Mcb Energy Let Through Rating

[Steve100]

I know that MCB's are grade with an energy let through rating of 1, 2 or 3 and 3 being the highest. I also know that it relates to the sizing of the cpc. However that is about where my understanding ends, I have read a few threads and documents, but still having trouble getting my head around it. Could someone please explain this principle in simple layman terms so I can understand?

Also in the current OSG it states that for MCB's of <=16A at a short circuit capacity of 6KA the min cpc should be 2.5mm2, but a lighting circuits cpc is 1.5??

Thanks

 

[binky]

6ka is a misnomer, CPC sizing is based of PFC against the live conductor size. Read up on sizing of CPC v PFC for the circuit. I would explain but I've forgotten the details.

 

[Steve100]

It was from the green OSG page 120 and its a table which details min cpc size in relation to the energy let through rating of the protective device?

 

[wozz]

will grab my osg later, but are you getting mixed up between energy limiting class and energy let through?

I am no expert on either.  

 

[Steve100]

Just read page 120 again and yes it is the energy limiting class I need explaining please?

 

[SPECIAL LOCATION]

If this link works....

make a mug of coffee then have a read of a few pages here...

around page 118ish..

{3rd page in the link..  but 118 on the actual printouts cuz they start at page 116.. if you get my drift?}

http://download.hager.com/Hager.ie/files_download/catalogue/Protection_Technical.pdf

:coffee

dunno if any of that starts to help with putting a few bits into context..?

 

[Steve100]

Cheers will have a read thxs

 

[Steve100]

Okay so had a read through and makes a little more sense, but going back to my original post, the table on page 120 refers to cpc size being a min size of 2.5mm2 for MCB's <= 16A. So if I was installing a lighting circuit in a house in general the cpc would be 1.5mm2 which contravenes this table?

 

[SPECIAL LOCATION]

Okay so had a read through and makes a little more sense, but going back to my original post, the table on page 120 refers to cpc size being a min size of 2.5mm2 for MCB's <= 16A. So if I was installing a lighting circuit in a house in general the cpc would be 1.5mm2 which contravenes this table?

Table B7 second column is the Fault level...  not the MCB short circuit capacity..

on your average house the fault level will not typically be anywhere near 3kA and certainly no where near 6kA..

Your B type breakers will be good upto a 6kA fault..

but what is your actual PFC and/or PSSC.. {'largest'}

with a fault upto 3kA a 1.0mm CPC is good on a type B breaker..

If you are expecting 6kA faults then possibly a 6kA type B MCB may not be an appropriate device to fit?

 

[Steve100]

The haze is clearing a little now, so the first column is the energy limiting class 3 device rating as apposed to its rated current In is that correct?

 

[wozz]

No I think the first column is the In.

See the note down the bottom go to 434.5.2 , this section in mine has a lot of notes around it, I think basically its saying calculate your own min cpc size with energy let through and thermal withstand.