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Unread 26-09-2014, 00:44
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Tristan Lall Tristan Lall is offline
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Re: Looking for slip rings and distributors

I had a look at the documentation for the Mercotac slip rings, and judging by the external design and description, I suspect they have very little mercury inside. Given how catastrophically mercury corrodes aluminum (like the housings), they're presumably designed to resist that failure mode—and the simplest way to do that would be to use just enough mercury to wet two closely spaced parallel plates with concentric traces, and then fill the housing with an insulating oil. (The aluminum housings are anodized, which helps, but likely not enough.) The amount of mercury needed to go around the circumferential contact would likely be on the order of microlitres. Judging by voltage ratings on the larger ones (maybe the small ones don't have oil?), that seems like a plausible method of construction.

If that's the case (and contact with their applications engineers ought to confirm/repudiate it), then we're probably looking at a level of hazard comparable to lead in batteries and soldered components, or hexavalent chromium and cadmium on plated components. While not ideal, there hasn't been a blanket prohibition on other components that contain metals that exhibit toxicity when ingested, and with form factors that permit accidental exposure to the metal.

As for the implications of a spill, I think that would depend a lot on how much information was immediately available about the quantity and toxicity of the substances released, and the stringency of the applicable regulations (as interpreted by the people in charge). If cleanup is necessary, it could be a fairly onerous process requiring external assistance.


As for the contact ratings, check with Mercotac's applications engineers if they are willing to publicly provide duration vs. current ratings, because typically, the failure mode is due to temperature rise. If they compare favourably to the trip performance of the circuit breakers, they may be usable for brief periods in excess of their constant current rating. Similarly, if they can provide ratings that presuppose a reduction in the expected life of the device, but won't cause catastrophic failure, perhaps you could get away with more current. In any of the above circumstances, you'd want an engineer's input to have documentation for the inspectors as to why the figures in the published datasheet are not applicable.

Last edited by Tristan Lall : 26-09-2014 at 00:46.