Oil filled junction box

Question

The project I am working within is considering the use of an oil filled junction box, water dept around 20 meters. The enclosure would include some moulded circuit breakers, voltage is 230V at 60Hertza and single phase. I know that Schneider circuit breakers aren't tested for mineral oil junction box applications. 
 There's no issues with the insulation subject to suitable routine testing of the oil, then replace if required. 
 => Is there any issues submerging molded circuit breakers in mineral oil? Does any one have experience or can share any relevant case studies? 
 
 Thanks for your feedback and sharing experience! 
 Andrew McGeachie

AJJewsbury · Accepted Answer

Going off a a tangent perhaps ... what would be the environmental impact if the oil leaks? (even if it's not toxic like the old PCB types, oil in the natural environment generally isn't approved of). Or the effect if water leaks in? (I gather they go to some length to make sure that transformers are totally dry before introducing the oil). 
 The other thought was a overcurrent device that was naturally sealed with no moving parts ... e.g. HBC fuses ? And then encapsulating in a conventional potting compound or gel? 
 - Andy.

Jam · Answer

Oil filled switchgear is largely deprecated, at least in the UK, on safety grounds. Most operators with oil-filled gear only operate it off-load, if at all. The HSE has some guidance on this, which includes failure modes. To be fair, this is primarily HV. 
 I am open to being proved wrong, but I very much doubt any standard MCB is rated for use in a oil-immersed environment as it&rsquo;s a completely different mode of operation to in air&hellip; for example contacts won&rsquo;t be able to move as fast, may need more of a wiping action to ensure a solid contact and I suspect the bimetallic thermal element may not operate as intended. That said I&rsquo;ve never known it be tried. 
 Would it not be easier to design a suitably sealed enclosure with motorised breakers in air? Or even better, have the switchgear on the surface, with, if necessary, something simple for isolation at depth? Or separate the functional switching and protection functions, eg solid state relays coupled with fuses?

mapj1 · Answer

seconded on the design of under-oil contacts being nothing like ones for gas or air operation at LV or HV. The oil forces its way between the contacting surfaces and prevents reliable contact, so contact forces have to be much higher - its the same physics that sets the maximum side load on a bearing as being when the lubricant breaks down, but you want the opposite effect! . Cooling is of course better, so any thermal timer would be compromised while the quenching of arcs is faster, so you can run at higher voltage for a given gap until you get air in there, and then kaboom ! 
 Another detour but in oil filled spark gaps used for pulse power switching , it is usual to pump the oil and filter it, so that it is replaced with clean oil between the electrodes for each strike. 
 Oil filled static things like transformers are good, but these days the switching is usually in the dry though it may not be as simple as open to the air. 
 Mike.

Oil filled junction box

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