Adiabatic Equation

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DYCHE4230

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Hi All,

My first post on this site so here goes....

Can anybody help....Trying to establish if I would have to change the mec on a TNS system....

Ok So S=sqr(I2xt)/k

When establishing the value of 't' do I use the characteristics of the distibutors main fuse or the main switch in the CU or I have missed the point entirely?

And when establishing 'I' do I use 230v or 240v

Some direction in this matter would be most helpful.

Many Thanks in advance.

 

Admin1

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I know that all calculations on Singe-Phase has to be done using 230V.

 

PAUL M

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Hi All,My first post on this site so here goes....

Can anybody help....Trying to establish if I would have to change the mec on a TNS system....

Ok So S=sqr(I2xt)/k

When establishing the value of 't' do I use the characteristics of the distibutors main fuse or the main switch in the CU or I have missed the point entirely?

And when establishing 'I' do I use 230v or 240v

Some direction in this matter would be most helpful.

Many Thanks in advance.
Hi dyche,for this equation mate,t is the operating time of the OVERCURRENT device in seconds and can be obtained from the graphs in Appendix 3 BS7671,to calculate I we use Uoc/Zs this gives you your PEFC,Uoc is taken as 240v.the 230 everyone is refering too is Uo.only one letter but makes a difference.hope this helps.if you want to check this out mate page 114 GN3.

 

DYCHE4230

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t is the operating time of the OVERCURRENT device in seconds .
Thanks PaulM...So for the MEC am I using the graphs for the dno's fuse or the main switch in CU?

Cheers

 

PAUL M

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well i would say the main fuse but i might be wrong.Guiness DrinkGuiness Drink

 

steptoe

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WTF is MEC...????

does the OP mean the Earthing Conductor.?

sorry if that sounds rough, but if that is the case then is there also a secondary Earth conductor that will also need taken into account?

(if you have a main earth then that would imply there is another earth also)

that would possibly rule out likelihood of domestic single phase, more like 3phase with secondary supplies.

therefore requiring 400v equasions and correction factors.

 

Deano

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Hi All,My first post on this site so here goes....

Can anybody help....Trying to establish if I would have to change the mec on a TNS system....

Ok So S=sqr(I2xt)/k

When establishing the value of 't' do I use the characteristics of the distibutors main fuse or the main switch in the CU or I have missed the point entirely?

And when establishing 'I' do I use 230v or 240v

Some direction in this matter would be most helpful.

Many Thanks in advance.
You dont have to use the adibatic formula, what you need to astablish is the size of the incoming tails then use the table in the big red book of RCD's. Cant remember page number but if you look up 'earthing - conductor size' in the index you will find it. ; \

 

DYCHE4230

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You dont have to use the adibatic formula, what you need to astablish is the size of the incoming tails then use the table in the big red book of RCD's. Cant remember page number but if you look up 'earthing - conductor size' in the index you will find it. ; \
I know about the tables, Just trying to establish if the existing earthing is ok. If not then I'll change them as per table 54.7

Cheers

 

Carter

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It's about protecting the cable from the energy 'let through' in that interval when the system is effectively "in limbo" between the time a fault occurs and the time it is 'seen' by the protective device and the supply is interupted so it is the characteristics of the DNO's fuse that determine that. (unless there is a complete mismatch with the let through of the CU switch disconnector and you're expectig that to explode before supply is interupted!)

 

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