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Ze vs Zdb Street Lighting columns on private network.

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Ze vs Zdb Street Lighting columns on private network.

Posted by Larryinplymouth on Jan 16, 2020 1:25 pm

Okay, I have a scenario whereby I am working for an organisation with numerous lighting columns on private cable networks covering seriously huge areas. Currently there are no cable drawings available showing where the cable network is located nor its point of origin etc so we are working in the dark - excuse the pun. Hence why we are carrying out this work.

We are ensuring that the columns disconnect within the 5 second requirement of BS7671 and using the cut out at the base of the column as the point of origin as for the moment we do not know where the origin actually is - nor how many columns are fed from the individual circuits. Providing our overall Zs reading (including the Ze obtained reading for the column) does not exceed the requirements of the fuse from the tables in BS7671 then we have certainty that the fuse will actually take out the supply to the column in the event of a fault. Just as a side issue some of the columns located are fed direct from the DNO so there we have no issue identifying the point of origin.

Once we have completed the testing of each column, we are identifying those fed from the private supply (SWA) on a mapping system so that we can then return and trace the cable and hopefully therein locate the feeder pillar. Once we have completed this we will then test the private cable in its entirety.

Usually the Ze is the point of origin for the DNO supply but at the moment as stated we cannot locate the supply…. Nor do the DNO have records as to where they are located.

My question here is that ordinarily we would use the base of the column (cut out) as Zdb and the feeder pillar as Ze but in this scenario we are firstly confirming that the individual columns are safe before we start locating the cable network. Is it therefore right or appropriate in this instance to classify the cut out at the base of the column as the point of origin for record purposes….?  bearing in mind that we will be tracing the private cable at a later stage, but as they run under both highway and rail tracks, we have to programme these advanced works when the network is not being utilised.

I welcome any comments (that make sense!)

Re: Ze vs Zdb Street Lighting columns on private network.

Posted by Chris Pearson on Jan 16, 2020 6:07 pm

Would it be naïve to suggest that the column with the lowest EFLI is nearest to the origin?

Failing that, if you plot the EFLI of all of the columns on a map, a pattern might emerge. There might be rows or clusters.

BS 7671 seems to allow a great deal of freedom with respect to the term "origin" inasmuch as the definition of "(electrical) installation" is broad.

I have no experience of this sort of work, but I do have experience of handling data so I hope that this may be of some assistance.

Re: Ze vs Zdb Street Lighting columns on private network.

Posted by Larryinplymouth on Jan 16, 2020 7:18 pm

Hi Chris

Thanks for the input


Re: Ze vs Zdb Street Lighting columns on private network.

Posted by mapj1 on Jan 16, 2020 9:44 pm

Until you know which lamps are DNO supplied and which are yours you have to treat them the same.
I'd be tempted to put a number sticker in or on each lamp post if not already identified, then call the readings you take  Zs1,2,3 4   , or Zlamp 1,2,3,4  etc  or anything you can remember that you can reconcile with the numbers or letters on a map (you could call the lamps Anna Bertha Carla Dora Ena Flora etc, but if there are more than 26 I;d be tempted to stay with numbers).
Nothing stops you changing the name you call the reading later, so long as you know which lamp it was taken at.

Maybe even write the result on the inside  as well, especially if you can identify the ones that go off when your supply fuse xx is removed. 
Also while the lid is off note how many supply cables and of what size come up in each point-  there may be buried joints of course, but assuming the installers wanted minimum grief, they are more likely to be daisy chain looping through each post in turn. A chain may split into a Y or any other pattern of course. so any fittings with 3 or 4 cables are especially interesting, as controlled disconnection may reveal more (only on yours you can isolate, not the DNO ones though..).  A camera on a phone if allowed on site is another useful thing if you do not get pix muddled up

Some 'time team'; techniques may be useful - can you work out when were things built, a lamp will not be supplied fro a building that was not there when it was put in! It is however likely that the  cable route will have been 'minimal digging' so from the nearest to the next nearest if at all possible

If you have a lot to do then there are cable tracing units that inject a signal, for example at the lamp=post, and then you can use the other bot of kit to see in which of you buildings or nearby fittings  a supply may be pinched from the signal actually pops up. Such kit can be hired, as you only need it for a short period.
A colleague and a walkie talkie can speed that sort of thing up as well, or even a bike.

For now only those that look like the internal fuse would not blow in a sensible time need a priority investigation or a smaller fuse. Later you can add to the records as the cable routes and feeder arrangements become clear. You may be able to identify which of the 3 phases a particular lamp is on, but that is only part of the information.
regards Mike


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