Type d mcb

[Alanz1]

Poni

I think you explain that well.
It's starting to make sense to me.
Many many thanks for taking your time to explain this.

 

[Phoenix]

The magnetic trip on any device is more or less a vertical line, you either hit the threshold to trip it it, or you don't, and if you do the tripping time is as fast as the contacts can separate( Its actually a lot quicker than 0.4s, we often take it to be 0.1s but this is a rather pesismistic value, but we don't need to worry about that, its most certainly faster than 0.4s which is all we need for this), more fault current doesn't make it go any fasterThats why the tables for B and C devices apply to 0.4s and 5seconds

MCBs have a thermal element as well, and the time for this to trip does vary with fault current and this will be acting a lot slower than the magnetic release, the thermal part is the same across all breakers of the same current range, eg B25, C25 and D25, the (letter defines the magentic release. in terms of disconnection times)

the thermal part is irrelevent to type B and C at the times you want faults to disconnect in (sub 5s), as at the current required to have the thermal part trip in 5s, you have already hit the threshold for magnetci release and its gone out instantly, so it never comes into play.

With D types, however, if you don't require a 0.4s disconnection time, then it is possible to have a fault current that doesn't hit the threashold to release instantly, but uses the thermal element to meet 5 second disconnection times and the values are IIRC just a little bit more onerous than the type C values

 

[SPECIAL LOCATION]

My lecturer says look at the table, all the amp ratings seem to trip at the same time, I just don't understand how to work the table.

Hi Alanz1....

First I just want to say it is really good seeing someone ask a question..
But also put down as much info about what they know and/or think they know as well,
and then asking about what they don't understand...

Rather than far too many who come on here and expect to be spoon-fed answers by just asking what is the answer to 'XYZ'

So good questions so far.. Re the tables..

Some bits of them can be confusing as the scale lines are logarithmic and not linear, and can look a bit strange..
(Some of us older bods still remember using 'log-table-books' during our maths exams!!!)

I dunno if this will help or not...
But you need to know a fuse of circuit breaker will not immediately disconnect the power as soon as the designated current rating is exceeded

And some types of equipment have natural current surges when switching on, which need to be ignored..
whilst other current surges, due to faults, need to be responded to quickly to minimise the risk of electric shock.

So, looking at the table from a different perspective..
For simplicity assume a random circuit with your 25A type D breaker has a 40Amp current flowing.

Along the bottom line (prospective current rms amperes) after the 10A, next line is 20A then 30A then 40A..
follow this line up until it crosses the curved 25A breaker characteristics line..
which is probably somewhere around the 800 seconds line..
(depending on how good your eyes are?!!)

Which suggests with a continual 40A overload fault current it could be 13+ minutes before a type D 25A MCB trips..

So in your case with a 380+ Amps fault current you need to guestimate whereabouts a line proportional to 383amps would cross the curved 25A breaker line and go horizontally to see what time duration that would be...

BUT.. if you are looking for a 0.4disconnection time...
most sensible people just look at the table on the right hand side that says you need a 500A fault current to achieve 0.4seconds disconnection with a 25A MCB!! so don't bother wasting their time tracing lines up tables if they know the current is not sufficient to give a 0.4second disconnection time.

I hope that makes some sense..
As I have had some intoximicating incohlic afflunencing beverages after another busy day at work...

I am sure one or more of my esteemed forumbulator colleagues will correct me,
if I have missed any key points..
Or completely confused myself!!!!!!!

Stay with us Alanz1
And keep asking good questions!

 

[kerching]

Yeah, what he said. ^^^^

Basically (. I like basic )
Trip time depends on current flow
Big current = fast time
Small current = slow time

Current depends on resistance
Voltage is fixed
So......low res = high current = fast time
High res = smaller current = slower time

Time has to be fast enough to stop cable being thermal damaged

 

[Alanz1]

I now understand this much better.
Check you have the required current to trip an mcb within the appropriate time.
Chart lists each current required, in my example. 383 amps wouldn't trip the breaker, adding massive heat to cabling or worse, fire.

Many thanks to all who took the time to try explain in simple terms, how these charts worked and ohms law.

I stupidly assumed that Zs wasn't really a resistance reading as it included the whole property wiring and also the external wiring back to DNO transformer.

I think I'll hang around this forum as every day is a school day

 

[kerching]

every day is a school day

indeed it is BUT remember some of the teachers/experts on here are thicker than mince

 

[apprentice87]

Here is how i do it without messing about with charts or culculators or any of that nonsense..

Firstly, yes, i know that they brought in an amendment that supposedly allows for the volt drop under fault conditions but i could alter my method to allow for that if i wanted.

Anyway, we know that to trip in the required time a "B" curve requires a fault current 5 times its rated current, a "C" curve 10 times and a "D" curve 20 times...

We also know that MEASURED Zs must not be more than 80% of its calculated value to allow for the increased cable resistance caused by the heating effect of the fault current. So the reciprocal of 0.8 is 1.25.. Ok????

Right then, an example..

i bimble about and i come across a final circuit, it is fed by a 32A "C" curve MCB Does this circuit comply?? Ok, so, 32A x 10 = 320A

320 x 1.25 = 400A

I test the PEFC with my meter, it is 475A [say] YES!! all is well!! If LESS than 400A umm, we have a problem...

Who needs charts or calculators????

john..