Relationship of MCB size to cable size

Thank you, PrenticeBoyofDerry.
I got BS7671. As I suppose i should read Appendix 6.
I didn't find requirement 1.45Iz ≥ I2
 
I want to clarify. I started this topic to find out source of requirement 1.45Iz ≥ I2.

To be shure. I look "ON-SITE GUIDE BS 7671:2008. lEE Wiring Regulations 17th Edition". Does Appendix 4 have title "Methods of support for cables, conductors and wiring systems" ?
There is nothing about choosing MCB. :D
 
PrenticeBoyofDerry said:
In the meantime I will find where this information is within the OSG for you.
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Thank you. It's not necessary. I got BS7671:2008 Requirements for Electrical Installations. But it has red jacket instead green.
I'm reading Appendix 4 "Current-carrying capacity and voltage drop for cables and flexible cords" now.


This means that the operating current of the protective device must not exceed 1.45Iz.
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"operating current" what does it mean? Device rating (In) or tripping current (I2) ?

I found
The rated current (or nominal current) is the current the device is rated at for continuous service. The operating current I2 is the current causing operation of the device in conventional time.
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What can you say about this 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 < 14,5 A. Cable will be overloaded because I > Iz or 14A > 11A.

_______________________________
BTW
http://www.tlc-direct.co.uk/Book/3.6.5.htm
Thus, the current at which the protective device operates must never be greater than 1.45 times the rating of the smallest cable protected. For example, consider a cable system rated at 30 A and protected by a miniature circuit breaker type 3, rated at 32 A. Reference to (Fig 3.17 shows that a prolonged overload of about 38 A will open the breaker after about 104 seconds (about two and a half hours!). The ratio of operating current over rated current is thus 38/30 or 1.27, significantly lower than the maximum of 1.45. All circuit breakers and HBC fuses listed in {3.6.2 sections 2 and 3} will comply with the Regulations as long as their rating does not exceed that of the smallest cable protected.
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There is mistake - 38A verti&#1089;al line and CB 32A curve don't cross :mrgreen:
 
What you seem to be concerned about is that there is a range of currents at which the MCB will not trip, but the current is greater than the rated current-carrying capacity of the cable? Theoretically, that's correct, however the standards for circuit-breakers take that into account by providing sufficient safety margin that damage to cables is unlikely.
 
stillp said:
What you seem to be concerned about is that there is a range of currents at which the MCB will not trip, but the current is greater than the rated current-carrying capacity of the cable? Theoretically, that's correct, however the standards for circuit-breakers take that into account by providing sufficient safety margin that damage to cables is unlikely.
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Isn't it probably the other way around? - i.e. that the tabulations of maximum current-carrying-capacities that we usually work with (e.g. in Appendix 4 of BS7671) take into account the operating characteristics of circuit-breakers (as defined in the relevant Standards). In other words, if the tabulations say that the maximum CCC is, say, 10A, this means that the cable can safely carry 14.5A for up to one hour?

Kind Regards, John
 
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