Dual Source Electricity!

[polygonum]

Many servers have two power supplies. You have several options about how to use them but one I have seen quite often is to plug one directly into the mains and the other into a UPS.

Advantages include being able to run on UPS if mains goes down (obviously) but also to run on mains if the UPS cooks itself.

However, quite a lot of equipment only has a single power supply - things like routers, switches, and so on. Is there a relatively sane way of connecting these lesser devices to two sources (e.g. UPS and direct to mains)? Everything I come up with in my mind has potentially severe safety issues. Issue that made e think of this was the need to replace a dead UPS - server presented no problem but users were inconvenienced by the switch and routers having to be powered down.

[Andy Burns]

Yes, automatic transfer switch ... e.g.

[polygonum]

At that price I could buy another (low power but adequate) UPS for each device! :-)

[Clive George]

Posher routers, switches, etc have two PSUs. Pay for it :-)

The alternative is to have two devices, eg stacked switches, and cables going from the servers into each of them. Covers you in case the switch goes T/U for other reasons too.

[D.M.Chapman]

Decent switches will have two power feeds.

For stuff that doesn't (cheap nasty out of band ADSL modems being common) there are transfer switches. We use APC in our datacentres, but others exist.

Not cheap, but work well

Darren

[polygonum]

I am not talking about the crap end, but HP, Cisco and the like. A few might have two power feeds, but certainly most of them don't. This is in real-world locations like offices where they might have a 24-port HP switch and a Cisco router. Doubling up isn't an option on grounds of cost alone.

[Andy Burns]

They may not have dual main input, but I'd say most non-crap Ciscco switches *do* have a 48V input too.

OK a Cisco RPS isn't exactly cheap either ... in short no cheap way to do it ...

[polygonum]

That is something I might need to look at. Hadn't even thought of that - thanks.

[The Natural Philosopher]

isn't there a non interruptible type where the kit is permanently fed from a battery and its also permanently on charge?

[dennis]

My new £40 poe switch has 48v input. Why they had to use what looks like a 12v jack so you can fry some other kit by mistake is anyone's guess.

[polygonum]

Well - if it runs from the main UPS that is what will happen and, as far as it goes, that is fine. The trouble is that while you can swapout a dead UPS, and swapin the replacement, without affecting the server, you are rather stuck with the switch and router.

Most of the switches and routers run from kettle leads (not separate low voltage supplies) so need mains or UPS output.

[polygonum]

Might be worth me making it clear that "automatic" is not necessary.We simply want to be able to switch source, probably for at most a few tens of minutes, while doing things (like replacing a UPS). It would be fine if we could unplug one kettle lead from the UPS and plug in one from mains within, ooh, 1/100 of a second. :-)

[Andy Burns]

Sometime UPSes are wired using a bypass switch ... I've never asked the price, but the ones I've seen were into the tens of kVA range ...

[Clive George]

Doubling up is the right option for anything involving servers. You don't want to lose your service if a switch goes down.

[polygonum]

Which, if it were replacing the UPS batteries, might very well do the job. But when the whole thing needs replacement, it doesn't. :-(

[Andy Burns]

Most decent UPSes allow hot-swapping the batteries anyway, the bypass switch *is* there for the day you need to replace the UPS, it's not part of the the UPS.

[polygonum]

You still have to justify the cost. In many of these companies it would be very difficult to make the case for full doubling up of kit. Ends up that delivery and installation of a replacement switch can be achieved quickly enough most of the time and, thankfully, switches have proved to be very reliable, so it is a rare event. But it is very difficult to persuade the same companies to let you severely disrupt them, even for a lesser period of time, in order to do some maintenance.

[Andy Burns]

Some UPSes do have an inbuilt bypass mode though. But the bypass switches I was talking about are like the ones pictured here

The left switch controls power input the UPS, the right switch controls power output of the UPS to the kit, the centre switch sends the input power direct to the kit.

[Clive George]

Where the alternative is downtime of the main file server for an office of 100 people for a day while a key switch gets replaced? Not hard to justify.

True, networking kit is generally pretty reliable. If only the same could be said for the WAN links between said kit :-)

Most companies will accept some downtime, especially if you can do it out of hours.

Those who won't generally accept that they have to pay extra for the resilience.

OTOH I was at a presentation by a company which was very proud of their investment in their resilient setup - links, servers, etc, they could cope with one of pretty much anything going down. Their business was

24x7 and time critical, so uptime was important to them. Except when they said "Our datacentre". "Only one?". "Yes". Ummmm...

[Grimly Curmudgeon]

I have the essentials from the UPS - PC, monitor, LAN switch, router. So that if mains goes, and I'm nearly finished a download, I can at least get that before shutdown.