will discrimination be achieved?

[paulthemagpie]

hi peeps,

just need help with this quesion.

The proposed design for the installtion includes the following protection devices:

> BS EN 60898 type D circut breaker for the MIG welder
> an MCCB rated at 125A for the distubution circut to DB2

Determine, using manufatures info, weather discrimination will be achieved?

thanks guys if you can help me with this i will be in forever debt to you guys!

thanks

 

[paulthemagpie]

This is just a small part of a big pie! Dont worry i wont be a fully qualified electrician next week! Lol it going to take like 12 mOnths and they loads of workshops then onsite training

The tutor said it was because it was an assignment

 

[trev]

Worrying for you mate not about you

 

[Richard Burns]

for example using the table on page 137.

if we go along the MBN 16A and down the H125D at 125A it gives us a figure of 3.7.

but the no definition of what 3.7 is and weather that means discrimination is achieved

Because of the nature of the tripping curves at some point the curves do cross and at this point you no longer have discrimination because the up stream device (further away from the fault) will trip before the one nearest the fault. The table is saying that up to a fault current of 3.7kA the MCB will trip before the MCCB and so discrimination is achieved.

If your PFC is greater than 3.7KA then discrimination is not achieved.


Discrimination is the method of ensuring that in a cascade of protective devices the device closest to the fault will operate before the ones further upstream.
This prevents nuisance denergisation of items that are not in fault.
In a domestic situation (where discrimination is not so clear) you may have as a sequence
Suppliers main fuse...CU MCB...FCU fuse...internal equipment fuse.

If a fault occurs at the equipment then the internal equipment fuse should trip first and therefore clear the fault.
If there was a fault at the equipment and the main supplier fuse went first then the whole house would be off and require a DNO call out, so obviously not ideal.

 

[Richard Burns]

for example using the table on page 137.

if we go along the MBN 16A and down the H125D at 125A it gives us a figure of 3.7.

but the no definition of what 3.7 is and weather that means discrimination is achieved

I have just looked at the table and notice that you have given a value for the B type MCB and I think you were using a D type. The NDN rows on the table are for D type and give a fault current of <2.6kA for discrimination vs a 125A MCCB.

 

[honeymonster]

MK is always my favoured brand.