MCB tripping point.

[Floody]

Hi guys,

Ok, ill tell you where Im at with this question. Nothing to do with college, but its something ive just read in a Trevor Linsley book.

"MCB Type B to BS EN 60898 will trip instantly at between three and five times its rated current and is also suitable for domestic and commercial instaltions"
(text from book)

So for example, a lighting circtuit on a 6amp MCB, will need to pull between 18a and 30a for it to trip. Is this correct?

As I was under the impression that if the MCB went over over 6a, even in the slightest, it would trip.

 

[Paul.M]

So for example, a lighting circtuit on a 6amp MCB, will need to pull between 18a and 30a for it to trip. Is this correct?

As I was under the impression that if the MCB went over over 6a, even in the slightest, it would trip.

Ok somethings are for exams and somethings are for electricians, let me explain:-

The breaking point of an 6A MCB is 1.45 the ratting of the MCB for up to an hour (8.7A) and the 1.45 rule is for any MCB (60898).

The MCB it self monitors two things, 1 the ampage and 2, the heat. The MCB has a heat censortive plate that cuts out at a certain temperature this is where the 1.45 rule comes in. Think about current + resistance = heat??????

The 3-5 times is for a short circuit, just look at Fig 3.4 in appendix 3 for 60898 MCB's.

 

[Marvo]

As Paul says, an MCB offers 2 types of tripping protection. As current flows through an MCB there is a small thermal element that gets warm. If the overload current is marginal the thermal element heats up over time to a point where it cause the MCB to trip. This is a slow process and can even take hours for a very slight overload.

Once the overload current exceeds a certain threshold (determined by the letter of the curve, B type in the case you mentioned) a second type of tripping mechanism is activated which is magnetic. This mechanism is very fast and designed to prevent serious damage to the circuit by fault currents but it only becomes active when the overload current is several times the rated current of the MCB.


With a C curve MCB for example;

with a tripping current (I[SUB]t[/SUB]) greater than 5 times the rated current of the MCB (I[SUB]N[/SUB]) the tripping mechanism will be magnetic and will take just longer than 1 millisecond.

I[SUB]T[/SUB] at >10 times I[SUB]N[/SUB] it will take <1mS for magnetic tripping.

If I[SUB]T[/SUB] is less than 1.35 times I[SUB]N[/SUB] it would take over 1 hour to trip.

When you're looking at marginal overload tripping times it becomes important to consider the ambient temperature of the CU or DB enclosure. Warmer temperatures can significantly reduce tripping times at the lower I[SUB]T[/SUB] ranges. Likewise cooler ambient temperatures can significantly increase tripping times. This isn't usually the end of the world because at cooler temperatures the cabling can carry higher currents as well so it all usually works out in the wash so to speak.