Discrimination of fuses?

[LearnerDave]

Hiya, Just wondering if any of you would be able to help me understand the I2t curves for discrimination between protective devices. (I have read that if the major fuse is twice the rating of the minor fuse then discrimination is achieved, but I am trying to work it from the I2t curves)

I have read - Discrimination is achieved if the total I2t of the minor fuse does not exceed the pre-arcing I2t of the major fuse.

When I have look up manufactures data, some have the pre-arcing and total total I2t values in tables which seams straight forward to follow.

The problem I have is when I looked up an MK MCB Data, they show I2t as a table like:

Does this type of graph show pre-arcing and total values?
If so, would where I marked A be the pre-arcing point?
And marked B be the total I2t point?
Would discrimination be achieved if point B of the minor fuse would be less than point A of the major fuse?

Or am I totally off the mark! which I fear I am.

Cheers Dave.

 

[MarkieSparkie]

Hiya, Just wondering if any of you would be able to help me understand the I2t curves for discrimination between protective devices. (I have read that if the major fuse is twice the rating of the minor fuse then discrimination is achieved, but I am trying to work it from the I2t curves)

I have read - Discrimination is achieved if the total I2t of the minor fuse does not exceed the pre-arcing I2t of the major fuse.

When I have look up manufactures data, some have the pre-arcing and total total I2t values in tables which seams straight forward to follow.

The problem I have is when I looked up an MK MCB Data, they show I2t as a table like:
View attachment 8827
Does this type of graph show pre-arcing and total values?
If so, would where I marked A be the pre-arcing point?
And marked B be the total I2t point?
Would discrimination be achieved if point B of the minor fuse would be less than point A of the major fuse?

Or am I totally off the mark! which I fear I am.

Cheers Dave.

Hi Dave, Fuse to fuse discrimination is relatively simple. Discrimination is achieved if the pre-arcing I[SUP]2[/SUP]t of the upstream device is less than the total operating I[SUP]2[/SUP]t of the downstream device. The Rule of Thumb is "An upstream fuse will always discriminate with downstream fuse of half it's rating."

Circuit breaker to circuit breaker discrimination is more complicated because the 2:1 Rule of Thumb cannot be relied upon for guaranteed discrimination. The 2:1 ratio generally is valid for the thermal trip mechanism, but does not always apply to the magnetic trip mechanism.
The normal method of manual co-ordination is by plotting the relevant protective devise characteristics onto the same log-log graph paper. When this is done there is always a zone where discrimination may not be achieved. This zone is where the curves of the breakers run parallel to the current x-axis, usually below 0.1s and is the reason why this part of the curve is omitted from the graphs in BS7671 Appendix 3, replaced by a note stating refer to manufacturers data . Discrimination within this zone can only be determined by carrying out comparative tests, thus only the manufacturers can confirm discrimination is achievable.
This subject is confused by the practice in industry of cascading circuit breakers. Cascading is where downstream circuit breakers are “backed-up” by other circuit breakers. This technique is not a substitute for system discrimination co-ordination. Where cascading is used, the upstream breaker must have sufficient short-circuit breaking capacity to interrupt the fault, and the downstream breaker must be able to handle the through fault currents sufficiently long enough for the upstream breaker to operate.

Circuit breaker to fuse discrimination is used at certain fault levels where fuses operate quicker than circuit breakers in order to achieve a co-ordinated system discrimination scheme. The principle is the same as used for back-up protection, where a downstream circuit breaker is not rated for the prospective fault current at the point of use. Larger currents are interrupted by the fuse, whilst smaller fault currents are interrupted by the circuit breaker. This is best illustrated by the 16kA conditional rating given to BS EN 60439-3 distribution board assemblies, where discrimination is achieved between the 6kA or 10kA rated BS EN 60898 MCBs or BS EN 61009 RCBOs and the DNOs BS 1361, Type 2 cut-out fuse and thus allows the MCBs to be used in situation where the PFC exceeds their 6kA rating.

The information provided by MK is in a horrendous form, the characteristic curves appear unconventional because they give a comparison of the maximum let-through energy I[SUP]2[/SUP]t for various MCBs with a breaking capacity of 6kA, it's much harder to use these characteristics in the real world, bin them, use the conventional graphs in BS7671 Appendix 3 instead mate.

 

[LearnerDave]

Thanks very much Markiesparkie.

I had read about the rule of thumb of X2. I was just wondering what the I2t curves produced by the manufactures meant. and how you could use them. Any pointers would be great.

 

[MarkieSparkie]

Hi Dave, Re-read #2, my earlier post, as I've added more information.
The information provided by MK is in a horrendous form, the characteristic curves appear unconventional because they give a comparison of the maximum let-through energy I[SUP]2[/SUP]t for various MCBs with a breaking capacity of 6kA, it's much harder to use these characteristics in the real world, bin them, use the conventional graphs in BS7671 Appendix 3 instead mate.
I'm working on an explanation of how you actually use the BS7671 graphs for MCB to MCB discrimination and will post it here in a day or two.