Commercial EICR - Your opinion?

[Risteard]

Then why do the model forms provide individual boxes to record IR for each circuit? If they had intended us to measure and record global tests then surely the model forms would reflect this?

My understanding is that the boxes were intended to look nice. Hard to believe but apparently true. You could ask the same of the RCD test boxes. If the RCD is an RCCB it is likely to be protecting numerous circuits.

 

[Risteard]

That’s why we are tought the 1/Rt method of calculating the total resistance in parallel.
Scientific calculator at the ready

But that will give you an equally poor result than measuring the DB in parallel.

 

[davesparks]

My understanding is that the boxes were intended to look nice. Hard to believe but apparently true. You could ask the same of the RCD test boxes. If the RCD is an RCCB it is likely to be protecting numerous circuits.

But for RCBOs it is necessary to have individual results, which covers the majority of installations outside of domestics.
Where does this understanding come from because it is quite hard to believe?

 

[Ian1981]

The insulation resistance of a conductor is inversely proportional to its length that is, its insulation resistance decreases as the conductor’s length increases and vice versa. It follows that the insulation resistance of a complete electrical installation, or a section of it, will be less than that of a single circuit of that installation.
Global testing a large installation will mostly give you relatively low IR readings.
There’s no format to insulation resistance testing other than the overall resistance of the db circuits and any distribution circuit applicable to said DB is 1M ohm or greater.
The method you test it is down to the individual.

 

[DPG]

The insulation resistance of a conductor is inversely proportional to its length that is, its insulation resistance decreases as the conductor’s length increases and vice versa. .

To a point. But in practice, any length of new cable will have an IR of pretty much infinity, at least as far as any normal test equipment goes.

 

[Ian1981]

To a point. But in practice, any length of new cable will have an IR of pretty much infinity, at least as far as any normal test equipment goes.

It was more to do with carrying the test out during a eicr on larger 3 phase DBs.

 

[ipf]

Lets say that you measure 20 circuits at 10 Megohms each. The fact is these resistances are in parallel so would you wrongly consider that to be OK in spite of the fact that the requirement in BS7671 is for an installation/distribution circuit with all final circuits connected and not for any one circuit?

That depends on how far you want, or need in some instances, to break things down. As the thread indicates, 'your opinion'. There's an argument to every opinion... we can all pick straws.

 

[Risteard]

Where does this understanding come from because it is quite hard to believe?

From someone who was senior with the NICEIC at the time, as quoted on the IET forum on numerous occasions. (I don't know him personally.)