New approach to contact and ground faults using directional signal current

One of the difficulties to pinpoint phase to earth/ground in armoured cables is that if they are low resistance faults, and if a thumper/discharge is used to locate the fault, then often there is little discharge noise across the fault due to the low resistance.

Also where the fault is likely caused by cable damage caused by marking stakes, sign posts, bollards, crash-barrier pillars and Solar panel pillars there are opportunities for human or animal contact with the metal and unexpected exposure to the high voltage discharge is a thumper is used.

Very often, if the fault can be seen with a TDR, as a short or open circuit, then distance to fault prelocation is successful. If the fault resistance is higher, TDR results are harder to see and possibly the prelocation solution is to use a bridge. Once distance to fault is known, pinpointing is still required so that the fault can be excavated and repaired.

The first commercialisation of the directional current concept was by Radiodetection about 30 years ago. This came about from the need to identify fibre optic cable routes in US to avoid damage claims, and along with the RD432PDL locator, Radiodetection produced the first (Line management System)  LMS permanently installed transmitter to transmit locate signal over 20 miles. The early transmitter receiver kits used a 256Hz and 512Hz dual frequency, and during development facilities were added in the lightning discharge units installed at the telephone exchange central office to ground the far end of the cable during location times to ensure best signals.

One side effect of using the dual frequencies was that the phase relationship between them changed over distance, so the receiver had a reset function to overcome this, if the current direction frequencies phase shifted, and no longer produced reliable forwards or backwards arrows indicating the current direction. Generally using lower frequencies ensured less phase shift over longer distances.

Radiodetection extended the current direction method into the long distance Pipeline Current Mapper for long distance gas, oil and product pipelines with a location frequency of 98Hz or 128Hz. Current measurement loss measured by the PCM’s magnetometer attachment using the 4Hz frequency for the PCM transmitter, was a quick and easy way to find pipe coating faults and contact faults with other utility services. A directional arrow confirmed the correct pipeline. At the same time also added an accessory A frame to pinpoint coating faults, and that used the combination of the 4Hz and 8Hz to provide the direction to coating faults and the 98Hz or 128Hz for location the pipeline. I have commented on PCM and the Vivax DM in previous blog.

Seba KMT also produced their version of current direction and called it Signal Select and it was a different concept from current direction, could be used well at higher frequencies and never needed to be reset. Seba’s idea was to use the nominal frequency and frequency a little above and below nominal frequency, rather like the noise of a steam train approaching and passing the observer blowing its whistle. The receiver indicated direction by (+) or (-) and the lowest frequency used was 491Hz from Seba’s high power FLG50 transmitter. I have commented on this in previous blog.

Metrotech, as part of the SebaKMT group, used the Signal Select system in their ground breaking i5000 locator in 2004, and used the system to provide direction indication, and for the first time ever, an indication of signal distortion. 

The Vivax-Metrotech also uses the Signal Direction in their vLocProSD locator, and their new vLoc-5000 uses the Signal Select system like the former Metrotech i5000.

 

So – faults.

This trickery could be used with any of the locator brands that have a directional signal.

Some years ago I was with the electrical department of the fuel refinery in Omsk, Siberia.

They needed to find a fault on a 22kV HV cable, and with the old TDR they had, they knew that it was near to the start of the cable, but couldn’t pinpoint the fault. At the time I had an RD4000 kit so by connecting across the phase and neutral of the cable with the T10 transmitter, I noticed that the current direction signal stopped and became useless about 4M from the cable termination, and (conveniently) where it was lying on the surface across a concrete area, before going underground. This was their cable fault. The Omsk refinery was a dirty old place with broken and abandoned areas, and the guys said that if they had a problem with the refinery process, or an accident or fire, they just built another section and abandoned the old units. Nothing was new and shiny around except the odd empty Vodka bottle.

Some years later I was doing a cable fault demonstration with the street lighting department responsible for motorways around Manchester. We had the Seba KMT Digiflexcom TDR, Seba SFX5,  5kV 1000 joule thumper with distance to fault prelocation and also the Metrotech i5000 locator, so well prepared for the task.

We had the information of the known cable route and lengths and the access to the cable at a feeder panel near the top of a motorway bridge. The cable crossed on the bridge structure and then down the embankment and along the far side of the motorway. The Digiflexcom showed a short (not well defined, as expected from this sort of fault) at around 600M away.

The street lighting engineers had an idea that a new crash barrier had been installed at about that position, so this was a likely suspect for causing the problems. Connecting up the Seba SFX5 and cable testing (fortunately) would not build any voltage before the fault broke down, so suggesting a fault to ground that we would not be able to pinpoint with a digiphone as it simply would not discharge across the fault. – so we needed to take another approach, and also trace the cable route accurately on the far side of the motorway.

Using the i5000 transmitter connected across the faulty core and earth, and using the low frequency signal select directional signal at 491Hz, there was a good locate signal on the cable as it crossed the bridge, then we piled into their street lighting truck and drove the 600M. At the beginning of the new crash barrier the cable was located to the far side of the crash barrier in the scrub and bushes, as we walked along the hard shoulder, the cable route approached the crash barrier, much to the concern of the street lighting crew.

Then there was a moment of excitement as the direction compass on the i5000 receiver’s display turned around, its arrow pointing accusingly at a crash barrier pillar and then locate signal dropped dramatically half a pace further along the route, and the compass arrow pointed backwards. The street lighting crew went into a huddle, spray paint appeared and the crash barrier and the offending support pillar got its first coat of shiny spray paint. The half-filled bottles in the bushes certainly did not contain Vodka.

Recently we were asked to help with a cable fault at former RAF Lyneham, close to Calne. Lyneham was used by the RAF for all military air transport to anywhere, but was probably more publicised more for returning the brave tragically fallen, through Royal Wooton Bassett, until the base was closed down and moved to Brize Norton 4 years ago. Now it’s a training station, and housing, but also has gained a huge solar farm – well there was lots of space, no more planes, and we need renewable energy.

The solar farm designers had their plans of the site and had a survey of existing cables around their new site and located the existing 11kV HV ring main around the site. The ring main sections likely to be affected were switched off. Then it seemed that the design changed due to site requirements and when the pillars were installed and the panels with Solar panels on top, they forgot about the 11kv ring main until it was switched on and there were 2 nasty bangs as the circuit breakers disconnected…

Generally if you can see the fault with a TDR, as either open or short, and you have an accurate cable length to correct the velocity of propagation of the TDR impulse for the cable type, and repeating the test from the other end of the cable confirms the location your TDR results are as good as they can be, and on this old armoured 11kV cable, we got a good result..

If we had not a good result with the TDR, then the 12kV Easy thump was with us, and we could use it to test, prelocate, and pinpoint using the Easythump’s thumping mode and the Seba Digiphone plus to pinpoint the fault..but there was a possible safety issue of the danger that if the solar panel pillars had indeed been pushed through the cable, then the Easythump voltage discharge would be earthed through the panel structure, and any person touching the panels could be a current path! 

So the next step was to trace the route accurately and at the same time wheeling the route with a measuring wheel for distance to fault, using a vLocPro2 locator with the vLocPro 10 watt transmitter connected to the faulty cable core and an independent ground. Like the previous faults above, we made use of the Signal Direction frequency, to help both identify the cable all the way along the route (approx. 1800M) and all being well, pinpoint the fault where the signal died away. We traced the route and this took us under the lines of solar panel and their support pillars. My colleague was calling out the wheel’s distance reading and the route was clearly crossing a line of pillars, and then the route was directly in line with a pillar, and as I passed it the Vloc pro2 compass did its happy little pirouetted at the base of a pillar and the signal died away shortly after. Interestingly, the signal was also strongly on the pillar itself indicating nothing short of a direct hit! So the sun shined on our results.

 

Much the same result on the second section of cable with similar results!

It’s clear that if there is an obvious fault to ground and its possible to prelocate with TDR, then there is a quite simple fault location method, accomplished without the use of the usual thumping discharge at the fault point.

Although it may seem too simple to use a survey standard locator and find faults using this method, when compared to a less portable big thumper or an even more expensive cable test van, sometimes the simplest is the cheapest. Many electricity companies already own the Vivax phase to phase equipment and the same can generally be used for this method. So if you need advice and training please call our office.

Barney Walker 15 July 2015