R1+R2 Calculations from ZS

[Chr!s]

Heres a link to a document malcolm

https://www.electriciansforums.net/attachments/indg354-pdf.120195/

 

[malcolmsanford]

Heres a link to a document malcolm

https://www.electriciansforums.net/attachments/indg354-pdf.120196/

Many thanks Chr!s ...........scholar and a gentleman

 

[nickblake]

My hair looks very much like lennys profile pic during live testing , spot on sintra just What i think

 

[mickeyspark]

Thanks Guys, apparently we have to have a MWC, thought I was right about R1+R2 calcs, this web site is so good for scratching the head and finding info however trivial it may seem. Its a huge world of Electrical knowledge we have to have in our heads, we can't pretend to have all the answers.

Thanks Mickeysparks

 

[ganz]

If
zs=(r1+r2) + ze

cant you just transpose the formula to read r1+r2 = zs - ze ?

 

[Sintra]

In a word no.

 

[ganz]

how come?

 

[yellowvanman]

Because when you measure Zs, you could have parallel paths (pipework etc.). When you measure R1 + R2 you should be measuring just the cable resistance, ie. conductors not terminated at a busbar.
So generally worst case Zs = Ze +( R1 + R2), and when you measure Zs it will probably be less (due to the parallel paths). Thats the theory anyway!

 

[telectrix]

how come?

because if you are doing things right, you do not energise the circuit until you have proved it with dead testing, i.e. R1+R2 and IR test.

 

[ganz]

i know you are right, but in theory transposing the formula should lead to the same result

 

[telectrix]

except for the aforementioned parallel paths, resulting in a lower Zs than from calc. from R1+R2. if you calc. using R1+R2+Ze, that will give you the expeced value of Zs. if you then measure Zs and get a higher value, that's when you start worrying what effect has the MCB and RCD on the result.

 

[brizospark]

what way to measure when a circuit is RELYING on pipework/armour as the cpc?

 

[telectrix]

then you clip your green lead on to the armour/pipework

 

[brizospark]

but then you are still getting cross readings

 

[telectrix]

do you mean testing R1+R2 when due to containment being the cpc, you can't eliminate parallel paths?