resistance testing

[Karin]

afternoon all, been reading up on testing and just wondering about the different types of testers and how they work..

insulation resistance - creates a potential difference across two conductors and measures leakage current to calculate resistance in MegOhms?

low resistance ohmmeter - i've read that these need to be capable of suppling a no load voltage of 4-24V and a short circuit current of no less than 200mA. does this mean that they work in effectively the same way as an IR tester, dropping a known voltage across a resistive item and calculating resistance from current flow? and is the short circuit current limit a requirement to ensure there is low enough resistance to guarantee accuracy?

guess i wanna know if they essentially function in the same way but on different scales?

loop impedence testers - these "circulate a current line to earth". does this mean that they create a link line to earth of suitable resistance to allow a, say 20A for high current test, current to flow and use that resistance to calculate the overall resistance (impedence) in the loop?

now i've started this electrics thing i'm starting to think i should have done some studies in electromechanical engineering 'cause i just wanna know everything! or maybe it's just a slow sunday and i'm waiting for the ale to kick in..
cheers!

 

[Richard Burns]

Resistance is calculated similarly in all cases, i.e V/I = R.
For insulation resistance the current and the voltage applied between cores is measured and so the resistance can be calculated.
For low resistance measurements the voltage drop across the external resistance is measured and the test current is measured this allows calculation of the external resistance.
For EFLI the voltage used is the supply voltage and this has to be measured to avoid errors due to voltage fluctuation during the test. The voltage drop experienced over the external loop path whilst a measured current is applied and also the phase variation in voltage and current (to account for capacitance/inductance) are used to calculate the loop impedance.

 

[Lucien Nunes]

if they essentially function in the same way but on different scales

Yes. To measure DC resistance using DC, you have to either output a fixed current and measure the voltage, or vice versa, or somewhere between the extremes and compute.

The reason an IR tester outputs a high voltage is to test the resistance of a nominal insulator while under electrical stress. I.e. not just to measure its resistance, you don't need 500V for that, but specifically its resistance while trying to make it break down. The reason a continuity tester outputs a significant current is to test a nominal conductor - in the case of a PAT tester earth bond test (normally an AC test current but the logic still applies) a high current, again to test the resistance when subject to stress. The reason for wanting to stress the test subject is that these tests are mainly to prove safety.

Most of the world's resistance testing is not about safety. When I measure the resistance of some component in an electronic circuit, I don't want to 'push it' or prove how easily it withstands conditions, I simply want to know the resistance. In this case the lowest possible test currents and voltages are best, as they will not risk damaging the test subject. Multimeters therefore use a low test voltage that is a compromise between not damaging devices under test, and losing accuracy to noise and through having to be too sensitive. They can span across the ranges covered by continuity testers and insulation testers and everything in between, because they do not need to generate high currents or voltages.

 

[Karin]

excellent, thanks for that guys. that's just the kinda information i was looking for

i feel like a lot of what i've learnt so far from college is "press this button and record the infinity result", which isn't all that helpful when faced with real world fault finding. actually understanding the tests and results is quite important i think!