Oil filled junction box

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’s a completely different mode of operation to in air… for example contacts won’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’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?

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 sparck gaps used for pulse power switching , it is usual to pump the oil and filter it, so that it is clean 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.

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 sparck gaps used for pulse power switching , it is usual to pump the oil and filter it, so that it is clean 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.