time/current characteristics for mcb's

[graham1]

Hi guys, this might seem a bit of a newbie question, but perhaps someone out there can put my mind at rest! Here goes; consider a 4mm t&e radial circuit as described in brb p363 being protected by a 32a mcb. Now, according to p282, assuming ref method c, the current carrying capacity of that cable is 37a so all is good so far. However when looking on p249 the time/current characteristic for the 32a breaker is nearly 50a at 1000 secs before it will trip, so how is the cable being protected when its current carrying capacity is 37a?? Thanks in advance!

 

[malcolmsanford]

Because under fault conditions the fault current will reach many 100 if not 1000,s of amps in milli seconds and it takes just 160 amps to trip a 32 amp Type B BS 60898 MCB in 0.1 seconds (100 mS) and the cable is rated for such tolerances.

 

[Paul.M]

Looking at fig 3.4 page 249 a 32A mcb can handle 50A at 100 seconds, not 1,000.

 

[graham1]

thanks chaps for your replies, but i was thinking along the lines of an overload of the circuit, not necessarily a fault current, for instance a 32a breaker will take 70a for 100secs before it will trip!

 

[Silly Sausage]

Looking at fig 3.4 page 249 a 32A mcb can handle 50A at 100 seconds, not 1,000.

One of us is reading the graph incorrectly...
I'm seeing 1000secs @ 50A

 

[Paul.M]

I apologise to both of you, I looked at the wrong line. Thanks for picking that up Archy.

 

[malcolmsanford]

thanks chaps for your replies, but i was thinking along the lines of an overload of the circuit, not necessarily a fault current, for instance a 32a breaker will take 70a for 100secs before it will trip!

You getting cable characteristic confused with MCB characteristics Graham.

Appendix 3 in the back of the BS 7671-2008 are for cables.

An MCB is designed to run at 1.45 it's In or rating capacity for an hour so a 32 amp MCB will run at 47 amps for an hour and trip, once it reaches say 50 then it will trip instantaneously or near enough.

It is also designed to to run quite happily at 1.13 times it In or rating capacity. All cables have this 1.45 for an hour capability factored into them.

 

[graham1]

Hey, no worries, been staring at the damn thing for half an hour myself!!

 

[Silly Sausage]

I apologise to both of you, I looked at the wrong line. Thanks for picking that up Archy.

No need Paul. Those graphs just look like a tangle of spaghetti sometimes, to me anyway.

 

[BlueToBits]

Without any authority on the matter, but as no one else has offered up a reason, here's three that come to mind:

1. I am sure that the IEEE and BSI would have considered the duration of such potential overcurrents in publishing table data. We have little choice from a practical or regulatory viewpoint other than to accept their values and requirements. The published tabulated rating may therefore be different to the actual rating to allow for this.
2. The line has to be drawn between 'perfectly safe' and 'practicability', so we end up with something along the lines of 'acceptable risk'. In our situation BS7671 determines what is acceptable and what is not.
3. If it is likely that the circuit will be subjected to such overcurrent then the designer should allow for this. If the increased load was because of a change of use, then the installation should be inspected for suitability at the time of that change of use and any required modifications should be undertaken. The original designer cannot be held responsible for use outside the design specification but could be critisized for over-specifing (over spending) or rejecting aspects of the client's needs of the installation due to incorrect implementation of the wiring regulations. - for example, incorrectly saying "You can't have that, it's against the regs"

A similar situation occurs quite frequently when a DIY householder replaces his c1970 6.5kW shower with a 9.5kW unit and wonders why it will only fun for a few minutes before the breaker trips. The real danger here is that the householder may 'fix' the 'fault' by replacing the MCB for one of the correct rating for the new shower.