Zs on a lighting circuit?

[captaincargo]

When carrying out a direct measurement of the EFLI on a lighting circuit,i was told the measurement should be taken at the furthest point from the C.U on the circuit to find the worst case reading.
Now...would you say the furthest point was....

A...The rose/light fitting?
OR
B...The switch?

 

[Spark123]

Take both and find out, what are your R1+R2 readings like?

 

[Taylortwocities]

Well, the furthest point in a loop-in system will be the switch but most testers need earth, live & neutral to give you Zs. I did spot one manufacturer (cant remember who) that now makes a tester that u can do Zs on the light switch with just phase & cpc.

Otherwise its at the ceiling rose.

If you want to be perfect you could measure it there, use your tape measure to get the length of the cable to the switch and add on the calculated resistance for the switch dfrop to the measurement at the rose but life's too short for all that.

 

[captaincargo]

Hi Spark

Im not testing as yet,just reading and wondering at the moment.
Im reading various publications and they say as i was told,to test at the rose/fitting itself but im wondering....say the last rose is 20 meters away from the C.U and the switch drop from said rose is a further 2 meters away then surely the furthest point is at the terminals iof the switch?

Do you test at both rose AND switches when carrying out an EFLI test?

 

[Spark123]

Depending on wire CSA sizes it could be either, if you do the R1+R2 check it will probably reveal the highest value to be in the rose on the switched return to the lamp and the CPC. But like I say, if you do both R1+R2 checks you won't go wrong.

 

[captaincargo]

Hi Taylor...
So when carrying out a direct measurement of Zs at a rose a 3 lead tester is a must?

Would i not get a reading using a 2 lead tester between phase and cpc?

 

[Spark123]

It will vary according to manufacturer, YOU MUST CONSULT YOUR INSTRUMENT MANUFACTURERS INSTRUCTIONS ON HOW TO USE YOUR TESTER. The tester I have (Megger LCB2000/2) allows the measurement between the red and green test probes without the connection of the black lead. Saying that, my tester also takes a load of batteries!! Just because my tester operates in this fashion doesn't mean yours will.
Be aware that a normal EFLI test can draw in excess of 20A through the system, albeit only for a short period but 6A (type B) MCBs can pick this up and operate.

 

[securespark]

My Alphatek will do Zs's at a switch, but not with RCD in circuit.

 

[Spark123]

The megger one I have will do a soft test iirc 15mA in 2 or 3 wire configurations, the 3 wire being preferred owing to supply noise. It also has one or two fancy things on the RCD test side too like auto test and a variable RCD setting adjustable up to 1A (something I will probably never use).

 

[captaincargo]

My Alphatek will do Zs's at a switch, but not with RCD in circuit.

Hi Secure...
I also have an Alphatek (instaltest 61557)
Can i ask,if you carry out a Zs reading at a light circuit, do you use it as a 2 or 3 lead tester?

 

[ColJack]

actually, the furthest point is at the rose anyway.. as it's then 2 meters to the switch and 2 meters back from the switch.. if the switch wire goes down to earth at the rose ....

 

[festa]

In reality if you look at it, the CPC`s longest route ends at the switch but the phase`s` longest route is,as col says,at the rose.

Confusing if you dig deep

 

[ColJack]

in which case, put the switch wire into the neutral at the rose and test at the switch, making sure you don't switch it on...

and remember to put the switch wire back..

 

[Spark123]

What is that proving though?

 

[securespark]

My Alphatek will do Zs's at a switch, but not with RCD in circuit.

Hi Secure...
I also have an Alphatek (instaltest 61557)
Can i ask,if you carry out a Zs reading at a light circuit, do you use it as a 2 or 3 lead tester?

If there is an RCD in circuit, you use it as three-lead. I put a croc on the earth, squeeze it onto the terminal screw in the rose, then put live on the switchwire & neutral on the N.

It's important to put the live on the switchwire then you're checking polarity at the same time.

If no RCD, you can use two wire set up.