Help on IET Cert

[m4tty]

Hi,

Im filling out an IET Cert (the ones that are in GN3) and for Insulation Resistance there are 2 columns. 1 is for "between Live conductors" and the 2nd column is "between Live conductors & Earth".

For all my testing I have tested 500V DC

1) Line to Neutral

2) Line to Earth

3) Neutral to Earth

so for the 2nd column do I fill in the lowest result I got for Line to Earth and Neutral to Earth

Thanks in advance

Matt

 

[sparkytim]

Yes i suppose so, I use NIC forms which give space for all readings.

 

[Brian 157]

"Live" means current carrying so includes neutral. For Live/Live record the lowest reading Line to Neutral (or if three phase lowest of line-neutral or line-line) for Live/Neutral the lowest Line-Earth or Neutral - Earth, as you suspected Matty.

Sparkytim is right though, the NICEIC forms are better & give you somewhere to record r1, rN & r2 for ring circuits.

 

[green-hornet]

On the NICEIC forms there is a column for L/L this is normaly N/A for single phase installations. The next columns are L/N L/E and N/E which covers all insulation tests across all conductors and earth.

The lowest readings recorded are put in each column

 

[Mark2spark]

Hi,...for Insulation Resistance...

...for the 2nd column do I fill in the lowest result I got for Line to Earth and Neutral to Earth

Matt

Join the L & N in a choccy and test to E IMO

 

[a1spark]

If you cant get back to the job and do as Mark suggests above then would it not be correct to add together your Live to earth & Neutral to earth readings then divide by 2. ?

 

[sparkytim]

Not really better to record lower of the 2 if different

 

[a1spark]

But isnt the test meant to find out the overall Insulation resistance of both conductors in parallel to earth.. if you just record the lowest live to earth reading ,this will simply ignore the the other conductors influence on the true overall reading wont it ? ......Resistances in parallel !

 

[spinlondon]

No not really, because you're not measuring the resistance along the length of the conductors, you are measuring the resistance between the conductors.

 

[a1spark]

Yep I realise that . but arnt you required to be measuring thet total resistance of the line conductor to Earth Plus the resistance of the neutral conductor to Earth...i.e. Live to Earth......2 conductors here having seperate resistances to earth when in service resulting in a parrallel resistance calculation....L/E resistance + N/E resistance divided by 2 ?

 

[spinlondon]

When doing R1+R2, you would be measuring the total resistance along both conductors and then adding them together.

When measuring insulation resistance, you are testing between conductors.

The reading you obtain when the line and neutral are connected together and then tested to earth, should be the same as the lowest reading obtained when testing the line and neutral to earth separately.

 

[Risteard]

Yep I realise that . but arnt you required to be measuring thet total resistance of the line conductor to Earth Plus the resistance of the neutral conductor to Earth...i.e. Live to Earth......2 conductors here having seperate resistances to earth when in service resulting in a parrallel resistance calculation....L/E resistance + N/E resistance divided by 2 ?

1/Rt = 1/R1 + 1/R2

So Rt = 1/(1/R1 + 1/R2)

But as spin says for live/earth IR I would just use the lowest live conductor to earth. Your insulation resistance ohmmeter will agree with this.

 

[kme]

Agree with our spinny.

IF you cannot disconnect all loads (PIR, for eg.), you would "lim" the phase neutral test, and link `em as Mark suggested.

HOWEVER, it is ALWAYS better to do a test if possible; so if loads CAN be removed, test `em individually. You wouldn`t then link PN, and ins. test `em to earth - that is a "workaround", as, under standard operating conditions, those conductors are seperate. Record the lowest of the two readings mate.

 

[a1spark]

Risteard staed :

1/Rt = 1/R1 + 1/R2

So Rt = 1/(1/R1 + 1/R2)

..Thanks.

if for simplicity we say overall IR. readings were L/E . 10 Mohms & N/E. 10 M.ohms

then the overall Live conductors to Earth IR reading for the installation would be the much lower figure of 5 Mohms would it not ?.....AS the purpose of the test is to determine this total figure in order to ensure minimum values of Earth leakage current are adhered to then just using the lower of the two measured IR values would produce an artificially High IR value with a resultant artificial calculated low Earth leakage current for the installation......Total.Insulation Resistance is being measured here not continuity.

 

[professional]

When doing R1+R2, you would be measuring the total resistance along both conductors and then adding them together.When measuring insulation resistance, you are testing between conductors.

The reading you obtain when the line and neutral are connected together and then tested to earth, should be the same as the lowest reading obtained when testing the line and neutral to earth separately.

Incorrect - parallel paths and all that......................

 

[kme]

Risteard staed :1/Rt = 1/R1 + 1/R2

So Rt = 1/(1/R1 + 1/R2)

..Thanks.

if for simplicity we say overall IR. readings were L/E . 10 Mohms & N/E. 10 M.ohms

then the overall Live conductors to Earth IR reading for the installation would be the much lower figure of 5 Mohms would it not ?.....AS the purpose of the test is to determine this total figure in order to ensure minimum values of Earth leakage current are adhered to then just using the lower of the two measured IR values would produce an artificially High IR value with a resultant artificial calculated low Earth leakage current for the installation......Total.Insulation Resistance is being measured here not continuity.

Total insulation resistance - yes.

BUT....As I stated above, are phase and neutral connected together in normal use, as they are for the combined test?

NO.

Reference 1: GN3, 2.7.7 refers (P37)

Reference 2: Inspection, testing & certification, Chapter 8, (P241)

Reference 3: BRB 612.3 (P158)

So you record the lower of the two invididual tests.

The above notwithstanding, your version would (prospectively) produce lower readings of insulation resistance; and TBH if the readings were so low as to cause concern, both methods would produce low results, therefore I can`t see any reason NOT to do it your way, if you so wish

HTH

KME

Incorrect - parallel paths and all that......................

Indeed mate. :innocent

 

[a1spark]

Thanks for the input KME, much appreciated, in it you say :

Total insulation resistance - yes.

BUT....As I stated above, are phase and neutral connected together in normal use, as they are for the combined test?

No they are not connected together in normal use but they both produce their own seperate current leakage to Earth in normal use dont they ? and the purpose of the IR test is to determine the value of this Total Insulation Resistance [ in this instance L/E + N/E] and from this a Total Leakage current [Cable ] on the installation can be found. As we all know the value in respect of the O.P being 1 M ohm....which works out at about 0.5 mA .[ or pre- 17th Ed.......1mA ]

I understand the requirement to limit the Earth leakage on an installation originated from the old Electricity Supply regulations

 

[kme]

I think (mr "artois" is beginning to make his presence felt), I was trying to say that, although you will generate a lower reading by combining phase & neutral, which could be taken as the maximum insulation leakage of that cable, you must also remember that you are performing that test at 500Vd.c., Which will provide a lower reading than the "actual" leakage from the same cable under normal, operating 240 230Va.c.

Further, your neutralcable is usually at or very near the earth conductor in terms of impedance. Ergo, reference to a test simulating them 500V apart does not represent a cable within "standard operating parameters".

If that makes sense to me tomorrow, I`ll be amazed!

KME