As an engineer who designed sub-sea submersible electrical connectors, putting dielectric grease on the contacts...
As an engineer who designs electrical connectors, I'd expect you to differentiate between "contacts" which are used in switches & relays, vs. "terminals" which are used in connectors.
But then, I'd also expect such an engineer to realize that all submersible artifacts are sub-sea, and vice versa. It's tautologically redundant.
If the contacts are doing their job correctly they 'wipe' any oxidation on the mating surfaces off when the contacts are slid together.
If the connectors get slid apart & back together frequently (relative to their environment & purpose), then that would certainly be true. But since the topic was a fan relay that can be expected to remain plugged in for decades, I don't think that's applicable.
...any dielectric grease will have been removed from between the actual surfaces in contact...
If dielectric grease were ONLY grease, then that would probably be true enough. But it's not. It contains sand (glass beads) to aid in maintaining the dielectric potential between (for example) a heat sink & a circuit that might require a heat sink, but that would be adversely affected by any RFI or other induced eddy currents in the metal sink. So the sand maintains that gap mechanically that might otherwise be lost if pure grease was used. That's why a grease labelled & marketed (and, one would hope, engineered by an engineer who designs greases) as a dielectric should never be used where electrical contact of any kind is desirable. Or do you think you're the only engineer who takes his work seriously?
Unlike dielectric grease, oxidation can work it's way into the mating surfaces if unprotected.
IDK what you think protects the mating surfaces from dielectric grease working its way in, but grease does creep over time. We've all seen it - even those of us who are not sub-sea submersible engineers. So it certainly happens when the grease that has been wiped a few microns away is allowed to creep back in over a few months or decades of continuous connection.
Contrary to at least one person's opinion, Ford engineers are not God and do not know it all or are infallible.
Contrary to at least one person's opinion, neither are sub-sea submersible engineers.
There's attempting to understand the design intent, and then there's kneeling in supplication while facing towards Dearborn.
You're omitting at least one other likely possibility: there's also actual understanding of the design intent (regardless of where that designer worked), based at least in part on reading the MSDS for the chemical in question.
...too much grease...then becomes the conductor itself.
No, as I said initially: electrical grease is not conductive. It's actually silicone grease very similar to that used in dielectric grease, but with properties sufficiently distinct to make it particularly suited to maintaining electrical terminal conductivity, instead of preventing it like grease engineered as dielectric does. It's vaguely comparable to a steel sword and a steel scalpel; they're made of the same material & both have sharp edges for cutting flesh - but one is for killing, and one is for saving life. As similar as they might appear to someone not familiar with them, neither would be particularly good at doing the other's job.
...like dielectric grease not doing the job that electrical grease is made to do.
...friction connection (again, if that’s a proper term?)
I think "terminal contact force" might be what you mean, since I think you're talking about the normal (perpendicularl to the surfaces) force that holds the 2 conductors in-contact, allowing current to flow between them. Friction would be a tangential force (parallel to the surfaces) that resists them sliding against each other.
...to render the connection inoperable.
That's not the issue. The connection is likely to remain operable to some degree. But it won't be as good as if the correct material was used. Sort of like a surgeon trying to use a sword to save the life of someone who took a scalpel to a swordfight. Yeah, he might get the job done, and the patient might even survive for years. But it won't be as pretty as if he had used the right tool.
...the grease is simply to seal the metal connections from air and moisture, that what other wise result in long term oxidation.
It'll never seal out air or Oxygen. But it's exceptionally good at repelling water, which often contains salts that become acidic or caustic to refined metals. Even the CO2 in most water (even tiny droplets too small to be seen) creates a weak acid. That's the grease's primary function, even sub-sea. Making the plastic connector shells slide together & apart more-easily is just a side-effect.
...just like Steve83 wrote, I use conductive grease...
I've never owned, used, or advocated conductive grease anywhere. The grease I apply to alternators & their brackets is non-conductive silicone-based electrical grease marketed by Ford for that purpose. Just like in connectors; its function is to slow the corrosion of the metals in-contact (the Aluminum castings) by repelling water. Nothing else.
Ford lists the XG-12 electrical grease as conductive...
Where? Not on this page, nor on the linked MSDS:
www.fcsdchemicalsandlubricants.com
I would be inclined to only use it where a silicon grease is undesirable for some reason.
XG-12 is a silicone grease.
... I've worked on thousands ii'd say inconclusive
How many do you think Ford engineers studied, tested, & analyzed during their development of XG-12 before coming to a conclusion? Have you ever seen an original 2G alternator (built & installed before XG-12 was developed) catch fire? I have - firsthand.
At least twice that I specifically remember - it was exciting both times.
GM exterior lighting used greased sockets plenty of issues there...
That's just one of many reasons that I don't buy or enjoy GM vehicles - I think their engineering is noticeably inferior to Ford's in many ways beyond grease on electrical terminals.
...the contact force was more important to conductivity than was the area of the contact. ... I believe it, even if I don't fully understand it.
Not having seen it, my guess would be that (assuming the force can be maintained as the points of contact deform) the heat of an insufficient area's resistance would melt those points until they became large enough to carry the current below the metals' melting points. And if different metals were used, that would prevent fusing (ERW), allowing the contact to be broken later. But that's just a blind guess.
