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!)
