Using The Adiabatic

[securespark]

Guys n Gals,

When using the Adiabatic to calculate minimum csa of a main earthing conductor, which "k" value is chosen?

 

[jockspark]

hi page 129 brb table 54.2
143/133

 

[securespark]

Also, consider the following example: (I have done this from a 16th Ed. workbook as I don't have my 17th to hand, yet again...)

Protection via 40A BS88 fuse, chosen a 4mm² cpc. Zs =1.2. Is cpc large enough?

The answer is yes, the calculated csa is actually 2.46mm².

Now, by the calculation, a 2.5mm² cpc is seen to be adequate, so why don't we use that size?

 

[ColJack]

depends on the material...

 

[Spark123]

I'll vote for 143 too, as you are unlikely to be dealing with a conductor greater than 300mm² in a domestic situation!

 

[securespark]

jock - I have not got my book...Which "k" value would you choose? 115? If so, why?

Edit: sorry, missed your figures!

RF. Would you not choose 115?

depends on the material...

Go on....!

 

[Spark123]

Now, by the calculation, a 2.5mm² cpc is seen to be adequate, so why don't we use that size?

If it is big enough to satisfy the adiabatic then there is nothing from stopping you from using that size, unless you are going into the realms of PME which has additional req's.
Use k=115 if it Cu and is part of a cable or bunched in, such as you would for a CPC in a cable

Is RF posting invisible posts again

Main earthing conductor is usually a single conductor in free air so it isn't bunched in nor part of another cable so has less acting on it. If it is Cu and <300mm² then k=143.

 

[jockspark]

thermoplastic
copper
70c
also mi at 70c
thats the main 2 there is more tin soldered ect
what exactly are you after

 

[securespark]

Just querying something: I've been told to use the figure 115 from table 54.3.

Your 143 from 54.2 would actually lower the calculated csa compared to the 115 factor.

Maybe that's why: I've been given the factor that gives the larger csa -
a bit of built-in leeway.

 

[securespark]

Now, by the calculation, a 2.5mm² cpc is seen to be adequate, so why don't we use that size?

If it is big enough to satisfy the adiabatic then there is nothing from stopping you from using that size, unless you are going into the realms of PME which has additional req's.
Use k=115 if it Cu and is part of a cable or bunched in, such as you would for a CPC in a cable

Is RF posting invisible posts again

Main earthing conductor is usually a single conductor in free air so it isn't bunched in nor part of another cable so has less acting on it. If it is Cu and <300mm² then k=143.

On the first bit of your reply, what if you did reduce the cpc to 2.5? Wouldn't the loop then be higher, with resultant changes to the I & t values?

On the second bit, one of my queries with these "k" values is that they don't really allow for incoming supply cables.

What if your old TN-S supply cable is not thermoplastic?

 

[Spark123]

What sort of conductor is it, is it a main earthing conductor on its own in free air or is it a conductor which is part of a cable?

For the BS88 example you give above, Zs = 1.2
230/1.2 = 191.667A
A 40A BS88 fuse will disconnect in about 3 secs
S = ((191.667²x3)^0.5/115 = 2.88mm²

 

[jockspark]

table 54.3
k for protective conductor incorporated in a cable or bunched
table 54.2
k for insulated conductor not incorpotated in a cable
or bunched

 

[Spark123]

On the first bit of your reply, what if you did reduce the cpc to 2.5? Wouldn't the loop then be higher, with resultant changes to the I & t values?

Yes, but as you did the adiabatic at the design stage it wouldnt be a problem

On the second bit, one of my queries with these "k" values is that they don't really allow for incoming supply cables.

Nothing to do with us what the dno do.

What if your old TN-S supply cable is not thermoplastic?

Ditto

 

[securespark]

RF. Would you not choose 115?

Is RF posting invisible posts again

I could have sworn he did post...honest! Or am I just VERY tired?

 

[holmslaw]

..