Little help please?

[SimpleSimon]

Gents. Studying for the 2391 exam Thursday, I've come across some practice questions that I can't get, would you mind helping me please?
First one coming up

An insulation resistance test was carried out on a large complex installation before it wasput into service. The test values obtained between live conductors and earth was 10Mohms. State
A) two possible reasons for this value excluding connect equipment and cable damage.
B) whether measured value is acceptable

 

[pjhill]

Where u doing your exam to on Thursday Simon?

 

[parm]

I would say,
A) factors afeccting insulationresistance other than equipment and cable damage are; excessive electrical loading, age of cable, length of circuit,number of circuits connected in parallel and condition of install

B) Yes ad its higher than 1Mohm

happy to be corrected

 

[SimpleSimon]

Thanks Parm. Phil taking it in Southampton.

 

[Johno]

Is it individual circuits tested, or the whole installation, as this will make a huge difference.

 

[jeremy]

Those results are not unexpected for a large complex installation. From the question it sounds like it is lives/earth test on complete install. When you've got miles of L/N/E cable running alongside each other this is where miniscule drops in insulation add up to something measurable. It's still 10,000,000 ohms, 5x a fail so you've saved the time of testing each individual circuit. Remember that the result means that each seperate circuit has at least that level of IR.

 

[pjhill]

Cool, I've got mine tonight in Exeter.

 

[SimpleSimon]

good luck Pjhill. feel confident?

thanks for the responses gents. Ive got more that I'll put up as I'm revising, only the questions that i get really stuck on.
For this I put length of circuit and environmental factors such as ambient temperature.

 

[pjhill]

Yea not feeling to bad. Waht about you? I am doing some last minuet revision.

 

[SimpleSimon]

All day mate. Just going over questions. Stick on here, anything we get stuck on we can help each other

 

[SimpleSimon]

How bout this one

Two identical conductors are to be connected in parallel. Each conductor measures 60m in length and has a resistance of 1.15milliohms/metre. Determine the
combined resistance of the conductors. Show all calculations

 

[SimpleSimon]

Now I know for IR, all resistance is measured in parallel. So I'm assuming the same applies here??

Conductor 1= 60 x 1.15= 69 milliohms
Conductor 2= 60 x 1.15= 69 milliohms

Conductors in parallel= 69 x 69/ 69+69 (4761/138) = 34.5/ 1000 = 0.0345 ohms

 

[SimpleSimon]

Test result sheet indicates that the total Zs of a circuit is lower than Ze +(r1+r2). State three possible reasons for this

 

[pushrod]

Now I know for IR, all resistance is measured in parallel. So I'm assuming the same applies here??

Conductor 1= 60 x 1.15= 69 milliohms
Conductor 2= 60 x 1.15= 69 milliohms

Conductors in parallel= 69 x 69/ 69+69 (4761/138) = 34.5/ 1000 = 0.0345 ohms

Agree but they might prefer the formula of 1/Rt = 1/r1 + 1/r2 even though it gives the same answer.

 

[pushrod]

Test result sheet indicates that the total Zs of a circuit is lower than Ze +(r1+r2). State three possible reasons for this

1) parallel paths due to main protective bonding being connected for Zs
2) Ze being a distributor supplied value eg 0.8 or 0.35 and higher than real value
3).... still thinking - what do you reckon? not taken at true end of circuit???