A point to ponder.

[scotsparky]

I dont know the true answer to this to everyone is able to chuck ther opinion in.

When wiring an installation how do you prove Overcurrent discrimmination when using more than one type on over current device an example being.

Bs88 hrc main device
MCCB as overcurrent device at sub panel,
MCB for final circuits.

 

[jaresquire]

Their different response graphs provide time-discrimination?
Going for me tea now, please mock in my absence!

 

[scotsparky]

Bs 7671 does not produce time current graphs for MCCBS

 

[IQ Electrical]

I dont know the true answer to this to everyone is able to chuck ther opinion in.

When wiring an installation how do you prove Overcurrent discrimmination when using more than one type on over current device an example being.

Bs88 hrc main device
MCCB as overcurrent device at sub panel,
MCB for final circuits.

The only way you can prove it is on paper at the design stage with time/current graphs for each device, thereby justifying their selection.

MCCB curves from manufacturers.

 

[scotsparky]

Iq do you go my the BS tables or use manufactures details

 

[IQ Electrical]

Iq do you go my the BS tables or use manufactures details

Always manufacturers for MCCB's the differences are quite surprising.

 

[scotsparky]

This is where the confustion is. I have a basic grasp of the overlapping time current curves meaining no discrimmination but whats the ruleof thumb?


1 Where do you get the information
2 how do you work it


I may put up arguments for this just to see if there is a clear answer or is it experiance and understanding of the graphs.

 

[Guest123]

Manufacturers websites usually have all the info relating to their MCCB's.

As IQ said, the differences in values are quite interesting and BS7671 does state manufacturers data can be used instead of the book values.

 

[scotsparky]

lenny i agree info is out there but is there a genaral rule of thum or is it cross match manufacturers info.

 

[Guest123]

where are the MCCB values in BRB never seen them.

There are none, these must be sourced from the manufacturer.

I was referring to standard MCB's aswell, as the manufacturer will supply different values to the values printed in the book.

 

[IQ Electrical]

I got sidetracked with a different discrimination problem there!

Have a search around this link Schneider Electric UK | Electrical Distribution | Products and Solutions

Took me a while but I got what I was looking for there a while back.

This is still what's referred to as a 'desk' camparison, in the real world, devices in series often behave slightly differently to their 'paper curves'

 

[Guest123]

I have an MEM PDF which is very informative but it's too large to upload onto the forum.

I can e-mail it to you if you like mate & anyone else if they want it..