adiabatic equation help!

[schoe]

Hi all i was wondering if anybody could shed some light on this equation or part of it. I think i understand how to work out the equation its self but its this part i dont understand.

In order to apply the adiabatic equation we first need to calculate the value of I (fault current) using this equation - I=v/zs. I undersatand zs is the earth fault loop impedance but where does the zs figure come from in ordor for me to divide the volts by it. i understand how to do a zs test when cables are installed, but how do you come across the figure before you start running cables?

any examples or advice would be great,

Thanks Chris

 

[Guest123]

If the circuit length is known then the equation Zs = (Ze+(R1+R2)) may be used to find If

Alternatively the I2T values for the protective device from the manufacturer may be used.

 

[telectrix]

the only way to determine Zs before installing cables is to know the cable length and resistivity. then you can calculate R1+R2 from table 9A osg, then add that to Ze (at design stage, you would get this by enquiry) that gives you Zs.

 

[malcolmsanford]

On a design you have to use the equation of Zs = Ze + ( R1 + R2).

The R1 + R2 you have decided to use a 2.5mm /1.5mm singles for a radial circuit that is say 27 metres in length. So it would be

19.51 X 27 / 1000 = 0.53 ohms.

The trouble then with a design question is you need the Ze and in most cases this is not available from measurement and therefore you can only get it by inquiry. If you can't then you have to either A take a Ze from a next installation or use the guides, so for a TNC-s it would be 0.35 ohm

So now you will have Zs = 0.35 + 0.53 = 0.88 and there you have the Zs.

 

[telectrix]

fastest fingers Lenny..

 

[GLENNSPARK]

If you have the use of a loop tester you can measure direct but watch yourself as its a live test and you will be working close to energised components. If you dont have access to a loop/pfc tester then by calculation (as above) Dont forget that some loop testers dont have what we call a low current facility so when doing a Zs on an RCD/RCBO protected circuit the RCD/RCBO wont stay in. This is also the case for robin type testers that featured a device called DLOC.....the device would give the RCD/RCBO a shot of DC which had the effect of "freezing" it so it would stay in during measurement.....but modern RCDs will detect it and wont stay in as a result....you can wire em out however or do a Ze +R1 R2 as above

 

[schoe]

Thanks malcolm, regarding the 19.51X27 where does the divide by a 1000 come from? Thanks again

 

[Guest123]

regarding the 19.51X27 where does the divide by a 1000 come from?

The 19.51 value is in milliohms per metre so you need to divide by 1000 to convert to ohms per metre.

 

[brucelee]

and when using the calculated figures for design you should also use a Multiplying factor of 1.2 to take into account increase in resistance due to operating temp

 

[Guest123]

If you have the use of a loop tester you can measure direct

Not at the design stage he can't.:cool3:

 

[GLENNSPARK]

Not at the design stage he can't.:cool3:

Sorry lenny...well pointed out mate:cool3: