Insulation Resistance Test, trying to wrap my head around it

[Pegasus]

I am self-teaching fault-finding and testing, but I can't wrap my head around insulation resistance tests.
From my understanding you send 500V for domestics down the conductors using an Multi-Function Tester,
and the values should be over 1M Ohm, now my question is why should the resistance be high?

I get that it tries to find out if the outer sheathing of the copper wires is in good order right? But why do we expect
a high result.

Also, I am reading that everything needs to be plugged out, as well as FCU etc? if we take out the FCU, are we supposed
to connect the wires up with wagos or how exactly does it work?

 

[nicebutdim]

A high reading indicates that insulation between conductors is doing its job. If no insulation was present and conductors were able to touch, you would have almost no resistance (a dead short). Lower than expected readings might indicate damage to, or degredation of, insulation.

You wouldn't necessarily remove an FCU, but would test with it in the off position as neon indicators would affect readings. With something like a USB socket, it may be necessary to remove it and join the cable in connectors. There are other means of achieving the same ends, but from a learner's perspective, this would be the method taught.

 

[Dustydazzler]

A higher reading is better , on small domestic circuits I expect a very high read out

I don't know where the 1MO or 200MO came about from but IMO we should just tick a box to say IR tests were done and to the industry standard what ever that is at the time

 

[Pegasus]

A high reading indicates that insulation between conductors is doing its job. If no insulation was present and conductors were able to touch, you would have almost no resistance (a dead short). Lower than expected readings might indicate damage to, or degredation of, insulation.

You wouldn't necessarily remove an FCU, but would test with it in the off position as neon indicators would affect readings. With something like a USB socket, it may be necessary to remove it and join the cable in connectors. There are other means of achieving the same ends, but from a learner's perspective, this would be the method taught.

if nothing is plugged in, where does this resistance come from? Also what if you had for instance the Line sheathing broken but the conductors aren't touching, would you be getting a low resistance reading too?

 

[Pegasus]

A higher reading is better , on small domestic circuits I expect a very high read out

I don't know where the 1MO or 200MO came about from but IMO we should just tick a box to say IR tests were done and to the industry standard what ever that is at the time

fair enough, but I am trying to understand why it is the way it is.

 

[nicebutdim]

if nothing is plugged in, where does this resistance come from? Also what if you had for instance the Line sheating broken but the conductors aren't touching, would you be getting a low resistance reading too?

Yor're sending a voltage along two separate conductors and measuring the resistance in between. Given that the conductors are each encased in a layer of PVC insulation, resistance between the two is expected to be high.

It's the opposite of a continuity test in which resistance is required to be low, indicating good connections. You don't want anything close to an electrical connection between separate conductors.

 

[Dustydazzler]

fair enough, but I am trying to understand why it is the way it is.

In a perfect circuit you will get 999MegOms for your IR tests or whatever your meter goes up to

I honestly have no clue where the 200 or 1 Meg Ohm figure came from

 

[Pretty Mouth]

now my question is why should the resistance be high?

Turn the question around. Why would it be bad if the resistance between (for example) live conductors and earth was very low, say 0.01 ohm?

 

[Pegasus]

Turn the question around. Why would it be bad if the resistance between (for example) live conductors and earth was very low, say 0.01 ohm?

okay so if resistance is low between live conductor and earth wouldn't that mean that there is an earth leakage? I think maybe I have some misunderstanding about certain fundamentals or some knowledge is missing where things aren't clicking yet. if the resistance between line and neutral is low, it would indicate the current is flowing without interruptions, but how does lets say a tiny cut in insulation change the resistance? Unless I am misunderstanding what actual insulation damage means.

 

[westward10]

if nothing is plugged in, where does this resistance come from? Also what if you had for instance the Line sheathing broken but the conductors aren't touching, would you be getting a low resistance reading too?

You can have bare conductors adjacent to each other and yet still achieve a high resistance between them, think of overhead distribution cables or busbar chambers. You can have damaged conductor insulation and it makes no difference to your readings so what would change the resistance between conductors?

 

[Pegasus]

You can have bare conductors adjacent to each other and yet still achieve a high resistance between them, think of overhead distribution cables or busbar chambers. You can have damaged conductor insulation and it makes no difference to your readings so what would change the resistance between conductors?

so what does resistance mean? isn't it resisting the current flow? if we send 500V from one end of the conductor, what do we get from the other end in order for the MFT to show us a resistance reading?

 

[littlespark]

It’s self explanatory.

Insulation resistance is testing that the insulation (be that pvc or open air) has enough resistance to not allow current flow between two conductors.

Continuity test is in affect, the opposite. It is testing there is no break in what should be a continuous path.

All loads have some resistance, but not in the range of what should be “insulation”
This is where your ohms law comes in….
Voltage in, divided by the load resistance gives the current.
To make things simple…. 240v / 24 ohms of a heating element is 10A
240v / the resistance in the 200 MEGA ohms scale… that’s 200,000…. The current is negligible.

 

[Dustydazzler]

And if you don't want to do IR tests the bang test will soon let you know if you have poor IR

 

[Pretty Mouth]

okay so if resistance is low between live conductor and earth wouldn't that mean that there is an earth leakage? I think maybe I have some misunderstanding about certain fundamentals or some knowledge is missing where things aren't clicking yet. if the resistance between line and neutral is low, it would indicate the current is flowing without interruptions, but how does lets say a tiny cut in insulation change the resistance? Unless I am misunderstanding what actual insulation damage means.

Yes. In the case I mentioned above, there would be a dead short between the live conductor and earth, and current would stray from the circuit to earth.

A tiny cut in the insulation probably wouldn't reduce the IR value in any measurable way in itself. Although you may have exposed copper on 2 or more cores, the air gap is too great. However, splash water onto those exposed copper cores so that it bridges the gap between them, and now the IR value will drop right down, perhaps to something like 0.01 Mega ohms. Low enough to trip the RCD.

I like to think of the IR test as a 'high value continuity test', where you're looking for a tenuous link between two conductors (a low IR result), where there should be none (a high IR result).

 

[Dustydazzler]

Yes. In the case I mentioned above, there would be a dead short between the live conductor and earth, and current would stray from the circuit to earth.

A tiny cut in the insulation probably wouldn't reduce the IR value in any measurable way in itself. Although you may have exposed copper on 2 or more cores, the air gap is too great. However, splash water onto those exposed copper cores so that it bridges the gap between them, and now the IR value will drop right down, perhaps to something like 0.01 Mega ohms. Low enough to trip the RCD.

I like to think of the IR test as a 'high value continuity test', where you're looking for a tenuous link between two conductors (a low IR result), where there should be none (a high IR result).

Scribbling 200MO on a test cert is just a tick box exercise

IR testing really come into its own for fault finding