mcb tripping times...help fried my brain!!

[dyland22]

can any1 please explain to me the maximum tripping times on mcb's.....i have installed lighting and powerd outlets in a commercial job....everything in dado trunking....my lighting circuits are protected only by 6amp mcb no rcd.....been looking through the 17th and it states that a disconnection time of 0.4 seconds is required for 230v final circuits....i thought only an rcd could disconnect that quickly,or does an mcb disconnect that quickly olso?...cheers

 

[NMc]

The lower your Zs the more fault current. Time/current characteristics for 6A type B mcb is 30A so Zs needs to be less than 7.67

 

[netblindpaul]

dyland22,
If you cannot understand and design your installations to be assured of this then it is doubtful that you are competent to undertake this work.

 

[dyland22]

I am more than competent to design and complete the work,thank you very much...!!.. my zs values are well within range,worked out my cable sizes,volt drop... a simple answer of yes an mcb does disconnect in 40ms when zs values are within the amounts given in table 41.3 of the 17th would have been nice.. :yesnod: wots the point in having a table that gives a zs value for 5s and 0.4!!.. why not only 0.4!!

 

[Knobhead]

Got to agree with Paul on this one. If you have to ask the question then should you have undertaken the job?

 

[MarkieSparkie]

I am more than competent to design and complete the work,thank you very much...!!.. my zs values are well within range,worked out my cable sizes,volt drop... a simple answer of yes an mcb does disconnect in 40ms when zs values are within the amounts given in table 41.3 of the 17th would have been nice.. :yesnod: wots the point in having a table that gives a zs value for 5s and 0.4!!.. why not only 0.4!!

Because Table 41.4 (5s disconnection time) is for distribution circuits or for a circuit not covered by 411.3.2.2

 

[dyland22]

whatever boys...i am a fully qualified electrician...its only a simple question!!..looks 2me like you dont know the answer!!..i just answered it anyway so no worries...nice friendly people here tonight!!

 

[dyland22]

No...table 41.4 is for fuses not mcb's.. why 5s and 0.4s for mcb's?..

 

[mogga]

as MCBs can also be used for distribution circuits FFS

 

[SimpleSimon]

NMc has given the best answer here.
Does it not say a disconnection time of 0.4s for any circuit over 32a? Not 230v.

Sent from my GT-I9100 using Tapatalk

 

[dyland22]

No sh*t mogga...all i am saying is that if i use an mcb for distribution circuit it would be within the zs value so therefore comply with 0.4 disconnection time..so wot is the point of talking about 5s when 0.4 has already been acheived!

 

[robsparky1975]

The people that have responded to your posts are generally really friendly and respected guys,0.4 is not 40ms its 400ms,0.4s is for final circuits uoto 32a,above that 5s is applicable,so if you have less than the max zs permitted by bs7671 then it would comply with disconnection times.the 40ms applies to 30ma rcds for 5x readings.

 

[Deleted member 26818]

I am more than competent to design and complete the work,thank you very much...!!.. my zs values are well within range,worked out my cable sizes,volt drop... a simple answer of yes an mcb does disconnect in 40ms when zs values are within the amounts given in table 41.3 of the 17th would have been nice.. :yesnod: wots the point in having a table that gives a zs value for 5s and 0.4!!.. why not only 0.4!!

Are you, then why don't you know the difference between 400ms and 40ms?
Why don't you know that MCBs do not operate in 40ms?
Why don't you know that the instantaneous operating time for an MCB is accepted as being 100ms?

 

[dyland22]

it says for any final circuit not exceeding 32A....and then in the table anything from 120v - 230v TN system disconnection time is 0.4 seconds.

 

[netblindpaul]

Worrying is it not!
We are generally expected to be nice and help these Muppets on here take our work away when they are obviously incompetent.

I appreciate the forum ethic, but, why should we answer questions that should never be asked?

IF the OP had said, I have been asked to..., but I have never done so I am looking for advice... etc. then this is a different situation.

However to come on and say I have done this and I am competent and qualified etc. and I am this and that and the other, and ask such a basic question then as far as I am concerned they are not.