now I know what to say instead F in England on public if i would know that russian football team lost in game vs english team.

edit: and your english is very good.

Thank you!

And thank you, [URL="http://www.electriciansforums.co.uk/members/27755.html"]Boydy[/URL].

 

[Geoffsd]

It says In < Iz. It hasn't requirement I2<=1,45Iz. But for MCB (EN 60898) I2=1,45In, so 1,45In<=1,45Iz and In <= Iz. Same.

Yes. In the tables all Current Carrying Capacities of cables are given for using Miniature Circuit Breakers so when using MCBs you do not have to worry about I2.

MCBs have a fusing factor of 1.

If using rewirable fuses (BS3036) which have a fusing factor of 2 then the CCC of the cable must be derated by 0.725 (CCC x 0.725).

This is the same as CCC x 1.45 / 2.

 

[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 .... 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.

 

[Mids]

Yes. In the tables all Current Carrying Capacities of cables are given for using Miniature Circuit Breakers so when using MCBs you do not have to worry about I2.
MCBs have a fusing factor of 1.

Ooh! It can be close. I see you refer to Appendix 4. Does protective devices (BS 88, BS EN 60898, BS EN 61009-1) have same I2/In=1.45? Maybe that devices shouldn't have it same because have diffirent principles?

Where I can find CCC tables, which I can use with BS7671?

 

[Darkwood]

The tables for CCC are the same regardless of front end protection and as above when working out the final ccc after all you have done all the cable calcs the 1.45 factor will already be allowed for if your device you use is mentioned in 433.1.2 and you comply with (i) and (ii) of 433.1.1

 

[GLENNSPARK]

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.

and the best way to ensure this is to keep cable runs away (above) the level of any future applied insulation.....this could be by clipping cables to rafters or in the birdsmouths (perimiter)....just a little forward thought.....

the problems come when cables have to enter ceiling mounted accessories (lighting, pullswitches etc).....not too bad where lighting is concerned.....shower cables though a different matter...

 

[Mids]

Appendix 4, BS7671:2008

This means that the operating current of the protective device must not exceed 1.45Iz. Where the protective device is a fuse to BS 88 or BS 1361, a circuit-breaker to BS EN 60898 or BS EB 60947-2 or a residual current circuit-breaker with integral overcurrent protection to BS EN 61009-1 (RCBO), this requirement is satisfied by selecting a value of Iz not less In.

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.

 

[Darkwood]

The key here is BS .... British Standards ... they may not be the same and/or comply in Russia if the circuit design is for a system in russia you need to follow their own standards.

 

[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.

 

[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.