Very Old MV Switches

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charlie b

charlie b

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I visited a project site yesterday that is over 100 years old. I don't know if all the electrical equipment is that old, but some of it might be. I was told of a concern raised by an EE a few years ago. It involves medium voltage switches that have oil surrounding the switch contacts. The concern was that the oil presented a safety hazard, in that it might explode in reaction to the arcing of the switch contacts if opened under load. The load is an immediately adjacent step-down transformer.

My immediate response was to declare this concern to be nonsense. My understanding is that the oil's role is to quench any arcing. Therefore, the manufacturer would not have selected a type of oil that was flammable or that risked exploding when exposed to an arc. But what I don't know is how manufacturers built such switches (let us say) 50+ years ago. I would like to put this concern to bed and assure the on-site personnel that there is no risk of explosion. But I need more information.

Can anyone offer some factual basis for calling this a non-problem?
 
I can't offer much directly, but has the oil ever been tested? Does if contains PCBs? If no PCBs, then it's probably a modern oil and you can pursue it from that direction. (IIRC there was a discussion of this in the last few months here, with a single case of an oil-filled switch exploding.)

As for risk of explosion, IMHO there's always a risk when switching but it may be minimal. (That is, nothing is 100% "safe", maybe only 99.9999%.)
 
While quenching an arc, some carbon deposits are formed in the oil and if not taken care of over time by regular maintenance would have consequences as feared by the EE. Moreover such type of breakers are obselete and no replacement parts for the breakers may be available.
So It is high time you replaced the breakers with vaccuum or SF6 or Air circuit breakers.
 
Are you saying that the oil might not have been supplied by the manufacturer, but rather was added to the switch during initial installation? That had not occurred to me. It begs the question of whether the oil is periodically replaced as a normal maintenance task. I will send a few questions to the field.
 
I get that arcing can happen, and can lead to arc flash events. I get that the act of opening or closing an MV switch carries with it a risk of arcing. My essential question is whether the oil itself adds to the hazard. Can one type of oil merely quench an arc whereas another type could catch fire and result in an explosion?
 
charlie b said:
I get that arcing can happen, and can lead to arc flash events. I get that the act of opening or closing an MV switch carries with it a risk of arcing. My essential question is whether the oil itself adds to the hazard. Can one type of oil merely quench an arc whereas another type could catch fire and result in an explosion?
Click to expand...
 
Yes, the oil itself with its carbon impurities born out its very act of quenching arcs would lead to arc flash event.
 
charlie b said:
My essential question is whether the oil itself adds to the hazard. Can one type of oil merely quench an arc whereas another type could catch fire and result in an explosion?
Click to expand...

I believe there was basically only one type of electrical insulating fluid, the same as what was used in oil-filled transformers of the day. Just as transformer oil needs to be tested for degradation, including explosive gases, the breaker/switch fluid should also be tested/maintained.

Don't forget about the other nasty stuff like PCB that we used to put into insulating fluid.
 
I have seen these blow up before, they need to be properly maintained and the oil needs to be sampled regularly. Every time that switch is operated the dielectric properties of the oil are degraded a little bit.
 
Put another way, LIQUIDS don't explode and in oil immersed switchgear, the oil was used specifically to quench the arc, so arcing is not the problem. So long as the contacts are completely immersed in the oil, there can be no explosion. But liquids that turn to gasses or are sufficiently volatilized can explode, as can any flammable material. If left with no maintenance and/or checking of dissolved gasses, conditions within the arc chamber can absolutely become dangerous. What can happen is that severe contamination leads to less arc quenching, which accelerates damage to the contacts until they fail, which may lead to the oil leaking out during a fault and THAT is what can catch fire or explode under the right (wrong) circumstances.

If however the oil was replaced in the 1980s in a PCB abatement program and the new oil was selected as a non-flammable version such as Hatcol, that is less likely. But if the end user doesn't know and/or has no records, you cannot assume the best, you have to assume the worst.
 
Thanks for all the information. I have sent a few questions to the site project engineer.
 
In the oil you will have carbon deposits along with dissolved hydrogen and acids. PCB’s are another environmental disaster area. GE made some replacement VCB trucks for us to replace 60 year old OCB’s, not cheap but a damn site safer.
 
I was told by the director of a Massachusetts utility at a seminar that over time as oil switches are used, little particles of metal get into the oil from the force of switching non and off. With higher voltages, these particles can carry an arc through the insulating oil. He brought it up to substantiate why his utility replaced oil switches with vacuum switches.
 
The switches under discussion serve as the local disconnecting means for small (under 100 KVA) transformers. I sincerely doubt they would get operated more than once a year. But then, some of them have been in service for I don't know how many decades. So it remains an open question in my mind. I have sent the local engineer a few questions about PCB replacement and periodic oil testing. I do not yet have any answers.
 
We changed on out three or four years ago that was 6 years from being in service for 100 years...
I know for a fact the oil hasn’t been changed in at least 40 years.
It has been tested annually, with mineral oil added every five or ten years.
the buildup of acetylene was the one we worried about for explosions..
it was a 100kV breaker though.
 
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