Time / current characteristic curves...

[junksamiad]

Just trying to clear something up for myself, hoping someone can put it into layman terms for me.

I'm looking at the time / characteristic curves in 7671. For example, 60890 Type B curve, p. 370.

Taking the example of a 32a breaker, the line flattens off vertically at 160amps. All understood ok there. But does this mean that a 32a breaker will trip anytime between 0.1 and (approx) 13 seconds, as these are the times in seconds that the verticalness of the line correlates to on the actual graph.

I am confused as the graph seems to show as I have explained above, whilst the quick reference box on the right of the page seems to suggest the trip times as 0.1 - 5 secs.

How should I be interpreting this information correctly here?

Any help and advice appreciated greatly as always.
Thanks again.

 

[Baddegg]

Depends on ya level of fault current....Type b will operate between 5 & 10 times Max rating, dont have my book to hand, but if you take the given/accepted Ze values and do the maths, 230/Ze Given or measuring you’ll see it will break quicker than 0.1 ish....

 

[Ian1981]

Just trying to clear something up for myself, hoping someone can put it into layman terms for me.

I'm looking at the time / characteristic curves in 7671. For example, 60890 Type B curve, p. 370.

Taking the example of a 32a breaker, the line flattens off vertically at 160amps. All understood ok there. But does this mean that a 32a breaker will trip anytime between 0.1 and (approx) 13 seconds, as these are the times in seconds that the verticalness of the line correlates to on the actual graph.

I am confused as the graph seems to show as I have explained above, whilst the quick reference box on the right of the page seems to suggest the trip times as 0.1 - 5 secs.

How should I be interpreting this information correctly here?

Any help and advice appreciated greatly as always.
Thanks again.

The reference box gives information for the current to provide instantaneous disconnection of the ocpd (160 amps in your example of a type B 32 amp mcb) which is at 0.1 seconds, for fault currents in excess of those providing instantaneous operation shown, then the manufacturers let through energy is required for disconnection In less than 0.1 seconds. Your required disconnection times for fault protection are generally required for 0.4-5 seconds disconnection times respectively for TN arrangements or 0.2-1 seconds for TT arrangements , see table 41.1 for the variety of voltage bands and disconnection times and regulations 411.3.2.3 and 411.3.2.4.
As 5 seconds is generally the maximum time allowed for certain types of distribution and final circuits, then by giving us the information in a simple reference box, we can be confidant that disconnection times are met without looking at the graphs.

 

[buzzlightyear]

Oops.

 

[Ian1981]

Time in sec t=k2 xs2-I2 calculate 434.5.2.

That’s for limiting time it takes for the live conductors to be damaged during short circuit conditions, not for when the ocpd disconnects.

 

[pc1966]

Taking the example of a 32a breaker, the line flattens off vertically at 160amps. All understood ok there. But does this mean that a 32a breaker will trip anytime between 0.1 and (approx) 13 seconds, as these are the times in seconds that the verticalness of the line correlates to on the actual graph.

The MCB has two trip mechanisms,:

• There is the thermal trip that is the curve up to around 5*In in the B-case which is intended to allow moderate surges/overloads to pass by in short times as they are not threatening the cables
• Then there is the "instantaneous" magnetic trip and when it fires the breaker typically opens in under 30ms, greatly reducing the fault energy that can pass.

The difference between B/C/D curves is all down to the trip point for the magnetic part.

So to answer the specific quection, not you get around 13s or < 0.1s as you cross the magnetic trip point.