Many many (> 20 ) years ago I worked for a small ISP in New York. The official disaster recovery plan included:

"Drive over to Warren's house {Address]. Under the deck, you will find a key hanging on a nail. Use this to open the closet and take out the generator [SAFETY WARNING: it is heavy]. Put it in your car, and drive back to the office. Please in the big cable <picture>, *then* start the generator and let it idle for a few minutes. Then flip the big switch marked TRANSFER."

Anyway, hurricane Floyd comes along and knocks out one feed to the office/datacenter, and we initiate the DR plan. One of the employees has a station wagon, so he's the one designate to go fetch the generator -- but, first we have to unload the nine 30lb bags of dry cat food which he inexplicably has in the back of the car. I still have no idea why, but... Anyway...

The "datacenter" is 8 relay racks in the front of the office, powered by around 25 consumer/soho style UPSs. Of course, the rear of the racks are ~4 feet from the wall, and the UPSs are buried under many feet of cable, etc... oh, and the room lights have no battery backup.

We immediately start stumbling around behind the racks with flashlights, trying to shuffle things around, powering off unneeded devices (PM4s and TNT MAX draw much power), etc.

We then start cycling out UPSs with low battery levels for more charged ones (move the second PSU to a charged UPS, unplug the first one, move that to new UPS, etc) and using some UPSs to recharge other UPSs connected to devices that don't have redundant power supplies, etc.

This is all a huge mess of wires, we only have flashlights, there isn't much space, etc -- and somehow someone manages to hook the output of UPS A -> UPS B -> UPS C -> UPS D. This all worked OK... right up until someone managed to hook the output of UPS D back to the input of UPS A.

UPSs might claim to have "Pure Sine(TM)" output, but, well, they don't... and so they all start clicking like a swarm of angry wasps, and then they all simultaneously let out the magic smoke, as well as much fire and noise...

I'm still convinced that I managed to jump right over the racks when this occurred.

Code enforcement is on the way. :)

-mel

On Aug 31, 2021, at 7:50 AM, Josh Luthman <josh@imaginenetworksllc.com> wrote:

Is this conversation really taking place on NANOG?

Don't backfeed power. Got it. Stupid people are going to be stupid, we won't solve it here.

Josh Luthman
24/7 Help Desk: 937-552-2340
Direct: 937-552-2343
1100 Wayne St
Suite 1337
Troy, OH 45373

On Tue, Aug 31, 2021 at 10:41 AM Mel Beckman <mel@beckman.org> wrote:

Mark,

But you said “Gas-fired furnaces or heaters should not have an impact because the only electrical requirement is to fire up the pilot light.” There is no gas-fired furnace I know of that doesn’t require a blower fan. How else does the heat get out of the furnace?

To answer your question, you need to understand that this safety system has two components. The first component, the furnace interlock relay, is designed to interlock the blower with the forced-air system, which also includes an outside air supply valve. When the blower is energized, a circuit inside the furnace gets power. The blower and furnace operate continuously when this circuit is energized, and the supply valve opens and closes as needed to ensure the air doesn’t get stale.

The safety second component is the limit switch, which primarily turns the blower fan on and off, but also has a safety role. When the temperature in the air supply plenum gets too hot, the limit switch turns off the furnace burner (or boiler, in a water-based system) to prevent damage, and possibly a fire, from overheating.

The actual state mechanics are thus not as simple as “if the blower fails the furnace won’t light”. And it’s because of these complex state mechanics that furnace electricity is hard wired.

Without AC power, no furnace can operate in a power outage. So that’s certainly not “no impact” from a utility failure. But the many thousands of deaths that occurred in homes and offices before these safety systems were put into the code is why you need a generator transfer switch if you want heat (or A/C) in your home during an outage.

-mel

> On Aug 31, 2021, at 7:15 AM, Mark Tinka <mark@tinka.africa> wrote:
>
>
>
>> On 8/31/21 16:06, Mel Beckman wrote:
>>
>> I think you’re forgetting about the all-important blower fan in a gas-fired furnace.
>
> Well, I was referring to a pure electric furnace, not one that uses a blower over a gas-fired one :-).
>
> In that case, the blower is not a major draw on power.
>
> But again, we don't have those things here, so :-).
>
>
>> That said, the reason the code requires furnaces to be hardwired is to ensure that the blower interlock system can’t be bypassed. An electrical interlock ties a heat recover ventilator to circulation air blower operation of a forced-air furnace system. This ensure that the blower circulates supply and return air within the structure. A plug-in power source leads to the possibility that this interlock could be accidentally defeated, resulting in an overheat within the flame box.
>
> Makes sense.
>
> Does this, then, mean that if the blower itself were to fail, the gas burner would not light?
>
> Mark.

“If you have an important point to make, don’t try to be subtle or clever. Use a pile driver. Hit the point once. Then come back and hit it again. Then hit it a third time-a tremendous whack.”

-- Winston Churchill