Nulling Test Leads

Can I just double check that the only test you need to Null your test leads on is the continuity test ? So you don’t have to null leads on insulation resistance ? 

Think it through logically.  Say your leads have a resistance of 1 ohm. 

If doing continuity, perhaps the circuit may be 4 ohms, so an error of 1 ohm is significant.  So set the instrument to cancel out, (null), the lead's resistance.

Now, consider doing insulation. Let's say you are looking to measure greater than 4 Mohm, (four million).  Ask yourself if that 1 ohm test lead resistance is relevant. 

A little known fact but, the leads on a fluke ( well on my 1654b) have to be mulled for a Ze/Zs test

also,remeber that they should be 're-nulled ' if you change probes/clips or anything that will affect the resistance

Geoff1946 said:

Think it through logically.  Say your leads have a resistance of 1 ohm. 

If doing continuity, perhaps the circuit may be 4 ohms, so an error of 1 ohm is significant.  So set the instrument to cancel out, (null), the lead's resistance.

Now, consider doing insulation. Let's say you are looking to measure greater than 4 Mohm, (four million).  Ask yourself if that 1 ohm test lead resistance is relevant. 

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Thanks Geoff when you put it like that it’s simple  

You REALLY want to null your test leads if using a long lead!

OP - if you find you have to null your leads onsite regularly then its probably time to buy a new set of leads - I find mine last about 12 months - but I do a lot of fault finding so my MFT is used a lot

I have a Kewtech KT63..... the frigging thing needs to be nulled all the time.. not because the leads are fubared,,,, if it goes into power saving mode the null is wiped unlike my old Megger 1502 which stored the null

NozSpark said:

I have a Kewtech KT63..... the frigging thing needs to be nulled all the time.. not because the leads are fubared,,,, if it goes into power saving mode the null is wiped unlike my old Megger 1502 which stored the null

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You not tempted to just remember the value and subtract it manually each time?

Murdoch said:

OP - if you find you have to null your leads onsite regularly then its probably time to buy a new set of leads - I find mine last about 12 months - but I do a lot of fault finding so my MFT is used a lot

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I seem to end up grabbing a set of leads every 4 or 5 months, with the newest ones going on the loop tester as that is very sensitive to the leads starting to fail and the parameters at the DB can easily be knocked a long way off, they then get used on the continuity tester where its easier to zero (the loop tester can take an offset value but no way to automatically detect it) when the amount they are zero's by starts to become more than a little or massivly varies they then get chucked out. I seem to spend at least 75% of my time at work doing testing though, so leads in use for on average a good 6 hours a day, so I suppose its not real supprise they wear quickly