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home :: tech :: power

Apr 04, 2007

Switch UPS

Wellingtons had some up's and down's with respect to power in the central city over the last few months - just before Christmas half the Terrace was knocked offline for 4 hours and a few weeks ago we had rolling power spikes for an afternoon.

For a major outage theres not a whole lot you can do other than having a really good UPS on your core servers (or hosting in a data-center) - the spikes generally aren't a problem for your server room as the UPS will condition the power.

Most PC's actually handle spikes quite well too - the kicker is that your distributed switches will either reboot or pass on any spike to your Power over Ethernet equipment - which means as well as the lights dimming your VoIP phones cut-out and reboot (ditto your Wireless Access Points if they also use PoE).

Now you start to think about some UPS's to cover your distributed switching gear (if you have the luxury of structured cabling all the way back to your server room then you're really luck!).

From an expert (not me!) -

"A general rule of thumb is that no UPS should be loaded more than 70% to 80% of full capacity to minimise the risk of compromising the protection due to unplanned or temporary overloads. In my calculations I have divided the total load by 0.8 to give a 20% headroom. It is then necessary to establish the VA rating of the UPS. Our UPS's suited to this application have a 0.7 output power factor, so the total watts requirement is then divided by 0.7."

So if you have four 24 port Catalyst 3550 switches and a PowerDsine PoE Injector -

Cisco Catalyst 3550-24-PWR: 525W x 4 = 2100W

PowerDsine POE Injector: 525W x 1 = 525W

Total Load: 2625W

Total UPS Watts Requirement with Headroom Allowance (/ 0.8): 3281

Minimum VA rating of UPS (/0.7): 4687

Which equates to a sizeable UPS. A 6kVA unit will last about 15min under full load but its primarily there for power conditioning and to buy a little time to cut phones over to another location in case of power-cut.

On the subject of Power Conditioning versus a UPS - again more expert opinion -

"Power conditioners were commonly used for protecting against brownouts - they would hold the voltage up for a few cycles. However off-line or line-interactive UPS's have now become lower priced than power conditioners and do the job adequately in virtually all cases. An on-line UPS regenerates the AC power so it is always perfect and constant irrespective of the incoming power."

"Most UPS's have spike protection too. However this is minimal and may become exhausted with one or two spikes, and there is no indication of this. If spikes are a special concern then dedicated surge diverters with a good practical surge capacity and low surge let-through voltage plus status indication."

So watch your switches :-)

[/tech/power] | [permalink] | [2007.04.04-00:38.00]