Type d mcb

[Alanz1]

Hi
Firstly, I am not a spark, merely mechanically qualified to trade, struggling with an electrical question at college.

Could anyone assist with this question ?.

A 230vac circuit with a Zs of 0.6 ohms on a TN system.
5kw load/25 amp type D MCB.
Would a 0.4ms be possible ?.

I have used
25x20 =500
230/500=0.46
0 46x0.8 = 0.368

To me the answer may be yes as 0.368 is less than 0.4 seconds, but I'm a bit lost.

I don't understand the tables showing 0.4/5s trip times for type D mcbs.

even more confusing is there are different figures used on the 2 tables below.











Can anyone help out ?.

Many thanks.

 

[poni]

I don't understand the tables showing 0.4/5s trip times for type D mcbs.

this just means that the current will be the same for both 0.4 and 5s , it will operate so fast that its the same time curve,

Would a 0.4ms be possible

are you sure its 0.4ms and not 400ms or 0.4s

if its 400ms then you are wrong , if its 0.4ms then you are wrong again ,

the bottom table is based on 80% to allow for the cable heating up during the fault, normally you test the cable at room temperature so the rule of thumb is 80% under the top table


your Zs = 0.6 ohms but you can only have 0.36 ( at 80% of 0.46)ohms



I don't know what this is or where you got it from?

I have used
25x20 =500
230/500=0.46
0 46x0.8 = 0.368

 

[Alanz1]

Many thanks Poni for your advice.

My question states 0.4, so I assume that's 0.4 seconds.

If the circuit is supplying a 5 KW load from a 25 amp type D MCB state whether a 0.4 disconnection time is possible and justify.

Am I correct in thinking that my 0.36 ohms is acceptable ?.

Been at this for 6 hours watching YouTube

 

[kerching]

How did you arrive at that calculation?

 

[Alanz1]

How did you arrive at that calculation?

To be honest, I'm not sure.
I used 20 times 25 (upto 20 for type d mcb times 25 for current).

Any pointers ?

 

[poni]

your Zs = 0.6 ohms but you can only have 0.36 ( at 80% of 0.46)ohms

this is the answer
the max Zs you can have for a 25A D type MCB is 0.36 Ohms (using role of thumb) , and you have a Zs of 0.6 Ohm, so NO it will not disconnect in 0.4s

 

[Alanz1]

Could you comment on my formula please Poni ?.

25x20 =500
230/500=0.46
0 46x0.8 = 0.368

Many thanks.

 

[poni]

Could you comment on my formula please Poni ?.

25x20 =500
230/500=0.46
0 46x0.8 = 0.368

Many thanks.

I think I seen that before ,but I have never had to use it


I see what you have done,
20 X I (25A) gives you a fault current,
so you need 500amps to flow
V/I=R
80% of R= 0.368Ohms

where do you get the multiplier from? have you a table that you use?

 

[Alanz1]

I'm using 20 as the multiplier for a type D mcb (10 to 20 recommended)
5 is for type B
10 is for type C

25 a mp is current.

Does that sound OK?.

 

[poni]

where have you seen that multiplier ?

I'm using 20 as the multiplier for a type D mcb (10 to 20 recommended)

who recommends this figure ? and why 10-20 ?
who says if you use the 10 or use the 20?

have you seen it written down in a book ?

 

[Alanz1]

I can't find in in the 18th regs book, so I think I got it from YouTube videos.

I have used 230/0.6 for Zs reading, giving 383 amps.

Now I'm trying to see if this current is suitable for a type D mcb. I'm looking at Characteristics table in regs trying to understand it

 

[poni]

Now I'm trying to see if this current is suitable for a type D mcb

but you already know this
you need the Zs to be below 0.36 ohms and the Zs you have is 0.6 ohms
why do this twice?

 

[poni]

get yourself an on site guide for the 18th ed

 

[Alanz1]

I have the 18th edition book.
I don't understand the characteristics table for type D mcb.

I spoke with an experienced electrician today, he doesn't understand what the question is actually asking for.

 

[monkey5]

Think about your 0.36, what units is that? You don't need to do any sums the answer is there in the table, but you have worked it out too. You've been given 0.6ohms as your Zs? I think your over thinking it.

 

[poni]

I don't understand the characteristics table for type D mcb.

what bit don't you understand?

 

[Alanz1]

My lecturer says look at the table, all the amp ratings seem to trip at the same time, I just don't understand how to work the table.

 

[poni]

state whether a 0.4 disconnection time is possible and justify

you need the Zs to be below 0.36 ohms and the Zs you have is 0.6 ohms

to disconnect in less than 0.4s you need a current of 500Amp to flow
to get 500Amps to flow you need a Zs of less than 0.36 Ohms
you have a Zs of 0.6 ohms that will only let 383Amps flow, that is 117Amps less than you need to trip the MCB in 0.4s and disconnect the supply

 

[kerching]

I spoke with an experienced electrician today, he doesn't understand what the question is actually asking for.

In that case I would be questioning his experience
In its most basic of forms the question is asking if a given number is more /less/same as a number in a table/calculation, isn't it?

 

[poni]

My lecturer says look at the table, all the amp ratings seem to trip at the same time, I just don't understand how to work the table.

they do all trip at the same time, but at different currents,
MCBs are made to trip at 0.4s to meet the regs,
if they did not trip in 0.4s they would not meet the regs and we could not use them ,
just like when you have a high Zs and insufficient current flows the MCB will not trip in 0.4s
look at your 383amps and see what time your MCB will trip it will be a lot more than 0.4s