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.