Am I doing this right? (R1+R2 for lighting circuits)

[Tuttle]

Sorry to ask such a fundamental question but this has been bothering me for a while now, I haven't come across an answer yet, and I want to make sure I get it right ...

For continuity of cpc, R1+R2 and polarity of domestic lighting circuits I was taught (at least this is what I wrote in my notes) to do the following:

1. Link the line & cpc of the circuit at the board
2. Check for continuity between line & cpc at ceiling roses - the test meter going beep is enough
3. Check for continuity between line and cpc at switches - again a beep is all we looked for
4. Measure the resistance between the switch return and cpc at ceiling roses to obtain R1+R2 for the circuit

Is this correct?

The reason I ask is that if I actually measure the resistance between line and cpc at switches I usually (if the cpc in the switch drop is smaller than the line conductor) get a slightly higher reading than the R1+R2 measurement I make at the ceiling rose. Also, for two way switching the resistance from each of strappers to the cpc at the switches depends on which switches are switched on and which are switch off.

So is it sufficient to just measure R1+R2 at a ceiling rose or should I be taking a measurement at the switch? Do I need to switch two-way/intermediate switches and repeat measurements or is any one of the ON switch combinations ok for R1+R2 measurements (it has little effect on measurements taken at the ceiling rose)?

Also, what do you do for ELV lighting where transformers and 3 plate connections are in the ceiling and can not be easily reached due to carpets and furniture, e.g. when testing following a consumer unit replacement?

Thanks in advance

 

[telectrix]

generally measure at the switched L at the ceiling rose with the switch closed. on 2 way, operate both switches and record the highest reading. by opening switch/es you lose the reading thus proving polarity.

 

[ezzzekiel]

between sl and cpc at the last point in the circuit which should be the highest value (wouldnt bother with beep around the circuits may as well take a reading as you go)

 

[GaryM]

The highest reading is generally at the switch point of the last room on the circuit, assuming t&e 1.5/1.0mm and loop-in at ceiling rose method.

 

[Tuttle]

Thanks guys, that helps a lot.

So how would you cope with lighting transformers tucked away out of reach above a ceiling when changing a consumer unit. Do you have to get into the upstairs floor and take a measurement at the transformer or is it acceptable to just take a R1+R2 measurement at the light switch and make a note on the EIC, e.g. providing that the Zs you end up with is well below the maximum permissible value for the breaker?

It seems a bit drastic to start pulling carpets up, or maybe that's what has to be done? What generally happens in these situations?

 

[telectrix]

on a CU change i would take readings where i could without pulling up floors and remaining within the client's budget for the CU change. anything i can't get a reading on, i enter N/V, as , at the end of the day, i'm replacing a CU, not doing a complete installation test.