Discontinued OCB 11kV Cable Termination Box Failure 1

[KKiiwwii]

I have fallen heir to what I am sure is a historically, incorrectly installed 11kV suite of OCB switchgear.

An old plant in the 1970’s had installed a suite of Yorkshire ‘So Hi’ OCB’s as a ring main unit. The suite was originally cabled with 3C 125mm² Cu cables but the cable box for a unit that has now had a cable-termination failure was never filled with compound. I am now convinced from reading the manual, that it should have been. The problem had also been exacerbated with a change-over to a 3C 250mm² Al cable which meant that far much more bulky terminations were now in the box

My reason for saying this is that the cable box that has failed has, I am sure, the configuration for compound- filling, as I have been able to discover from manuals available on the internet. I am sure that if the failed cable box had been left as it was originally cabled, there was obviously just enough distance in the internal clearances for the box to have ‘squeaked through’ and not undergone an arc-flash event, although it had not been filled, as required, with cable-box compound.

All the other boxes have not had any similar problems to date, but as far as I can determine, they have not been re-cabled either. As the rest of the suite is live I cannot check whether the other boxes are compound-filled and therefore problem-free, or if they are also only-just squeaking through internal clearance requirements.

Can any readers familiar with this older style and now discontinued line of switchgear, please shed light on this matter. Has anybody had any similar experiences with this same style of switchgear, who could share them with me.

 

[waross]

If the circuit was out energized, but no current was flowing then condensation may have contributed to the failure.
It does not take much heat to keep the interior of an enclosure above the dew point. Often the small I²R losses of the loaded conductors will generate enough heat to keep the interior of a junction box dry. (There are exceptions.)

Another possible issue causing condensation build-up is the absence of a drain hole. Over the years I have encountered a number of supposedly sealed enclosures that were soaking wet inside. Despite the supposed sealing, changes in atmospheric pressure will pump ambient air in and out of the enclosure. I have seen a number of control cabinets and junction boxes that were dripping with water inside. A 1/8 inch dia hole in a bottom corner allowed the enclosure to breath and the enclosures stayed dry after a drain hole was drilled in the bottom of the enclosure.
If the enclosure is tight enough to hold potting compound it may have been subject to damp air being pumped in by changes in atmospheric pressure.
Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Marmite]

Just because the cable box was originally designed to be compound filled, doesn't mean it has to be. If the clearances are sufficient it is possible to use a dry termination, such as heatshrink or cold shrink. As Waross correctly points out, it is necessary to have a breather hole on the cable box, preferably fitted with an insect screen. It should be fairly easy to tell whether your failed termination was meant to be compound filled, and someone forgot. If the cable is paper insulated, the cores would be wrapped with fabric tapes before filling the box with compound. If the insulation is polymeric then the cable is unsuitable for bituminous compound, and you either need a dry termination or a suitable compound such as guruflex.

Please find attached SOHI manual, and also a table giving the clearances required for an air termination.

Regards
Marmite

http://files.engineering.com/getfil...4a-8fdf-e73f5408be5e&file=Yorkshire_SO-HI.pdf

http://files.engineering.com/getfil...408f-47ea-ac01-845c5876af25&file=Spacings.pdf