Spiking HV and LV cables

[benjho]

Hi everyone,

I'm a mechanically trained engineer but im currently training to be an electrical Authorised Person for HV and LV systems. Im looking at cable spiking at the moment. What kind of ohmic values would I be looking for before and after a cable is spiked on both LV and HV??? And also would these vary depending on the earthing system i.e TNS, TT, TNCS???

Some advice and info wold be great thanks.

Ben

 

[Marmite]

Cable spiking is part of a procedure to positively identify the cable to be worked on at the point of work, which may be a joint hole in the ground somewhere along the cable route. Change of core insulation before and after spiking is one method, but you can also use loss of signal where you apply a signal generator to the cable and use a listener to detect the signal in the joint hole prior to spiking. After you spike the cable the amplitude of the signal diminishes significantly downstream of the spiking gun. This method is better when you have more than one cable in the ground as it helps you to provisionally ID the cable prior to spiking. What you are looking for with the change of core insulation method is a significant change in the resistance before and after spiking, rather than absolute values of resistance. So for example a clean unfaulted HV cable between two open switches would tyically have a resistance of several megohms or even giga ohms to earth or between phases, but after spiking this would drop to Ohms rather than megohms. It's a similar thing with LV cables but there may be other cables jointed to the cable or connected loads which diminish the resistance to start with. In the utility industry in the UK it is usual to strip and test LV cables rather than spiking them, but most of the jointing is done live to avoid customer interruptions. Spiking is obviously very destructive. Regards Marmite

 

[PHovnanian]

The before/after insulation resistance values of a spiked cable will depend on shielding/grounding. It will also depend on the condition of the cable before spiking. A faulted cable might show a low resistance to ground due to a fault. This may not change much after spiking and could be an unreliable method of identifying the cable from its termination ends.

Back when I was in the locating/repair business (working alongside line crews), spiking was used more as a safety measure to ensure the cable to be worked on was de-energized. The termination ends could usually be identified by switching/parking the cable under repair, verifying that its ends were de-energized and finally grounding them. At this point, it becomes necessary to identify which cable or phase is the one of interest in a hole in the ground. We would do this by various techniques. Non contact voltage sensors were of some use, although a bunch of nearby live runs can couple a signal into the dead cable. Injecting a locating signal into one end of a run was pretty reliable. Finally, when we were relatively certain we had the correct phase, the crew would spike it with a temporary ground. On the few occasions we had mis-identified the cable, a fuse at the source end would blow.

 

[benjho]

Thanks guys, much appreciated. I don't suppose there is an example any of you could draw on that I could use for my technical question in my examination?

The system I am working on will be 11 KV delta config on a TNS system if that helps?

Thanks again.
Ben