Instantaneous faults and discrimination

[Nchika]

Hi,

I am currently working on a project where the client has asked us to complete a discrimination study.

The primary area of concern involves an upstream miniature circuit breaker (25A type C) and a downstream 10A gG fuse.

The type C circuit breaker time current characteristics within BS 7671 (Appendix 3, Fig 3A5) have been reviewed and it appears that the curves only exist for breaking times greater than 0.1s. I have a few questions regarding this:

1. Why do the curves not exist for time frames below 0.1s?
2. Is it correct to infer from the lack of data below 0.1s, that all instantaneous faults will last at least 0.1s?
3. The time current characteristics of the 25A type C MCB and 10A gG fuse do not overlap. Is this sufficient to conclude that discrimination/selectivity is achieved for all fault levels or do other things need to be taken into account? I am particularly concerned about instantaneous faults.

 

[Darkwood]

Hi welcome to the forum.

Looking at the mcb curve in the regs you are given a box in top right of the page, this will stipulate a specific trip time related often to various regulations, so if you have a circuit that has a 5second allowance then you can refer the rating of device to the current required in a fault condition to ensure it trips in time, having information extending past 0.1 seconds serves little purpose as this will be classed as instantaneous.
Discrimination between devices means when you have a fault condition past a downstream device then that fault will take out the downstream device before any upstream devices, where devices are of a different nature you are best looking up a discrimination chart between different devices.
In a fault on your scenario I suspect only partial discrimination will be achieved, it can be however an issue to achieve full discrimination if your supply size to the premises is a limiting factor, in your case this not an issue relative to the small rating of the 10amp fuses, you may need to upgrade you supply and increase the 25amp though to ensure full discrimination, I cannot say though without comparative charts.

As an example of full discrimination, mccb - mcb then to ensure full discrimation on a mcb dist' board you would need a 160amp rated mccb.