I have recently been tasked with carrying out some Inspection & Testing on a critical infrastructure graded bridge as part of a short term contracting position.
I was sent out with the companies "Lead" electrician who is in the practice of making up paperwork/test results and trying to "look" busy whilst finding as much time as possible to dedicate to internet browsing. It put me in a bit of a pickle because I couldn't be a part of that when I'm part of the team responsible despite my signature not being asked for.
So I took the apprentice and decided to do some test & inspection. The particular job was a 3 yearly Inspection of the lighting and power circuits on the towers part of the bridge. There are three phase power circuits, three phase lighting circuits (phases split in the bridge but haven't yet found where) and single phase 110 volt sockets.
The bridge is extensively metal. Every location where internal circuits are installed one is surrounded by box sections of steel bolted together.
The supply arrangement confirmed only by measurement (0.12 ohms) is TN-C-S fed from an adjacent 11KV transformer inside the bridge bowels sat on a concrete plinth in the basement.
The inspection and testing has proven challenging due to scale, access and lack of prior adequate records and I am still gathering evidence between juggling our other basic maintenance and reactive maintenance tasks. We were allotted 2 weeks to complete the lighting and power testing and had no schematics or location drawings to aid us everything has been visually verified and proven by testing. The area these circuits covers is huge and original installation between 1983 and 1985 with additions over the years.
Wiring is almost entirely SWA, potentially some MICC left in the circuits.
Anyway that's the background. My concerns are as follows:
1. A reading as high as 730 mili amps - Taken with a clamp meter on the main earth supply from the HV/LV Transformer to MET.
2. Suitability of a TN-C-S supply when surrounded by steel work and stood on a potentially great conductor.
3. I have not been able to verify visually, or by test, the presence of main bonding conductors for the structural steel - This is due to the cable tray behind the main switch gear being inaccessible for visual inspection however I can see an unsheathed copper conductor, either 35mm or 50mm sq leaving the void. I have not worked out where the void enters the structure and require a long wander lead to carry out further investigation.
4. Due to limitations I had to test the final lighting circuit I.R at 250v with line & neutral connected together to earth. We have old 3c +N PILC sub mains cables where the armouring is the CPC excellent I.R results > 700 Mega ohms some better than 999 Mega ohms. However on the PVC swa final lighting circuits we have readings as low as 0.4 megaohms and lots of them. Some are fine at 37-50 Mega ohms. The circuits are protected by BS88 10 amp fuses and have a mixture of LED and Fluorescent lighting.
5. There are a myriad of "end of line" points because the circuits have not been wired from point A-Z. Instead we have a main trunk in the towers with innumerous spurs off the main branch. Initially I was carrying out ZS readings (Live using insulated gloves) for convenience and to get a rough idea of Zs condition. However at some point I started to suspect the potential for parallel paths was almost certain given that many of the old fluorescent lights are metal clad, have bonding straps to the case from the cpc and are fixed to the steel by Unistrut fixings and similar providing earth continuity to the structural steel.
6. I do not know if there is any continuity between the bridge structure and "true" earth, either by an intentional electrical conductor or by the natural design of the structure.
7. Reduced voltage readings were observed at several lighting junction boxes between 83 and 153 volts. No labelling and no way to determine circuit and phase. 415 is present in some JB's
8. Multicore swa cables some with 8 conductors, have not been identified in anyway. I am most concerned that neutral and cpc's have been mixed and the bridge is forming a neutral.
This inspection has me wanting to pull the main fuses and hand it to the client in a sealed skull and cross bone bag. If I were to go off coding, individually there are no C1 faults. But as a whole, without other records, design data and further verification I have reasons to suspect it could be extremely unsafe and due to the age, vibration and hot cold fluctuations if a fault does occur, at this stage I cannot say how risky it is.
I have all my guidance notes here and am preparing a customised report with photographs, test results etc.
I am seriously concerned with the possibility of the PEN conductor being lost, and where would that 700 or so miliamps flow to? I have no reference earth to use.
My main recommendation is that the bridge requires a robust bonding system, and a back up TT earth system which is tested regularly due to general public pathway on the bridge (Being metal) contractors and employee's traversing the internal bare metal structure daily.
I would really appreciate your thoughts
