2391 question multiple choice IR help to break down

[sid521]

During the construction of an installation, insulation resistance tests have been carried out between live conductors and earth on the individual circuits and the L to E results are shown below. What is the expected value of insulation resistance L to E when the whole installation is tested?




can someone explain how to break down this question?

thanks

 

[westward10]

What are the four multiple choice answers?

 

[sid521]

didnt show answer

 

[sid521]

there is another with the answer

The insulation resistance of two circuits is of 40MΩ an 36MΩ respectively. When tested together what is the total insulation resistance:?

answer is 22mohms

 

[sid521]

What are the four multiple choice answers?

33.6mohms is correct answer

 

[DPG]

Bear in mind that when all the circuits are connected to the consumer unit, they are effectively in parallel. This should give you a clue as to how to answer the question.

Even without a calculator, there is one value that it will certainly be less than.

 

[westward10]

didnt show answer

For multiple choice there should be four answers. As above, with the answers you will require no calculation although it can be done by calculation.

 

[westward10]

https://www.electronics-tutorials.ws/resistor/res_4.html

 

[sid521]

ah yes is it the median 130 value divided by the number of circuits 4 so 130 divided by 4 equals 32.5 (correct on multiple choice) thanks

 

[Risteard]

During the construction of an installation, insulation resistance tests have been carried out between live conductors and earth on the individual circuits and the L to E results are shown below. What is the expected value of insulation resistance L to E when the whole installation is tested?

View attachment 119857


can someone explain how to break down this question?

thanks

1/Rt = 1/R1 + 1/R2 + 1/R3 + 1/R4...

 

[Risteard]

33.6mohms is correct answer

32.23 Megohms unless I punched something in incorrectly. Probably just a rounding error actually. (Maybe they were only going to 2 decimal places or something at each step.)

 

[sid521]

1/Rt = 1/R1 + 1/R2 + 1/R3 + 1/R4...

Thanks alos there is a similar question here

The insulation resistance of two circuits is of 40MΩ an 36MΩ respectively. When tested together what is the total insulation resistance:?

 

[Avo Mk8]

Thanks alos there is a similar question here

The insulation resistance of two circuits is of 40MΩ an 36MΩ respectively. When tested together what is the total insulation resistance:?

Risteard gave the formula, which becomes:
1/40 + 1/36 = 1/R which to 2 decimal places is:
0.025 + 0.028 = 1/R
Work out R from that

Consider - if the two IR figures were both 40Meg, then in parallel that would make 20Meg.
But one is a lower resistance than 40, so your answer is going to be a lower number than 20.

 

[sid521]

Risteard gave the formula, which becomes:
1/40 + 1/36 = 1/R which to 2 decimal places is:
0.025 + 0.028 = 1/R
Work out R from that

Consider - if the two IR figures were both 40Meg, then in parallel that would make 20Meg.
But one is a lower resistance than 40, so your answer is going to be a lower number than 20.

yes thanks makes sense now

 

[sid521]

Risteard gave the formula, which becomes:
1/40 + 1/36 = 1/R which to 2 decimal places is:
0.025 + 0.028 = 1/R
Work out R from that

Consider - if the two IR figures were both 40Meg, then in parallel that would make 20Meg.
But one is a lower resistance than 40, so your answer is going to be a lower number than 20.

so with the 0.053 you then move that 2 decimal places?

 

[Risteard]

so with the 0.053 you then move that 2 decimal places?

No. Your answer is 1 over 0.053, so you need to calculate that.

 

[Avo Mk8]

so with the 0.053 you then move that 2 decimal places?

No. Sorry - missed the post above!

0.025 + 0.028 = 1/R
Or 0.053 = 1/R
Some basic maths here!
If 1/R = 0.053, how do you work out R? Use a calculator?
One way of explaining it - Multiply both sides of the equation by R gives 1 = 0.053R
So R = 1/0.053. That you can do on your calculator.
Or by long division, as if if anyone can remember how to do that!

 

[sid521]

No. Sorry - missed the post above!

0.025 + 0.028 = 1/R
Or 0.053 = 1/R
Some basic maths here!
If 1/R = 0.053, how do you work out R? Use a calculator?
One way of explaining it - Multiply both sides of the equation by R gives 1 = 0.053R
So R = 1/0.053. That you can do on your calculator.
Or by long division, as if if anyone can remember how to do that!

thanks

also do you know where i would find the maximum permissible residual operating current for rcd on a TT system . in the bs7671 or gn3 and where about ? thanks