Difference between 250v/500v insulation resistance and 15ma/200ma continuity test?

[ppelec100]

Hi I hope everyone is all well.

I’ve built a rig at home on some ply to do some testing and fault finding to prepare for my exam and just learn in general…

I put 100v then 250v through L & N through a B22 led lamp and also the spur neon to see what reading I would get. They were both the same at 100v: 1.35Mohm and 250v: 0.195Mohm
I didn’t want to do it on 500v for obvious reasons.

Can someone enlighten me on the test voltages please. I know insulation resistance test is like a pressure test between conductors to check for breakage of insulation deterioration and short circuits.

Q1. Why is there different readings when you use different voltages? Also if you have any other info about insulation resistance testing that will be useful to me please tell.

I also noticed there is a button to switch from 15ma and 200ma on the continuity test. From my 2391 course these past couple weeks I know we test using 200ma.

Q2. What is the 15ma option for? Can someone go into the differences in a bit of detail please?

Thank you

 

[ppelec100]

Anyone…?

 

[mainline]

Hi I hope everyone is all well.

I’ve built a rig at home on some ply to do some testing and fault finding to prepare for my exam and just learn in general…

I put 100v then 250v through L & N through a B22 led lamp and also the spur neon to see what reading I would get. They were both the same at 100v: 1.35Mohm and 250v: 0.195Mohm
I didn’t want to do it on 500v for obvious reasons.

Can someone enlighten me on the test voltages please. I know insulation resistance test is like a pressure test between conductors to check for breakage of insulation deterioration and short circuits.

Q1. Why is there different readings when you use different voltages? Also if you have any other info about insulation resistance testing that will be useful to me please tell.

If you inject a higher voltage into a circuit, it will alter the current.

I also noticed there is a button to switch from 15ma and 200ma on the continuity test. From my 2391 course these past couple weeks I know we test using 200ma.

Q2. What is the 15ma option for? Can someone go into the differences in a bit of detail please?

15mA is a low-test current suitable for testing inductive components such as motor windings and coiled cables.

 

[Simon47]

Voltage.
With your neon, if you test below the strike voltage you will read open circuit (or at least a high resistance). At low voltage the neon is just two bare conductors in a non-conducting gas.
When you test at a high enough voltage, the neon will strike and act as a semiconductor which (roughly) clamps the voltage at a lower level where the arc is maintained.
When used as an indicator, the neon has a series resistor that controls the current.

The LED lamp can behave in different ways depending on it's design.
Some have a long string of LED chips plus rectifier and current limiting resistor or capacitor. A resistor type will show open(ish) until you reach the knee voltage for the string of LEDs, and then show a reducing apparent resistance with increasing voltage. A capacitor type will initially take a little current and then settle back to open circuit on DC. This latter effect can be seen on "press & hold" meters as the needle kicks and then settles back as the caps charge to the test voltage. Something more automatic (like my MFT) just gives some spurious reading as it doesn't apply the test voltage long enough. For that reason, alongside my MFT I also have my dad's old BM10 which can do IR tests on circuits with capacitance in them.

If the LED lamp has electronics then there are many possibilities. Typically, it'll try and charge a cap and when there's enough voltage the electronics will try to turn on the LEDs - at which point the LEDs will flash and the cap will discharge as there isn't the current to keep them going. But if there's a voltage dropping cap, that will (like the basic version) block the DC from your meter and it'll drop back to open circuit.

Going to 500V would show lower apparent IR for both your neon and LED. The tester (assuming it's a small battery powered job and not something resembling an industrial welder) won't have the ability to push enough current to reach 500V anyway so won't damage the neon and won't damage basic LEDs.

For completeness, if you test a linear load (e.g. a heating element) at different voltages then it's resistance should remain constant. This may not be what you might calculate from measuring voltage & current with it operating and resistance changes with temperature. This is especially the case with incandescent lamps where the cold resistance can be an order of magnitude lower than when hot.

But, the 100V & 250V ranges on your meter aren't designed for low resistances. Below levels where you'd say "IR test failed", it just won't be able to push out the current needed to reach the test voltage.

 

[Tom256]

But, the 100V & 250V ranges on your meter aren't designed for low resistances. Below levels where you'd say "IR test failed",

If you watch the instrument it looks to ramp up the voltage to enable no more than 1mA test current. If it cannot the reading collapses.
Basic Ohms law on this. For 100 volts test if needs 0.1M ohms, 250 volts - 0.25M ohms, 500 volts - 0.5M ohms, 1000 volts - 1M ohm to place the installation under full test voltage.

Years ago we used wind up meggers and if you had a damp circuit you would wind up the voltage slowly in a 'dry out test run' in order to eventually to see if it was possible to subject the cabling / motor to the required full test voltage. You would then hold that in the form of a dielectric test.

 

[Simon47]

If you watch the instrument it looks to ramp up the voltage to enable no more than 1mA test current. If it cannot the reading collapses.
Basic Ohms law on this. For 100 volts test if needs 0.1M ohms, 250 volts - 0.25M ohms, 500 volts - 0.5M ohms, 1000 volts - 1M ohm to place the installation under full test voltage.

That would depend on the instrument. On my old BM10 when you press & hold the button it just powers up the inverter circuits to generate the selected voltage for as long as the button is down. It'll make an LED lamp glow.