HELP! calculating ze

[Ste69]

Is it possible to calculate Ze?

I am in the middle of an assignment, and have this question to answer.

When confirming automatic disconnection of the supply it is necessary to determine the external earth fault loop impedance(Ze) of the system. State three methods by which this can be achieved.

i have 2, inquiry and measurement (my answer is a little more in depth lol)

to my problem, i want to say calculation and the tutor has said this is correct BUT when i look into it i can not find the calculation! i can work out (Zs) i.e Zs = Ze + (R1 + R2)

so am i right in thinking the question is wrong? or am i being an idiot and missing something here?

sorry for the long winded question.

 

[Zebra Stripe]

Come on dont you?
Just to see if everybody else is as useless, or perhaps even more usless hopefully.....cos most of the time I think Im just playing at this, even faking it or cheating by looking in the book and one day I'll do it properly.

 

[ArseyDee]

You enjoy reading works of fiction then?!

I must admit I do enjoy......and one day I will send my collection to the relevant schemes as proof of their assessment shortcomings.

 

[Zebra Stripe]

And they will do nothing cos, how can you prove the results were made up and they too interpret the Certs completely different each time....I was told to put in the SoTR under max Zs required by BS7671 the 80% value one year and then the design value the next

 

[ArseyDee]

Proving the results......that's true, but there's varying levels of bull****

 

[Zebra Stripe]

Ah, but the canny drive by specialist is well versed in the art of confussion and may only reach severe levels of BS when at full stress levels, ie: 1hour for an EICR or £150 for CU change. Once this point has been reached he reverts back to semi-comatose state and continues to support Manure without any real signs of disease or distress

 

[ArseyDee]

confussion

I'm confussed!

 

[Zebra Stripe]

Hey just realised I had no idea what this thread was about................so can ya calc Ze yes, should you calc Ze when not in C&G land No, why - cos you'd have no idea whether you had an adequate earth for the the installation.....maybe the C&G question wants you to say 1) Measure, 2) Enquire from DNO and their vast records of external loop impedence values for all the properties within their area, and 3) Max Design values

 

[GLENNSPARK]

Hey just realised I had no idea what this thread was about................so can ya calc Ze yes, should you calc Ze when not in C&G land No, why - cos you'd have no idea whether you had an adequate earth for the the installation.....maybe the C&G question wants you to say 1) Measure, 2) Enquire from DNO and their vast records of external loop impedence values for all the properties within their area, and 3) Max Design values

try `em then....they will ask you what the earthing arrangements are....you will give them that information and they will reply either .35 or .8

 

[Zebra Stripe]

try `em then....they will ask you what the earthing arrangements are....you will give them that information and they will reply either .35 or .8

Hey did you not note the hint of sarcasm in "vast"......

 

[GLENNSPARK]

hmm......

 

[Zebra Stripe]

Well, this is I think, a C&G question.
And in their eyes the DNO do hold this info, and I know in reality it is the design values which would be given out, but hey 3 ways of getting a Ze for a C&G question.....although having said that the 3rd answer is probably calc it, but as we all know this would not be a good idea.

 

[Silly Sausage]

To calculate a Ze, you would require an accurate Zs (measured) for starters and know precisely how it was measured, taking into account parallel paths etc.

So, if you were measuring a Zs you may as well just measure Ze!

 

[mark80]

Theoretically you can ring the DNO, but they will just tell you the maximum ze permissible, and a PSCC of 16kA. Knowing the length and size of the cable into the property from the transformer, it could in theory be calculated, but you'd need to know the resistance through the transformer.

I've got a table from a guide (can't remember which off the top of my head) but that gives rule of thumb values for ze from cable length and conductor CSA. I'll try and post it up at some point if you want? It's just for estimating values at the design stage of a project though