Another question

[Rather be Fishing]

Sorry for all the questions

The maximum tabulated value for a protective device is 1.85 ohms.

And the measured R1 + R2 is 1.03 ohms

What would the maximum ze 've expected to be allowed.

A) 0.35
B) 0.82
C) 21
D) 0.21




For some reason I typed in

1.03 x 0.8 on the calculator as I had the rule of thumb figure in my head and it came out a 0.82

So I want with that and that's what the answer was


Does that mean I applied the right maths

Or did I just get coincidentally lucky?

 

[loz2754]

Sorry for all the questions

The maximum tabulated value for a protective device is 1.85 ohms.

And the measured R1 + R2 is 1.03 ohms

What would the maximum ze 've expected to be allowed.

A) 0.35
B) 0.82
C) 21
D) 0.21




For some reason I typed in

1.03 x 0.8 on the calculator as I had the rule of thumb figure in my head and it came out a 0.82

So I want with that and that's what the answer was


Does that mean I applied the right maths

Or did I just get coincidentally lucky?

No.
Yes.

The formula for calculating Ze is:

Ze=Zs-R1+R2
Ze=1.85-1.03= 0.82 ohms

 

[Rather be Fishing]

No.
Yes.

The formula for calculating Ze is:

Ze=Zs-R1+R2
Ze=1.85-1.03= 0.82 ohms

Thank you @loz2754

Good job I asked but what a fluke that was

I don't suppose you know what this 0.8 rule of thumb figure I have in my head is for or applied to do you

 

[loz2754]

Yes the Zs is normally measured when the cables are at room temperature of around 20 degrees.
The Max Zs permitted is for when the conductors are at maximum operating temperature which for PVC is 70 degrees, at which temperature the resistance will be higher. So the measured Zs is multiplied by the 0.8 factor to account for this difference.

 

[Dustydazzler]

Been 25 years since I was at school but isnt it only the OSG that you apply the rule of 0.8 to and not to the Big regs book or V.V ?

 

[loz2754]

Been 25 years since I was at school but isnt it only the OSG that you apply the rule of 0.8 to and not to the Big regs book or V.V ?

The OSG figures are already temperature corrected.
BBB values are at maximum operating temperature.