Max Zs for a 50A BS3871 type 4!

MilesSparks · Mar 18, 2015

Can someone help confirm what the max Zs for a 50A BS3871 type 4 Mcb?

Am I correct to use the equation 230 / (50xIn)?

Using that equation it comes to 0.092.

Theres not much help in the regs!

Leesparkykent · Mar 18, 2015

Looks good to me, type 4 have an instantaneous trip of 50

graham123 · Mar 18, 2015

Manufacturers data for type 4. What make/model is the Mcb? I have a few data sheets

MilesSparks · Mar 18, 2015

Federal Electric!!

graham123 · Mar 20, 2015

A 50a type 4 stablock at 5sec is 1.37 and at 0.4 sec 0.45 I have the data sheets if you want a copy

MilesSparks · Mar 20, 2015

Yes please Graham

Engineer54 · Mar 21, 2015

If these are those old black bakelite FE MCB's i'd be advising the client in favour of a CU/DB replacement. I can't think of a worse system than those old Fed Elec boards and OCPD, especially the stab lock design, which were basically taken from the US... From what i can remember they were nothing but trouble and often didn't accurately follow the type Time/Current curves.... Crap with a Capital C!! lol!!

MilesSparks · Mar 21, 2015

Totally agree with you on that 54! I've got a block of 50 retirement flats with them in! I'm trying to find concrete reasons why to change rather than just recommend it.