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

 

[Sparky Ninja]

any other information available?
Rating of type D device?
Rating and type of MCCB?

A couple of variables otherwise.

 

[paulthemagpie]

working out the type d device will need to be a 16A

they rating of the MCCB is 125A

I can make reasonable assumtions. so i can pick the type of MCCB which i feel is most suitable as long as a document it.

thanks for your help

 

[Sparky Ninja]

Well if ADS is your protective measure, then you will be requiring a fault current of 320A for the 16A type D device.

So you need an MCCB that will disconnect in the required time. As it's a distribution circuit, this will be 5 sec for a TN-C system or 1 sec for a TT system.

So select a device that requires more than 320A to achieve 5 sec or 1 sec disconnection, otherwise the MCCB will operate before the type D device.

 

[paulthemagpie]

i was going with a TN-C system!

 

[Brightspark2]

The question says determine using manufacturers info, so you need manufacturers info....

 

[Sparky Ninja]

The question says determine using manufacturers info, so you need manufacturers info....

With the limitations of the question, it's fairly simple.
Manufacturers information will be too defined will so little reference info.

It may state it in the questions, but it's not necessary. Unless there's a load of other information the OP hasn't given.

 

[paulthemagpie]

as it hasnt provided the manufacutes info, im suppose to find it myself.

i have spent hours trying to find any manufacture info regarding this and cant fidn anything. Can anyone help me out?

 

[Richard Burns]

This is a section of manufacturers data from Hager covering discrimination and has the information you need to determine if discrimination is achieved. The information is probably pitched above the level to which you are currently working so do not get too bogged down in it.
But probably useful to know! The table on 137 shows the fault current to which discrimination would be achieved comparing MCB to MCCB.
View attachment Hager selectivity and discrimination Protection Devices.pdf

 

[paulthemagpie]

Hi, thanks for sending that bur i xant make heads or tail of it

Can help me understand it and apply it to this question

 

[paulthemagpie]

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

 

[Guitarist]

Do you have no tutor available to help you on this course? Surely a mentor has been assigned to help you.

 

[paulthemagpie]

im training provider is a load of **** to be honest mate! they gace me a loads of reading matertial to read which is a load of crap. I was told a tutor would then help me when i come to the centre. Then when i get here the tutor saying he not allowed to give me any input. Now its an open book exam. Im fuming with them to be honest. He wont even point me in the right direction with it.


Im very greatful to the guys in this forum. Im a guy that only has to be told something once then i remember it forever, so its not im some thicko its just my tutor wont help me. Once i get the help from this forum i then know it. When someone just gives me an answer i reverse engineer it to understand it.



The only good thing about it is this is my last question i need answering for this unit.

 

[Dom.]

304 again?

Get a clear plastic sheet & a pen.

Turn to page 301 BGB trace the curve for 125A
Turn to page 303 BGB trace the curve for a 16A*

You will see the two lines never cross, regardless of the time it takes to trip or the amount of fault current ergo "discrimination" is achieved. (When we discriminate we separate things/people out.. We have separation here. The MCB for the Welder protects the welder, nothing else has to step in**)

Dom.

*I'm putting my welder on a 16A based on http://www.electriciansforums.net/e.../53724-suitable-wiring-system.html#post488744

**I'm talking as one student to another. If someone can offer a nice simple way to explain discrimination please do! :bigear:

 

[trev]

What qual will you have assuming you pass this course Paul? Some of the things you're saying are a little worrying. Why was the reading material crap? What grounds did your tutor give for him not being allowed to give you any input? Atutor is just a teacher with a different title, it's his/her job to teach

 

[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.