MCCB Max Zs

[sparkydude]

Hi all , Simple question for you all, current job has MCCB panelboards fitted with adjustable trips which allow the tripping characteristics to be altered to 40% of maxim full load current. Manafacturers data sheet has max Zs values , but only for the origanal current rating of the unit ie 400A ,250A etc.

The client has done a protection study and has tweaked the breakers to suit his needs , Obviously now they are not set at maximum so how do i find the Max Zs for each device now?? Would the manafacturers be of any help or is it a case of performing some calculations to establish them ?

ANy pointers welcomed as normall work on HRC fuse panels which obviuosly have a set value of Zs .



Cheers

Nick

 

[malcolmsanford]

You will have to be guided by the manufacturers data on this. I would imagine that the data sheet would have a tripping curve and you would need to work off that.

 

[Engineer54]

Use the Max Zs given by the manufacturer. As these are adjustable, there is nothing stopping anyone from adjusting the setting to x1. ....If the max Zs is good for the rated maximum current of the MCCB, then it's good for the lower setting selected.


You always use the ''Max Zs'' as stipulated on the manufacturers data sheet, with any adjustable type MCCB...

 

[Deleted member 26818]

Yes if you look at Tables 41.2-4 in BS7671, you will note that the maximum allowable Zs increases as the CPD rating decreases.