testing

[snowplough]

Hi pushrod,

Man Thanks,

Yes, i do understand that the worst reading on the entire lighting circuit would be your true reading for your R1 and R2 reading as this would decide how much current would flow under fault conditions.

Im just having difficulty understanding why it said that the last ceiling rose is the furthest point on the circuit when the switch/s that come from it usually as the earth terminating there , and the rose as the line terminating there ,so to me if you looked at a diagram of them both , they have either the line or the cpc terminating there so to me the are both as long as each other?.

Please let me no your thought on this.

Hope our on site guide is still there when ou get back, i always take mine home with me, i need the revision.

Many Thanks

Snow Plough

 

[pushrod]

Hi Snow , obviously not looking at the diagram that you are - just a thought though - imagine if it is a pendant light at the rose with a 3 core flex. If that is the case, bearing in mind that the earth path through the cpc will be as short as it can be, then the route at the switch will definitely be shorter than from the actual lamp holder.
Get what you are saying though about a possible longer cpc and shorter line route versus a shorter cpc and longer line route.

 

[snowplough]

Hi Pushrod,

Sorry not quite sure what u were meaning with the pendant etc, just a thought tho, i wonder if it i omething to do with the path of the line wire going in and out of every switch and so b the time it gets to the last rose on a light circuit say there would be alot more cable ued compared to that of the earth if it only goes as far as each swich ,where as the earth goes in and out of each switch back to the rose if you get my drift, i wonder , because i have just been thinking about the one light and switches we do on our test board in college, but in a house say ,b the time u get to the last rose in the top bedroom sa , ou must have used alot more line cable.

I still wonder tho i have a picture in m O.S.G of the probes for the R1 and R2 test being positioned at the switch instead of the rose, interesting.

Welcome your thoughts

Kind Regards

Snow Plough

 

[pushrod]

Hi Pushrod,

Sorry not quite sure what u were meaning with the pendant etc, just a thought tho, i wonder if it i omething to do with the path of the line wire going in and out of every switch and so b the time it gets to the last rose on a light circuit say there would be alot more cable ued compared to that of the earth if it only goes as far as each swich ,where as the earth goes in and out of each switch back to the rose if you get my drift, i wonder , because i have just been thinking about the one light and switches we do on our test board in college, but in a house say ,b the time u get to the last rose in the top bedroom sa , ou must have used alot more line cable.

the line will go in and out of the two switches on a 2 way lighting circuit - (what i said quite a few posts ago), but it does not go in and out of every switch *, which you appear to be saying. With the usual way of wiring lights the line will loop in from one rose to the next, to the next and so, and not go via the previous switch.

Hopefully tomorrow i can get my OSG back and actually see which diagram you have been looking at.

* you sometimes come across lights wired by looping in switches but it is not so common now and i don't think it is ever now taught.

 

[snowplough]

Hi Pushrod,

yes your quite right, back to the drawing board for me as i said earlier tho regarding the (shorter cpc in the rose, and the longer line wire in the rose) i think you can see what i mean about them both being the about the same in the furthest point on the circuit , so i would assume you could do your Ri AND r2 reading at either if im correct, why i wonder they say the ceiling rose ?.

I nam at college tom night ,i might try ask my lect again although he doesnt seem to explain it very well,or its prob me thats not seeing wood for trees.

If you get chance have a look in your O.S.G and let me know what you think .

Kind Regards

Snow plough

 

[pushrod]

I now have another OSG and can see the picture that you were looking at (P79) and yes in that particular example of one way lighting there could be a fractionally higher reading at the switch for R1+R2 because the cpc in T&E will be smaller and so have a slightly higher resistance than the line.

Remember though that it is not so much where the reading is taken, but what is the value of the highest reading, so that you can work out Zs (Zs = Ze +[R1+R2]), that is important. You can then work out your highest prospective fault current and check that swtichgear and mcbs etc can cope with it.

hope this helps.

 

[snowplough]

Hi Darkwood,

Good to hear from you,

Yes i think all i was trying to say was that because the line ends in the rose,and the cpc ends in the switch to me they are both as long as each other so i would think ou could take either two as our fourthest points, if they are tthe last switch and rose on the circuit, but i suppose if it was wired so the cpc an line both finished in the last ceiling rose the that would be our true furthest point.

As you say tho i suppose ou could get a hire ohms reading somewhere else in the circuit which might not be the furthest point but could a bad connection or a dodgy switch or whatever else, and that would be your true R1 and R2 reading.

Regards

Snowplough