CPC size help

[silverfox1]

Hi all been following this thread with interest,thanks Lenny for a final outcome. I enjoy reading this forum always learning something.

 

[pencilpusher]

Yep, this is a good thread. I get the idea of this now Lenny so thanks for explaining; I was just about to knock an outside wall down to get at my T&E !!

I must admit, I always thought the protective device would not "let through" 0.8kA when its a 20 amp breaker. That is the confusing thing about high currents and time graphs. Surely, a 0.8kA fault current would trip the breaker in under 0.1 secs and the cpc would not "have time" to melt?!

Is that what manufacturers "let through" data is concerned with, how much actual fault current, the device will accept before it actually trips and stops any current causing problems?

Cheers

PP

 

[Deleted member 26818]

Here's a suggestion:
Using the tables in Appendix 3, for devices other than MCBs.
Starting at 5s, conduct the adiabatic equation using the values in the tables and work your way down to 0.1s.
I think the result will surprise most people.

 

[pencilpusher]

I am done with calcs today Spin. Whats the surprise?

 

[telectrix]

basically, we are fitting far too large cables for various jobs, thus fuelling the ridiculous price of said cable to the delight of the manufacturers and the south african copper mine owners.

 

[pencilpusher]

So what IS the answer to this?

 

[Guest123]

AFAIC the answer I've given using the manufacturers data is the answer, end of.

Spin is referring to fuses in his post, not your MCB.

 

[pencilpusher]

Lenny, I wasn't questioning your answer just the answer to the copper miners taking the p**s. Same thing i suspect!!

The let through data I have found on t'internet (not easy) shows a figure of 19.5. I suspect it is 19.5 something which is why you have quoted as 19500?

 

[Guest123]

The let through data I have found on t'internet (not easy) shows a figure of 19.5. I suspect 19.5 something which is why you have quoted as 19500?

Yes, it's 19.5KA = 19,500A.

 

[Deleted member 26818]

I am done with calcs today Spin. Whats the surprise?

Other than the required CSA decreases as the PEFC increases, there isn't one.

 

[pencilpusher]

Other than the required CSA decreases as the PEFC increases, there isn't one.

...and I wonder why I am confused!

 

[Deleted member 26818]

In general, protective devices operate faster, the greater the current.
As such as the current exists for a shorter period of time, there is less heating of conductors.
This results in a smaller CSA being required.
MCBs are an oddity, because they utilise two methods to operate.
An electromagnetic coil and a Bi-metallic strip.
In the tables in Appendix 3, these are represented by the straight line for the electromagnetic coil and the curve for the Bi-metallic strip.
The tables only show the amperage required for the MCB to operate within the times specified by BS7671, 0.1s to 5s.
Without having the manufacturer's time/current curve available, it is quite likely that this Amperage will also cause the MCB to operate instantaneously (within 0.01s).
When you start picking and mixing the values, choosing one that is measured, and another that is tabulated, you will when conducting the adiabatic equation obtain a result that produces a higher value for CSA than is actually required.
If you wish to use the measured value of PEFC 800A, you should not use the tabulated time values in the tables in Appendix 3, but those of the manufacturer.
If you wish to use the tabulated time values from the tables in Appendix 3, then you should also use the tabulated current values.

 

[pencilpusher]

So basically, the adiabatic equation should be used during the design stage. If already installed and a measured PEFC taken, the manufacturers data should be used for I2t?

 

[Deleted member 26818]

You can use either the manufacturer's data, or that contained in the tables in Appendix 3.
Just not a pick and mix.

 

[La Poste]

Great thread I didn't know about the I2t and k2s2 equation before.

I²t must be less than or equal to K²S², really helpful.

I just wonder where I am going to get the energy let through figures for a BS3871 old push button type 1 Wylex MCB.

Any one got any ideas?
If not I guess I will have to phone them up.

Look at page 243:
http://books.google.co.uk/books?id=...QWLh-isCA&ved=0CD4Q6AEwAw#v=onepage&q&f=false