Why do you need to null the test leads on an instrument?

[urim]

Hi there.

could anybody help me.

I am doing an electrician course, and at the moment i am stuck on one question,

maybe anybody could explain the answer for me.
the question is :

Why do you need to null the test leads on an instrument?

thank you for help.

 

[ElectricDunc]

Accuracy. If you don't null (zero) the test leads when you measure the resistance of your circuit you will also be measuring the resistance of your leads at the same time giving a false/inaccurate reading.

 

[TicklyT]

As above, accuracy, and also a quick functional check of your leads and tester.

Imagine tracing a fault that seems to appear everywhere you look. Three hours later, you find out the fault is in your meter leads

 

[bernardgreen]

Surely this question could be answered from the basic electrical theory that I assume is given at the beginning of the course.

Or is it now not considered necessary for modern day electricians to know the basics of electricity ?

 

[ericmark]

I was invited to sit in on 2330 course where students did not know iron was a conductor!

If you work out the earth loop impedance using R1 + R2 and you are out by as little as 1 ohm it could be difference between pass and fail.

 

[ban-all-sheds]

Or is it now not considered necessary for modern day electricians to know the basics of electricity ?

Apparently it's not considered necessary for modern day electrical course tutors to be able to explain the basics....

 

[Spark123]

Why do you need to null the test leads on an instrument?

As has been said it is to discount the resistance of the test leads.
There is however nothing stopping you measuring a resistance without nulling the leads then subtracting the resistance of the leads afterwards.

 

[tim west]

Or is it now not considered necessary for modern day electricians to know the basics of electricity ?

Apparently it's not considered necessary for modern day electrical course tutors to be able to explain the basics....

why does it hurt when i stick a wet finger into a lampholder?

 

[ColJack]

As has been said it is to discount the resistance of the test leads.
There is however nothing stopping you measuring a resistance without nulling the leads then subtracting the resistance of the leads afterwards.

that's what I have to do on my meter.. there is no facility to zero the leads on it.. ( well none that I can see or work out how to do.. Robin 1610 )

 

[JonoPashley]

I was invited to sit in on 2330 course where students did not know iron was a conductor!

If you work out the earth loop impedance using R1 + R2 and you are out by as little as 1 ohm it could be difference between pass and fail.

Pretty sure its the tutors and students at fault not the course.

 

[securespark]

why does it hurt when i stick a wet finger into a lampholder?

Ah, well, it doesn't always. If your finger were in contact with the neutral or live terminal alone and you were insulated from earth, there would be no shock and therefore no pain.

 

[ElectricDunc]

why does it hurt when i stick a wet finger into a lampholder?

Ah, well, it doesn't always. If your finger were in contact with the neutral or live terminal alone and you were insulated from earth, there would be no shock and therefore no pain.

You must have small fingers to not touch both pins!

 

[ban-all-sheds]

He wouldn't know - it's all surmising, as it isn't his finger he's in the habit of sticking in there.

 

[roberta1000]

why does it hurt when i stick a wet finger into a lampholder?

Ah, well, it doesn't always. If your finger were in contact with the neutral or live terminal alone and you were insulated from earth, there would be no shock and therefore no pain.

You must have small fingers to not touch both pins!

Yep!!!

Once upon a time I used to routinely check the springiness of the terminals when changing a lightbulb by squeezing them a couple of times with my finger. Stopped doing it after a couple of hefty belts though.

 

[ban-all-sheds]

Stopped doing it after a couple of hefty belts though.

Bit slow on the uptake there.....