Max zs values

[davesparks]

but OP question is interesting why is there a table for 0.4 and 5 secs for one but not the other?

Look at the tripping curves, the B and C curve MCB's are already into their instantaneous tripping point at or before the 5s mark

 

[Mark Wright]

Look at the tripping curves, the B and C curve MCB's are already into their instantaneous tripping point at or before the 5s mark

exactly it's straight down from 10 seconds that tells us nothing ?
the only thing I can see is 5 sec is for distribution circuits of & above 32amp

 

[pc1966]

exactly it's straight down from 10 seconds that tells us nothing ?
the only thing I can see is 5 sec is for distribution circuits of & above 32amp

It tells us that you meet both disconnection requirements at that current.

 

[Mark Wright]

It tells us that you meet both disconnection requirements at that current.

Yep but what 5 second requirement I can't see one except 5 sec is for distribution circuits of & above 32amp ?

 

[buzzlightyear]

BS7671 isn't that relevant here Max Zs is just Supply Voltatge/Trip Curve * Cmin

i.e (230/Mcb 32A*5) * 0.95(rule of thumb if you want 0.80) =1.37 or 1.1 ohms etc

I use the big blue book for max zs not the OSG.

 

[Mark Wright]

I use the big blue book for max zs not the OSG.

yeah both aren't really needed if you just do the simple equation.

 

[pc1966]

Yep but what 5 second requirement I can't see one except 5 sec is for distribution circuits of & above 32amp ?

Those are your cases for a 5s disconnect time, subject to thermal limits on the cable (e.g. adiabatic check).

Normally you would always aim for 0.4s or below even if not explicitly required, but there are cases when you look at the 5s limit. For example:

• For high current sub-main or fixed circuits you may not be able to meet 0.4s due to Ze anyway
• On a sub-main you might push the upstream OCPD as high as the 5s or adiabatic thermal limits allow to get better selectivity with the down-stream DB MCBs.

[automerge]1590238260[/automerge]

yeah both aren't really needed if you just do the simple equation.

Except when the simple equations is very wrong. Yes, if you know it is a B-curve MCB, but in other cases you can get it wrong by a factor of 2 or so.

 

[buzzlightyear]

yeah both aren't really needed if you just do the simple equation

yeah looking at the book is much quicker then pulling a calu out .

 

[pc1966]

Those are your cases for a 5s disconnect time, subject to thermal limits on the cable (e.g. adiabatic check).

The OSG book in Appendix B has tables that include the typical adiabatic limit for the common CPC sizes so it is your easiest option for selecting an OCPD when desiging something like a garage CU feed, or verifying an existing OCPD and measured Zs is reasonable.

 

[Mark Wright]

has anyone seen this video ? 35 is the Magic Number

View: https://m.youtube.com/watch?v=URBD4llLU6Y


at first I was thinking, what! this makes no sense see post 10 but it does make sense and 35 is the magic number - bit of fun figuring out why if anyone wants to have a crack.