help needed and soonish :)

[lee1982]

hey people,

Ive got my c&g 2391 test and inspection exam at 1 today, im all ready to go except im confusses about one thing.

Continuity of ring final circuit condutors
When carrying out step 3 which is connecting the live (line) and the cpcs together on a ring main to carry out the figure of 8 test to check for interconnections ect, you are getting your R1+R2 reading, do you have to divide this reading by 4? im getting confussed one book says you do and another there is no mention,
do i just put down my reading for R1+R2 on the test forms. then if i want to work out my zs, i then have to divide R1+R2 by 4 then add my ze.

Thanks for any help in advice

 

[ericmark]

If you are referring to 2c in diagram. Then Book 3 does say divide by 4 to get Zs-Ze.

Measure cable resistance

• ericmark
• 26 Mar 2009
• 1

 

[davelx]

If you are referring to 2c in diagram. Then Book 3 does say divide by 4 to get Zs-Ze.

NO IT DOESN'T. It (GN3 2.7.6)is not very well written, but the difference between Rn and rn is crucial.

Step 1, using fig 2a, gives you the phase, neutral and cpc loop (ring) resistances, r1, rn & r2 respectively (and for 2.5 T+E, r1=rn and r2=r1*1.67)

(Step 2 fig 2b is for L+N and nothing to do with determining R1+R2)

Step 3, using fig 2c, the meter reading is (r1+r2)/4 which is the same as R1+R2 (Ohm's Law - resistances in parallel) which is the figure you then add to Ze to get Zs,