ZS in a Ring Final Circuit

[Hefferl]

In a pure Ring circuit, assuming Line and CPC conductors are of equal CSA, would R1 + R2 at each outlet be equal and thus ZS at each outlet also be equal?

Many thanks!

 

[JUD]

Even if the line and cpc were different csa you would still expect all readings to be equal (or within 0.05ohms) in a ring circuit. The readings should be the sum of your R1 end to end reading plus your R2 end to end reading divided by 4.

(R1 + R2) / 4

Anything higher will be a spur

 

[Risteard]

(R1 + R2) / 4

Sorry to be pedantic but that should actually be (r1 + r2) / 4 = R1 + R2.

Zs should be tested at mid-point of ring.

 

[JUD]

Sorry to be pedantic but that should actually be (r1 + r2) / 4 = R1 + R2.Zs should be tested at mid-point of ring.

So it should. I knew what I meant :red card

Zs should be tested at every socket (incl. spurs) and highest reading recorded.

 

[ADS]

In a pure Ring circuit, assuming Line and CPC conductors are of equal CSA, would R1 + R2 at each outlet be equal and thus ZS at each outlet also be equal?Many thanks!

Yes. And as Jud stated, the CSAs don't have to be the same.

And the same R1 + R2 reading can be read at your cross connections - all the same value under ideal conditions.

Zs should be tested at every socket (incl. spurs) and highest reading recorded.

Agreed.

 

[SPECIAL LOCATION]

In a pure Ring circuit, assuming Line and CPC conductors are of equal CSA, would R1 + R2 at each outlet be equal and thus ZS at each outlet also be equal?Many thanks!

Despite what the others say....

NO if you do a Zs reading at each outlet on a ring it will get progressively higher until you reach the mid point of the ring furthest from the CU.

I think they are confusing the three step test method to verify the integrity of a ring circuit described in section 10.3.2 On Site Guide.. Pages 80 - 82..

With the above cross-over test method you will get substantially equal continuity results around a correctly wired ring, (i.e. No fig '8' bridges across the ring)

BUT

this is NOT a Zs reading....

With the ring correctly connected at the CU the ZS will increase as you go around the ring proportional to the effects of the parallel resistances down the two lives and two CPC back to the CU..

Consider a hypothetical example of the parallel resistances of a pure equally spaced ring with identical conductor size using "easy numbers"..

A CU and Five socket outlets 'A', 'B', 'C', 'D', 'E' ('C' would be the mid point of the ring)

lets say each conductor is 5ohms between each outlet.

so Live and CPC from CU to midpoint 'C' would be

5ohm 'CU' to 'A'

5ohm 'A' to 'B'

5ohm 'B' to 'C'

e.g. 15ohms up each leg to mid point...

and total loop round ring CU to CU would be

5ohm 'CU' to 'A'

5ohm 'A' to 'B'

5ohm 'B' to 'C'

5ohm 'C' to 'D'

5ohm 'D' to 'E'

5ohm 'E' to 'CU'

e.g. 30ohms round each leg.

so mid point R1 is 2 x 15ohms in parallel = 7.5ohms

and R2 is 2 x 15ohms in parallel = 7.5ohms

So R1+R2 = 7.5+7.5 =15ohms

which IS as expected equal to r1+r2/4

30+30/4 = 15ohms.

BUT consider the values you would get at outlets 'A' and 'B' (same as 'D' & 'E')

So 'A' has an R1 with 5ohms in parallel with 25ohms = 4.17ohms

as our conductors are the same R2 will also be 4.17ohms back to the CU

So R1+R2 = 4.17+4.17 = 8.34ohms.... which don't = our mid point 15ohms!

the same calcs at outlet 'B' are 10ohms in parallel with 20ohms = 6.67ohms

thus R1 + R2 becomes 6.67+6.67 = 13.34ohms... again NOT equal to our mid point 15ohms!

So NO Zs is NOT the same at all outlets on a pure ring!

If you wanted to put in some 'realish' numbers do the same exercises but assume 20m of 2.5mm (Singles then T&E) between each of the five outlets

SINGLES

20m of 2.5mm = approx 0.15ohms total r1=0.90

20m of 2.5mm = approx 0.15ohms total r2=0.90

r1+r2/4 = 0.45ohm

But at outlet 'A' this would calc out to 0.25ohms

and outlet 'B' would calc to 0.40ohm

T&E

20m of 2.5mm = approx 0.15ohms total r1=0.90

20m of 1.5mm = approx 0.24ohms total r2=1.44

r1+r2/4 = 0.59ohm

But at outlet 'A' this would calc out to 0.33ohms

and outlet 'B' would calc to 0.52ohms

Obviously each of these figures would add onto you Ze for the Zs

but again in ALL cases Zs increases as you proceed around the ring to its furthest midpoint...

(No spurs involved)!

:coffee

 

[ADS]

You blow me away, Specs

 

[superspark]

Me too :Applaud

have you thought of getting out more specsGuinness

but respect due

 

[SPECIAL LOCATION]

Me too :Applaud have you thought of getting out more specsGuinness

but respect due

I think about it a lot.......

But Admin has me locked in this cellar..

with only a small glint of daylight through the gap in the padlocked shutters!

I am only allowed out to the walled and barbwire protected garden on the 12th of the month... if it happens to be a Thursday!

 

[Hefferl]

Thanks Special for a very clear, understandable explanation - I questioned my lecturer on this recently and he was unable to explain this concept!!

Despite what the others say.... NO if you do a Zs reading at each outlet on a ring it will get progressively higher until you reach the mid point of the ring furthest from the CU.

I think they are confusing the three step test method to verify the integrity of a ring circuit described in section 10.3.2 On Site Guide.. Pages 80 - 82..

With the above cross-over test method you will get substantially equal continuity results around a correctly wired ring, (i.e. No fig '8' bridges across the ring)

BUT

this is NOT a Zs reading....

With the ring correctly connected at the CU the ZS will increase as you go around the ring proportional to the effects of the parallel resistances down the two lives and two CPC back to the CU..

Consider a hypothetical example of the parallel resistances of a pure equally spaced ring with identical conductor size using "easy numbers"..

A CU and Five socket outlets 'A', 'B', 'C', 'D', 'E' ('C' would be the mid point of the ring)

lets say each conductor is 5ohms between each outlet.

so Live and CPC from CU to midpoint 'C' would be

5ohm 'CU' to 'A'

5ohm 'A' to 'B'

5ohm 'B' to 'C'

e.g. 15ohms up each leg to mid point...

and total loop round ring CU to CU would be

5ohm 'CU' to 'A'

5ohm 'A' to 'B'

5ohm 'B' to 'C'

5ohm 'C' to 'D'

5ohm 'D' to 'E'

5ohm 'E' to 'CU'

e.g. 30ohms round each leg.

so mid point R1 is 2 x 15ohms in parallel = 7.5ohms

and R2 is 2 x 15ohms in parallel = 7.5ohms

So R1+R2 = 7.5+7.5 =15ohms

which IS as expected equal to r1+r2/4

30+30/4 = 15ohms.

BUT consider the values you would get at outlets 'A' and 'B' (same as 'D' & 'E')

So 'A' has an R1 with 5ohms in parallel with 25ohms = 4.17ohms

as our conductors are the same R2 will also be 4.17ohms back to the CU

So R1+R2 = 4.17+4.17 = 8.34ohms.... which don't = our mid point 15ohms!

the same calcs at outlet 'B' are 10ohms in parallel with 20ohms = 6.67ohms

thus R1 + R2 becomes 6.67+6.67 = 13.34ohms... again NOT equal to our mid point 15ohms!

So NO Zs is NOT the same at all outlets on a pure ring!

If you wanted to put in some 'realish' numbers do the same exercises but assume 20m of 2.5mm (Singles then T&E) between each of the five outlets

SINGLES

20m of 2.5mm = approx 0.15ohms total r1=0.90

20m of 2.5mm = approx 0.15ohms total r2=0.90

r1+r2/4 = 0.45ohm

But at outlet 'A' this would calc out to 0.25ohms

and outlet 'B' would calc to 0.40ohm

T&E

20m of 2.5mm = approx 0.15ohms total r1=0.90

20m of 1.5mm = approx 0.24ohms total r2=1.44

r1+r2/4 = 0.59ohm

But at outlet 'A' this would calc out to 0.33ohms

and outlet 'B' would calc to 0.52ohms

Obviously each of these figures would add onto you Ze for the Zs

but again in ALL cases Zs increases as you proceed around the ring to its furthest midpoint...

(No spurs involved)!

:coffee

 

[Lee321]

hmmm debate ROTFWL

why does the onsite guide say substantially the same?

there is a note for singles saying it will increase towards the midpoint if an error in connection is made.

 

[sparkytim]

hmmm debate ROTFWL why does the onsite guide say substantially the same?

there is a note for singles saying it will increase towards the midpoint if an error in connection is made.

Are you getting mixed up with Ring Main CONTINUITY tests?

When cross over connections are made then readings taken at each point should be the same, with SINGLES it is easy to get mixed up which leg is which unless marked when installed so if "error in connection" is made then readings will increase to mid point then decrease.

For Zs readings on the ring (Live test) then the same will be seen as you are getting further away from the supply the reading will increase with Max at mid point OR a spur of the ring.

 

[Lee321]

ah ha yes.

double barrel question isn't it.

r1 + r2 will

but live testing zs won't

 

[Nicky Tesla]

every leccy should know what spesh has explained its all pretty basic especially for a scientist like me

 

[SPECIAL LOCATION]

ah ha yes.double barrel question isn't it.

r1 + r2 will

but live testing zs won't

Its doing the cross-over connections that makes the difference...

e.g. during dead tests

Live cable1 is joined to CPC cable2

Live cable2 is joined to CPC cable1

But during live (real working connection) tests

Live cable1 is joined to live cable2 (both at MCB)

CPC cable1 is joined to CPC cable2 (both at earth bar)

No longer crossed over!

 

[kme]

Thank gawd......

I started reading the thread..........................

stopped.

read it again.

stopped again....................

thought about it a while..............................

decided everyone was deluded..........................

carried on, to get to the end and cuss you all for being daft.............................

And, as if by magic............................

Trailer-boyo appeared, with his explanatory post................

(I sincerely hope you`ve rewarded him with scoobs? Else he be at my Guinness again)

SL -