Low Iz Reading...

[Netto]

I have a periodic inspection report stating "low IZ reading on neutral to earth on D/S lighting circuit although above minimum set by 17th Ed Regs.

I know this means its safe, according to 17th Ed, but what does it actually mean in english!!?

 

[EFLImpudence]

What is meant here is that the insulation resistance between the neutral and earth conductors is not as high as would be liked.

That is your cables are getting on a bit and the insulation is breaking down.

A reading of more than one mega-ohm is acceptable according to the regulations. Any reading under 2Mohms should be investigated.

New cable will have insulation resistance near infinite - most meters only read upto 200 or 500 Mohms.

 

[Netto]

What is meant here is that the insulation resistance between the neutral and earth conductors is not as high as would be liked.

That is your cables are getting on a bit and the insulation is breaking down.

A reading of more than one mega-ohm is acceptable according to the regulations. Any reading under 2Mohms should be investigated.

New cable will have insulation resistance near infinite - most meters only read upto 200 or 500 Mohms.

Ah!!! All is now clear - the reading is '5', which is a shocker considering all other readings on the other circuits (same age of wiring) is >200!

Cheers!

 

[EFLImpudence]

That is relatively low (everything is relative) and likely may be just at one point where the insulation has been damaged.

Where this is could be found but might be a bit involved.

 

[Netto]

That is relatively low (everything is relative) and likely may be just at one point where the insulation has been damaged.

Where this is could be found but might be a bit involved.

Thanks, think it will be just as easy to replace the wiring on this circuit. I need a new consumer unit anyway, and wouldn't want it to keep tripping out!

 

[ban-all-sheds]

"low IZ reading ..."

Really?

 

[EFLImpudence]

Had to think for a while before realising what was meant IZ Iz I2, then like the sun it dawned on me - I Insulation Z resistance.

 

[Risteard]

Minimum insulation resistance values are for the installation, not the circuit. As such consideration should be given to the effect of the resistances in parallel.

The whole installation, or where appropriate each distribution board, should be tested collectively and not individual circuits except to find where low readings are coming from.

 

[holmslaw]

..

 

[Risteard]

Like ignoring regs that say 2MΩ is an acceptable reading.

[...]

I think you need to think outside the box (house).

Would you do that in a 20 storey office block?

No reason why a resistance of two million ohms across the installation isn't safe.

As I said, for simple installations the installation should be tested as a whole. For more complex installations each distribution board should be tested (so I dealt with office blocks).

If individual circuits are tested then the effects of resistances in parallel must be considered, i.e. 1/Rt = 1/R1 + 1/R2 + 1/R3 ...

Read the Regulations. The values stated are not for circuits but for installations.

 

[holmslaw]

..

 

[ban-all-sheds]

Had to think for a while before realising what was meant IZ Iz I2, then like the sun it dawned on me - I Insulation Z resistance.

But that's not what Iz signifies.

I would seriously question whether anybody who might approach, for example, 433.1.1, believing that Iz signifies insulation resistance has sufficient competence to be allowed anywhere near design or testing work.

And Z is not the symbol for resistance anyway.

 

[SgtTrojan]

IR therefore I"ZED" it's not right. Hmm took me ages to try and make that joke

It will be a handwriting mis-interpretation.