Discuss Insulation Resistance Test, trying to wrap my head around it in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

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?
 
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.
 
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).
 
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
 

Reply to Insulation Resistance Test, trying to wrap my head around it in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

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