Redundant power UPS with ground isolation

[BrunoGelli]

Hi my fellow nerds,

I'm participating in the design of a nice particle physics detector. One interesting thing is that we have 2 types of power: Quiet power, low noise with controlled ground impedance for measuring equipment; Common power, for common and high power application. In the event of a power outage, only the common power is backed up using a generator.

Most systems have a UPS, but the critical ones have to have redundant power as well. That is, the UPS would have to be able to charge from the back-up generator power in the case we have long power outage. 

The thing is, there is one system which requires redundant power and also quiet power. And this is a problem, because if we use a simple dual input UPS, we are crossing the ground connections in both circuits (quiet and common) which defeats the purpose... And it is not acceptable for the noise level we are trying to achieve.

The currently solutions is to have people babysitting the experiment. In the case of a power outage, the person would physically change the outlet the UPS is connected to. There got to be a better solution!

Do you know a UPS which is capable of fully switching between 2 circuits? Including grounds!

I guess another options would be to create a simple relay circuit, or even better would be a motorized switch like they have for solar+mains+backup. But the approval process for "homemade" gear is a pain...

[JimboJack]

hi,

You can get add input and output isolation transformers to UPS depends on the quality of the vendor.  all the top UPS guys sells them.
https://www.riello-ups.co.uk/questions/50-what-is-galvanic-isolation


ATS switching you pays for quality , emerson , siemens, be your ideal choice.

J.

[jonpaul]

DPDT mains coil contactor.

Cost ~ $50

j

[ejeffrey]

I've never seen a UPS that switches the ground connection and I kind of doubt one exists as having switches in the safety ground is not generally allowed.  Maybe there are situations where that is allowed and desirable where there is a commercial solution but I haven't seen it.

Instead use an external isolation transformer.  There are a few ways to do it but the easiest is probably to put it on the auxillary / generator input to the dual source UPS. Then, at the isolation transformer bond a ground conductor to the neutral output and connect that to the UPS ground. In this way, your sensitive equipment is always connected to the "quiet" ground and never to the generator "common" ground but still provides a fault path back to the source (the transformer secondary) when operating on generator power.

[BrunoGelli]

Thanks for the replies! They are all good suggestions.

Do you think a good Automatic Transfer Switch would work? I can go barbaric and simply cut off the ground pin of the auxiliary/redundant supply. (Or buy a cable that doesn't have it)

[tridac]

Switching grounds seems foohardy to me, and potentially dangerous. Would not do that for the home lab, never mind something others may come into contact with. Analyse the overall system in more depth, then implement isolation *only* for  the circuits that need it, at the output side of the ups/generator. Small isolation transformer, or isolated power supplies, for example...
 

[NiHaoMike]

Use an inverter/charger (or UPS with external battery connection) with an external "floating" DC supply as a secondary charger.