Can you explain this please

[L Plate]

In our work shop last week , I was installing on my board a 2 way lighting circuit ( alternative ) during my testing I found out that I have a high reading which is I am really expecting a low reading. I am aware that maybe because the cable were too short ? less than 3 meters , its only on a small cubicle where we do our practical workshop.. could you please input me the reason why the reading will go high if the cable run is too short .

thank you

 

[Evans Electric]

What was this reading ? Insul. test live to earth , live to neut , neut to earth or loop impedance test ?

 

[sparkytim]

What reading were you taking?

CPC continuity? R1+R2? Insulation resistance?

 

[L Plate]

Upps ! sorry continuity and insulation test

 

[NozSpark]

Did you remove all the lamps???

 

[ianmacd]

Continuity should have been almost 0ohm unless you had a loose connection and IR should have been very high unless you had a short (for example like a lamp still in as Noz suggests or neons somewhere).

 

[L Plate]

Did you remove all the lamps???

there were no lamp at all > BTW correction , the cable run were not even 2 meters

 

[SPECIAL LOCATION]

Insulation resistance:-

With all new wiring Ins Res should be off the scale of the meter no matter how long or short the cables are..

e.g. > 200Mohms or >999Mohms are typical max values on commonly used meters.

so larger than > max readable value is still going to be 'OL' off scale..

you cant get a reading bigger than the biggest readable value.

Continuity:-

Did you zero the leads correctly?

Incorrectly zeroed leads can cause funnies on really short lengths.

e.g.

R1+R2 on 1.0mm T&E is 0.0362ohms per meter

Say two lengths one 5m and one 75m

5m actual R1+R2 would be 0.18ohms

75m actual R1+R2 would be 2.72ohms

lets say meter leads are 0.5ohms when zeroed correctly meter actually reads

{cable+lead-stored zero value}

0.68 - 0.5 = 0.18ohms and

3.22 - 0.5 = 2.72ohms.

BUT

If when zeroing leads you had a bad connection and the meter calculated for 0.75ohms of lead resistance....

when go back to you proper cable tests with a good connection meter would get

{cable+proper lead-incorrect zero value}

0.68 - 0.75 = -0.07ohms

3.22 - 0.75 = 2.47ohms

so on a longer cable length you just get an incorrect R1+R2

but on a shorter cable length you get "Minus 0.07ohms"

Minus figures can mess up some meters and they can end up giving incorrect higher values!

Guinness

 

[Evans Electric]

Who woke him up ????

 

[SPECIAL LOCATION]

Who woke him up ????

Clocks change this weekend!

Nearly time to come out of hibernation!

 

[L Plate]

Insulation resistance:-With all new wiring Ins Res should be off the scale of the meter no matter how long or short the cables are..

e.g. > 200Mohms or >999Mohms are typical max values on commonly used meters.

so larger than > max readable value is still going to be 'OL' off scale..

you cant get a reading bigger than the biggest readable value.

Continuity:-

Did you zero the leads correctly?

Incorrectly zeroed leads can cause funnies on really short lengths.

e.g.

R1+R2 on 1.0mm T&E is 0.0362ohms per meter

Say two lengths one 5m and one 75m

5m actual R1+R2 would be 0.18ohms

75m actual R1+R2 would be 2.72ohms

lets say meter leads are 0.5ohms when zeroed correctly meter actually reads

{cable+lead-stored zero value}

0.68 - 0.5 = 0.18ohms and

3.22 - 0.5 = 2.72ohms.

BUT

If when zeroing leads you had a bad connection and the meter calculated for 0.75ohms of lead resistance....

when go back to you proper cable tests with a good connection meter would get

{cable+proper lead-incorrect zero value}

0.68 - 0.75 = -0.07ohms

3.22 - 0.75 = 2.47ohms

so on a longer cable length you just get an incorrect R1+R2

but on a shorter cable length you get "Minus 0.07ohms"

Minus figures can mess up some meters and they can end up giving incorrect higher values!

Guinness

Thank you SL for the info.. It's really a big help to me