Max zs values

[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.

 

[Farmelectrics]

I like the 35 way. I've always done it this way but its longer . b type 5 times, c type 10 times, d type 20 times. so if its a c type 32amp it would be 230 divided by 320=0.71. 0.71x 80%= 0.56. 0.56 x 0.95 c mim =0.55, long winded so in future might youse the magic 35.

 

[pc1966]

What bothers me about "quick calculations" like 35/In is they end up being used and not understood. Yes, that works fine for a B-curve MCB but not for other cases!

So Farmelectrics method is better as it shows the key point - the 0.4s disconnection is achieved by ensuring the magnetic trip is reached under worst-case conditions (0.95 supply volts and 0.8 for cable temperature on R1+R2).

 

[Mark Wright]

Can't say I'm a huge fan of this guy, seems a bit harsh after posting his video but I get the impression he doesn't know why it's 35 - also on his site he uses little r1 for a reading on a Radial and few other things that a tutor should know better.