electricalforum.nz

Anyone have much knowledge or experience with doing discrimination studies?

I want to check the discrimination between an 63A gG HRC with downstream MCB in accordance with 2.5.7.2.3 (c) and wanting to know what the maximum MCB size I can use to provide discrimination

Do I need the actual manufactures data? Or can I use the time-curves etc from the applicable standards?

Manufacturer's data is usually best, some have it as an PC application and it is easy to use.

Standards are also Ok to use but will cost many dollars to get and you will have to draw your own tables to a common time v's current to a common scale so that you plot the two graphs on top of each other. Hint, draw the graphs on plastic sheets overhead to make it easy to compare the time v's current curves via visual means.

Just a further question in regards to this

My main reason for this for checking discrimination between the supply fuse and downstream MCBs in a standard home, because the clause indicated it as a mandatory requirement

But it got me thinking, although it's still probably a good idea, does 2.5.7.2.3 (c) apply in this situation when the supply fuse is prior to the PoS and not part of the installation?

Correct that only the installation, which starts at the PoS, is required (by ESR59) to comply with "3000".
Also consider that while NZ's ESRs require linesco to provide both short circuit and fault protection for mains; they don't have to proved overload protection.
And in some States of Oz; they have "unprotected" mains - no supply fuse at all.
So a good case can be made that we don't have to apply 2.5.7.2.3(c) just because there happens to be an HRC supply fuse upstream of our typical 32A mcb for a range circuit.


Fact is most Inspectors have no more idea about this stuff than you, (at least you are reading the book and thinking about the issues);
And we've been using such combinations in domestic & similar installations for many years, without problems.
So chances of getting into trouble for non-compliance are tiny.