Manual T-N-C-S calculation question

[creggors]

Hi lads, im wondering if someone could clarify a question I have regarding doing a manual fault loop impedance calc for the circuit shown below (t-n-c-s). As you can see the fault is taking two different paths back to the utility transformer, im trying to determine which path I should base my calculation on,in other words will I have to take the resistance on the main neutral conductor(R3)into account. Should I be assume that the neutral line will have more resistance therefore my fauly loop impedance should be taken from that? I know this is only my first post,but im doing a project and just found this forum.any help will be much appreciated,thanks

 

[sparkytim]

So are you trying to calculate the PFC(Prospective Fault Current)?

If so then this should be the MAXIMUM possible and that is dependant on the LOWEST Zs value.

So on that basis which path would give you that?

 

[steptoe]

this should be in the learning section?

 

[Manator]

this should be in the learning section?

I agree, thread moved.

 

[ADS]

Going by those drawings the fault will take both paths - it's pretty much impossible to calculate the Ze or the PEFC without further info.

 

[steptoe]

calc for drawing one is easy,

calc for drawing 2 is impossible,

calc for drawing one=

R1/1 + R1/2 + R1/3 + R2/2 + R2 + R3 = EFLI

drawing 2 is missing the value for earth to star point, (Ra) so is an impossible question to answer,

have you gave us the full info provided in the question?

or do we assume that max Ze of 0.35 ohms in in effect?

or max PSCC of <16Ka ?

more info required

 

[creggors]

Pscc has a maximum of 40ka at the transformer,which according to Amtech gives a Ze of 0.0058 ohms. Sorry I wasnt too sure about which section to post in. Im trying to compare a manual calc with Amtech for a project.

 

[sparkytim]

Is the Transformer in the front room?

 

[creggors]

Well due to the size of the load, im using a utility transformer, assuming a maximum of 40KA. Maybe I should have included the cable runs

R1/1 = 10meters of 185mm sq XLPE

R1/2 = 40meters of 120mm sq XLPE

R1/3 = 50meters of 4mm sq PVC

R2/2 = 50meters of 2.5 sq PVC

R2 = 40meters of 70mm sq XLPE

R3 = 10meters of 185mm sq XLPE

 

[Sidewinder]

Why 40kA?

Who is your DNO, what do they limit their max fault currents to by network design?

Or, is this a private transformer?

As far as the XLPE goes, I am guessing it is XLPE/SWA/PVC?

If so what are you using as the cpc, a core or the SWA?

As far as the 4mm & 2.5mm pvc goes is this Flat Twin?

 

[green-hornet]

Is this an exam question? or a working example? because like sidewinder says above I am a little lost here as to what you need to be answered.

 

[creggors]

This is a calculation I have to do regarding a college project.The 40ka is the maximum pscc that can be given by the DSO's utility transformer, please note this is based on an Irish system, however everything should be the same, out installation methods are the same as the BS regs. All XLPE's are SWA/LSF. 4mm and 2.5 shall be single core cables ran in trunking. Im wondering about the fault loop impedance path in the event of a phase to earth faullt, mainly, does the fault go to earth from the MDB or out through the main incoming neutral? thanks

 

[sparkytim]

The fault will take BOTH paths but proportional to the Impedance, hence as the Earth stake route will have a very much larger impedence it can allmost be disregarded and the solid Nuetral path used for calculation puposes.

 

[creggors]

Brilliant, thank you