Reminder: Never connect a generator to home wiring without transfer switch

Warren Kumari warren at
Tue Aug 31 22:54:02 UTC 2021

On Tue, Aug 31, 2021 at 12:17 PM Mel Beckman <mel at> wrote:

> It’s germane to NANOG. Just last week I visited a “data center” that uses
> a roll up generator and a cheater cord to power the racks. “Oh, this is
> safe”, they told me. “We have a policy that you must throw the main breaker
> before plugging in the generator. Since you have to open the garage door to
> wheel in the generator (!), you walk right by the switchgear and a sign
> that reminds you to throw the breaker.”
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...

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

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


> Code enforcement is on the way. :)
>  -mel
> On Aug 31, 2021, at 7:50 AM, Josh Luthman <josh at>
> 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 at> 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 at> 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
Perhaps they really do strive for incomprehensibility in their specs.
After all, when the liturgy was in Latin, the laity knew their place.
-- Michael Padlipsky
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