bs2871 type b mcb

[whatif]

question not so much about the mcb as the max Zs tables.
In the on site guide, it's the blue one I've got, on page 89 it begins the max disconnection times/Zs values table, giving 0.4 and 5s tables.

For bs3036, bs88, and bs1361 there are seperate tables(with seperate value) for 0.4 and 5 seconds. However for bs3871(bs en 60898) there is a combined table, which puzzles me a bit. How do we end up with the same value of Zs for 5s and 0.4s disconnections.

I know I'll feel stupid when I hear the answer, so go ahead who's going to be first to make my day?

 

[plugwash]

the thing to remember about a mcb is its essentially two breakers in one.

the thermal part has an action somewhat like a fuse though its actually a bit slower at higher currents.

the magnetic part pretty much either trips or doesn't there isn't really any speed factor to consider

with most mcbs (i belive type Ds may be an exception to this but using them without a very good reason is a BAD idea) you have to hit the mag trip to get a disconnect within 5s and once you do hit the mag trip the breaker trips almost instantly (the breaker spec requires 0.1 s but i belive modern breakers are much faster than this).

therefore the maximum earth fault loop impedances are based on the current required to gaurantee the mag trip activates.

 

[whatif]

ah I get it ........ the "curve" on an mcb between 0.4s and 5s is really a straight line.

Thanks for the explanation Plugwash.....appreciated