back to article A router with a fear of heights? Yup. It's a thing

Cisco's let users of its ASR 920 Series Aggregation Services Routers know they've got a fear of heights. In one of the odder field notices The Register can recall, Switchzilla has revealed that “AC Power Supplies (A920-PWR400-A) shipped between September 2015 and May 2016 are only compliant for usage in elevations up to 2000 …

  1. Platypus

    At a previous job we had a similar - but not identical - problem with a machine in Boulder. In our case there was one more step. Because of the thinner air, we got less cooling. The warmer temps made the PSU less efficient, causing brownouts which manifested as transient errors on our internal communications links. The fix turned out to be a slight adjustment the the ratio between temperature and fan speed. I was the guy on-site, but kudos to the hardware folks back on the east coast for figuring it out.

    1. frank ly

      I suspect that's at least part of the reason for the warning in this case too. Does anyone know how ambient air pressure affects flash-over beakdown voltage?

      1. Neil Barnes Silver badge

        It should *increase* the flashover voltage - if anything, air at altitude tends to be drier (unless you're above the dewpoint). I suspect a cooling issue, unless they've really gone to town with the voltage creep distances.

        And come to think of it, apart from the mains input, why would there be high voltages around?

        1. Roger Greenwood

          Yup - will be a cooling issue. If you remove all the air you remove the risk of flash - hence vacuum circuit breakers.

      2. Dave Pickles

        Breakdown voltage for a given gas, pressure and gap length is determined by a Paschen curve (ooer!). It generally decreases with reducing pressure then rises again at very low pressure.

    2. John Riddoch

      Sun M-Class

      The old Sun M-Class servers (M4000/M5000) had a setting in the XSCF where you could set altitude and it would (presumably) adjust fan speeds/cooling accordingly. Was never a problem for us as we weren't high enough above sea level to care, but the function was there.

    3. Alan J. Wylie

      Cheyenne Mountain Complex

      The State Capitol building in Denver, Colorado, has mile high (5280 feet, 1609m) elevation markers on its steps. Most of the more populated areas, e.g. Boulder and Colorado Springs are at about 1600m, but the Air Forces Cheyenne Mountain Complex (NORAD, and where the Stargate is kept) is at about 7200 feet / 2200m. We don't want that bursting into flames!

      1. Velv

        Re: Cheyenne Mountain Complex

        The State Capitol building in Denver, Colorado, has mile high (5280 feet, 1609m) elevation markers on its steps.

        Fnarr. Do they have, err, "problems" with office workers and visitor staying late for mile high activities?

  2. Anonymous Coward
    Anonymous Coward

    I'm just glad I don't have to live where 'crazy ants' swarm inside my computer and chew though the wiring.

  3. Nolveys

    Not much info.

    Too bad there's no mention of what specifically is going pop. The PSU isn't just one IC, is it?

    Interested because the atmos around here causes groceries that come in sealed packages to become rather round (before the roundness is transferred to myself).

  4. Your alien overlord - fear me

    Just as well mine are located in my underground lair at the bottom of an extinct volcano. Mwahahaha.

    1. Anonymous Coward
      Anonymous Coward

      Do you find issues with the pressure changes when the launch doors open? It's causing me no end of grief.

    2. Anonymous Coward
      Anonymous Coward

      Be careful with cat hair though as that can cause problems too.

  5. Mike Banahan

    Fear of heights

    A long, long time ago I was doing some work at the mathematics centre in Amsterdam (the home of the famous mcvax which became a hub of uucp email, so that must be 30 years ago at least). The DEC salesman was there to sell them a VAX and seemed convinced he had his sale (and these were big-ticket items).

    "Ah, there seems to be a problem" said the MC.

    "What?" asked DEC in fear of losing juicy commission.

    "It's specified to work at altitudes of 0 to 15,000 feet .... "

    ".... but we are below sea level."

    Gloomy look on salesman's face.

    "But don't worry, we'll put it on the second floor." Cue beers all round and laughter.

    1. Anonymous Coward
      Anonymous Coward

      Re: commission

      ""What?" asked DEC [salesman] in fear of losing juicy commission."

      Then he wasn't an employed-by-DEC salesman.

      DEC didn't normally pay commission to salespeople. DEC founder and boss man Ken Olsen thought that commission was likely to motivate salespeople to sell what was best for the salesperson rather than what was best fit for the customer. It's documented in various places.

      FWIW, I'm in the "it's almost certainly cooling, not arcing" camp on the original issue here.

    2. Robert Carnegie Silver badge

      Re: Fear of heights

      It'd be a brave supplier who warranted equipment to work below sea level if it isn't waterproof. "But it says..."

      Hail to "On Call" where someone wants to turn on their computer but their office is flooded. Up to fan level.

      An altitude problem was reported before, was it hard disks? That was air pressure, I think, and sealed disk units that got bulgy up a high mountain, such as at the better astronomical observatory, or a monastery that wants to write or print all the possible names of God (yeah, those guys again).

      Cosmic rays probably aren't bad, or not much worse than here, until you're actually in space. Which happens.

    3. Anonymous Coward
      Anonymous Coward

      Re: Fear of heights

      There are loads of datums around the world for sea level. Perhaps one of them can put Amsterdam at 0 (feet or met{re|er}s - take your pick)

      1. imanidiot Silver badge

        Re: Fear of heights

        Well, the 0 height reference used in the Netherlands is called NAP (Short for Normaal Amsterdams Peil, Normalised Amsterdam Level, which is used as the reference level for all height measurements in the Netherlands and several other countries.) SO it's not all that hard to argue Amsterdam (almost by definition) is at 0 meters NAP.

        1. Robert Carnegie Silver badge

          Re: Fear of heights

          The Netherlands explicitly comes under

          which means what it says: low. Even below sea level. Actual sea: they have dams to keep it out. Many people actually know this.

          On the other hand, geographical confusion exists between "Netherlands", "Low Countries", not the same thing, and "Holland", also not the same thing really, nor lexically - Wi'pi' says it means "wooded land".

          When everything belonged to the Wholly Roman Empire, it didn't so much matter.

  6. Anonymous South African Coward Bronze badge

    Bigger cooling fans then?

    Or liquid cooling (oil/water/cat pee/coffee) as a solution?

  7. TRT Silver badge

    You had me going for a minute there...

    when you said about liking it "low and slow" and "mountainous networks", I thought you must be referring to sudden peaks of demand causing supply instabilities. Seriously? It's a cooling problem? Wow. That's a tight tolerance. I mean, I know that capacitative trackpads had a problem with dry air / moist air once upon a time, and you had a correction factor for that. Well, you live and learn.

  8. Brenda McViking

    I also think it would be a cooling issue.

    My home projector has a "high altitude mode" which judging by the noise it makes after it is enabled just spins the fans faster. Manual also says it is reccommended to run it in this mode if ambient temp is above 30C.

  9. Anonymous Coward

    Less air to insulate a PSU

    "at altitude, there's less air to insulate a PSU which heightens the risk of arcing inside a PSU."

    Illogical captain, less air would lead to less arching. Perhaps the lack of air prevents heat being removed through convection currents.

    1. Paul Crawford Silver badge

      Re: Less air to insulate a PSU

      Wrong, the ionisation voltage drops with pressure until you get really low (like near-vacuum) when it rises again. Its a risk for satellite HPA design, for example, as high-Q filter coils and similar with high voltages can arc wile it de-gasses, but stops once it really is a space-level of pressure. Which is why neon bulbs are at low pressure...'s_law

      Also of note is the Chinese safety standards (stop laughing at the back!) specify to 5000m, not the more usual 2000m for UL.

      1. TRT Silver badge

        Re: Less air to insulate a PSU

        I thought we'd be talking about hundreds of kilovolts here... unlikely to be present in a router's power supply.

        1. imanidiot Silver badge

          Re: Less air to insulate a PSU

          Rule of thumb is about 1kV per millimeter of arc length at sea level. At altitude this can be several millimeters per kV, so if the design used has an inductor creating high voltage spikes in the rectifier circuit it's totally possible to get enough voltage somewhere to arc.

        2. Paul Crawford Silver badge

          Re: Less air to insulate a PSU

          Its voltage gradient that matters, i.e. (volts)/(distance). Going from 2000m to 5000m typically involves a 48% increase in creepage and clearance distances for PCB design, etc.

          Edited to add @imanidiot - its not just the operating voltage, which can easily peak to a significant fraction of 1kV in a SMPUS, but also the need to pass a 6kV lightning surge test for typical safety reasons. That is why most distances are several mm (e.g. 8mm or more) for mains clearance, etc.

          1. CommodorePet

            Re: Less air to insulate a PSU

            Pedantic - that's 6KV ESD (Electrostatic Disccharge, i.e. a static build up), not a lightniing surge.

            IEC 61000-4-2 is the relevant test.

            1. Paul Crawford Silver badge

              Re: Less air to insulate a PSU

              Nope, just checked and it is IEC 61000-4-5 for lightning and industrial surges. Category 4 is 4kV / 2kA surge typically modelled with a double-exponential 8us rise time and 20us decay time.

              Somewhere I remember reading that generally normal 220V/240V main is limited to around 6kV peak in any case as the wiring and sockets, etc, tend to flash over if you get more than that incoming (say farm at end of long overhead wires).

  10. Terje

    I'm in total agreement with the above commenters that arcing has nothing to do with it. The dielectric strength of air is pretty bad, but gets better the lower the pressure goes. So it's likely just a case of running hot under normal conditions and risk reaching to high levels at low pressure

    1. imanidiot Silver badge

      Actually no, it gets worse for anything humans would still consider breathable low pressures. Minimum breakdown voltage occurs at roughly .2 mbar. Only once you start getting close to hard vacuum does the breakdown voltage suddenly climb back to infinity.

      I really don't get why people think dielectric strength gets better at lower pressures. It's demonstrably not true and this has been known since the 1890s

      I would have to add though for a few 1000 meters to make a difference it would have to be a rather low margin design though..

      1. Terje

        Have an upvote for teaching me something new!

  11. Camilla Smythe


    Fortunately once we have implemented The Great Repeal Bill we can get back to LSD, forget about all this 'expert' stuff and become Globally Globally Global.

    Top Tip for Trump.

    If your microwave is spying on you then according to FCC15 you have to reorientate it so it does not.

    Better still... just get rid of the FCC.

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