Adiabatic equation??!!

[HappyHippyDad]

Hello all..

I have read some of the threads on the 'adiabatic equation' and still cannot understand it!

I have my assessor turning up on thursday to view my CU change, I have gone through everything with a fine tooth comb and am trying to cover everything he may find fault with. My ring main is 2.5mm[SUP]2 [/SUP]with a 1mm[SUP]2 [/SUP]cpc. OSG states min cpc should be 1.5mm[SUP]2 [/SUP]for a MCB 32A with less than 3kA PFC, but BS7671 says I can use the adiabatic equation.

The ring is on a 32A mcb. The PFC is 1.61kA.

I can see the adiabatic equation, but I'm not sure how to use it. Is I = PFC? How do I determine the operating time of my NHXB32 wylex MCB? Do I use 115 as my value for K?

I've looked at a couple of my neighbours houses and they both have 1mm[SUP]2 [/SUP]cpc on the ring (built in the 80's) so I'm assuming this must be ok, but I would like to give the assessor a proper answer if he spots this rather than just 'all the other houses have it and, from what I've read on the internet it seems ok'.

Thanks..

 

[trev]

http://www.google.co.uk/url?sa=t&rct=j&q=adiabatic+equation&source=web&cd=3&cad=rja&ved=0CDQQFjAC&url=http%3A%2F%2Fwww.tlc-direct.co.uk%2FBook%2F5.4.5.htm&ei=oaF0UNCcJYTK0QW4qoDQBw&usg=AFQjCNGJ6xlkPmiq0Z3mW_QsgV0VUYfRJw
A pretty straight forward explanation of it mate

 

[HappyHippyDad]

Getting more confused!!

PFC is 1.61kW , however it looks like I is calculated by using 230/Zs???

I've spent all day doing tests and drinking coffee!

I'm going to bed.

 

[Deleted member 26818]

You have to look up the time/current Tables in Appendix 3 of BS7671.
Look up the Table for your particular device, and use the values in the table.

For instance a 32A Type B MCB will operate in 0.1s at a current of 160A.
You take those values and use them in the equation.
160² = 25600 X 0.1s = 2560.
√2560 = 50.5964 / 115 = 0.3499mm².
115 being the value for K where the CPC is incorporated within a cable, such as in the case with T&E.

Some people mistakenly use the actual value of PFC, and then use the nearest disconnection time from the Tables.
This gives an incorrect value for CSA, as the disconnection time is incorrect.
For instance if we were to conduct the equation using your measured value, we would get:
16000² = 256000000 X 0.1s = 25600000
√2560000 = 5059.6442 / 115 = 43.996mm².
Obviously you would not use a CPC with a CSA of 44mm² for an RFC.

 

[Richard Burns]

Spin has answered simply and clearly (with some math errors) so I thought I would make it complicated!

Break it down a bit:
The OSG is making things simple for you:
If you have the conditions it states then a 1.5mm² will be OK without further calculation, easy simple.

If you have different conditions then you need to go back to the regs.
In your case as you say you need to use the adiabatic equation. t= (√(I²t))/k.

As Spin says you need to determine I for disconnection time t.
This can be done from the time current tables.

Or the description you had from the link says use I=Uo/Zs.
If you look up the max Zs for a B32 BSEN60898 MCB this is 1.44Ω
so I = 230 / 1.44 = 159.7 A
Pretty much the same as the table.

This is the best case scenario at maximum resistance for the circuit and gives the 0.4399 mm² csa and would be compliant.

(Damn typing too slow and have lost the post, start again more briefly!!)

In a real situation the resistance would be lower and can give a higher fault current especially for a short at the cables in the CU. the fault current would not reach your absolute maximum of 1610 A since the design of an MCB is such that it will limit the maximum current flowing through it.

So to use PFC as I and t as 0.1s to give a csa of 4.4 mm² would be wrong (but if it complied this would mean the circuit is well within parameters and could be used as a quick check).

Using the manufacturers values of I²t (the "energy let through") for a protective device (see the graph below for Eaton MCBs) would give an I²t value of about 5700 and using this in the adiabatic equation gives a value of 0.656 mm² so still compliant. Other manufacturers may vary.

If the inspector asks this then I would be very surprised!!!
View attachment energy let through MCB FAZ.pdf

 

[SJD]

If the inspector asks this then I would be very surprised!!!

I'd be suprised too, my experience is the inspector will be looking at things like connections in CU tightened, cables not resting on unprotected sharp edges, etc. And important stuff like the right labels! Plus get you to test something quick and easy to do, like Zs or RCD trip times.

 

[moospark]

http://www.google.co.uk/url?sa=t&rct=j&q=adiabatic+equation&source=web&cd=3&cad=rja&ved=0CDQQFjAC&url=http%3A%2F%2Fwww.tlc-direct.co.uk%2FBook%2F5.4.5.htm&ei=oaF0UNCcJYTK0QW4qoDQBw&usg=AFQjCNGJ6xlkPmiq0Z3mW_QsgV0VUYfRJw
A pretty straight forward explanation of it mate

Thats a handy site, cheers