3 ms·

> it's difficult to test this equipment Can't you just flip the circuit breaker or just pull the plug out? Or does a real power outage usually look different t

by cactus2093 6y ago

> it's difficult to test this equipment Can't you just flip the circuit breaker or just pull the plug out? Or does a real power outage usually look different to that in some way, like a surge followed by a shutoff?

WalterBright 6y ago
Around here the typical failure pattern is the power goes out for a second or two, then comes back on for a few seconds, then goes out for good.
- myself248 6y ago
  That's a recloser operating. Initially, there's a fault, say, a tree contacting a wire. This conducts a bunch of current to ground, the voltage in part of the network sags a fair bit, and a protective relay senses the overcurrent and opens the circuit. This happens in a second or two, and your power goes out. You may notice lights dimming or flickering in the instant beforehand. But, many faults are transient. Perhaps that tree branch finished falling, and is no longer against the wire. So the protective relay performs a "reclose" operation, where it turns the power back on for a moment and measures the current. There's normally a huge starting inrush as motor-driven appliances restart, so it waits a moment before sampling the current, and this is the period when the power comes back on for a few seconds. (It's probably pretty nasty power during this interval, too.) If things are good, hey, you're good! A brief outage while waiting for the fault to clear itself, and then everything's back. The operation will be reported to the dispatch center, and someone may come out to inspect the area in case there's damage. But if the fault is still there, and after a moment the current is still way higher than it should be, the recloser opens again, and this time it stays open. Gonna need a crew to move that tree, and then they'll manually try again.
zzz61831 6y ago
You can put your equipment on a voltage relay to make failure modes as simple as flipping a circuit breaker. It will turn off once voltage drops too much and will wait for it to stabilize for specified amount of time to turn it on.
bradstewart 6y ago
It depends. We built and deployed some IoT devices (controlling and reporting status of lights, HVAC systems, that sort of thing) to a bunch of greenhouses--one of which ran on generator power. The customer was running the generators right at the maximum capacity, and we'd see really, really odd things on the power lines. Some times a surge like you mention, sometimes changes to the frequency, sometimes changes to the voltage. We discovered all sorts of weird brownout conditions in our devices.
jcrawfordor 6y ago
It's difficult in practice to transfer load to generators for test especially if there's load shedding involved as is common in larger facilities (higher-power-consumption items like part of air conditioning capacity are cut off when transferred to generators). To help address this a lot of more critical facilities are equipped with a test load (really just a very large outdoor resistor with active cooling) that allows for running the generators at rated load for test and exercise without actually transferring. Of course, these tests do not always detect problems with the transfer equipment, and if things aren't designed and maintained well the generator may experience load under real conditions that behaves differently from the test load.
avh02 6y ago
Iirc (top of my head), some datacenters go off the grid regularly at peak (provider) load times and get more favorable rates for doing it. I guess it ensures your fuel doesn't go stale and all your transfer systems work.
- bluGill 6y ago
  Good point. Many power companies will give you a nice discount year round if you will disconnect from mains on demand. If you are on the fence about a good backup generator this will more than make up the difference.
gerdesj 6y ago
If done right it is routine. In our small data centre we have a board with five lights and a key. You turn the key and the mains is cut off. The lights give the status. The UPSs beep for a few seconds "on battery", the genny fires up in the boiler room and when it stabilises, it takes the load. We leave it running for 5-15 minutes, go out and check the fuel levels and top up accordingly. It's diesel so safe to hot refuel. When done, turn the key back and then the UPSs take the load again and then hand back to the mains. The genny runs for another 15 minutes and then shuts down. That should avoid flip flopping. You then fill in the test form on our wiki and job done. We do it weekly to fortnightly. An ESP8266 based thingie reports status back to the central monitoring (GPIOs and ESPHome -> Home Assistant)

WalterBright 6y ago

Around here the typical failure pattern is the power goes out for a second or two, then comes back on for a few seconds, then goes out for good.

myself248 6y ago
That's a recloser operating. Initially, there's a fault, say, a tree contacting a wire. This conducts a bunch of current to ground, the voltage in part of the network sags a fair bit, and a protective relay senses the overcurrent and opens the circuit. This happens in a second or two, and your power goes out. You may notice lights dimming or flickering in the instant beforehand. But, many faults are transient. Perhaps that tree branch finished falling, and is no longer against the wire. So the protective relay performs a "reclose" operation, where it turns the power back on for a moment and measures the current. There's normally a huge starting inrush as motor-driven appliances restart, so it waits a moment before sampling the current, and this is the period when the power comes back on for a few seconds. (It's probably pretty nasty power during this interval, too.) If things are good, hey, you're good! A brief outage while waiting for the fault to clear itself, and then everything's back. The operation will be reported to the dispatch center, and someone may come out to inspect the area in case there's damage. But if the fault is still there, and after a moment the current is still way higher than it should be, the recloser opens again, and this time it stays open. Gonna need a crew to move that tree, and then they'll manually try again.

zzz61831 6y ago

You can put your equipment on a voltage relay to make failure modes as simple as flipping a circuit breaker. It will turn off once voltage drops too much and will wait for it to stabilize for specified amount of time to turn it on.

bradstewart 6y ago

It depends. We built and deployed some IoT devices (controlling and reporting status of lights, HVAC systems, that sort of thing) to a bunch of greenhouses--one of which ran on generator power. The customer was running the generators right at the maximum capacity, and we'd see really, really odd things on the power lines. Some times a surge like you mention, sometimes changes to the frequency, sometimes changes to the voltage. We discovered all sorts of weird brownout conditions in our devices.

jcrawfordor 6y ago

It's difficult in practice to transfer load to generators for test especially if there's load shedding involved as is common in larger facilities (higher-power-consumption items like part of air conditioning capacity are cut off when transferred to generators). To help address this a lot of more critical facilities are equipped with a test load (really just a very large outdoor resistor with active cooling) that allows for running the generators at rated load for test and exercise without actually transferring. Of course, these tests do not always detect problems with the transfer equipment, and if things aren't designed and maintained well the generator may experience load under real conditions that behaves differently from the test load.

avh02 6y ago

Iirc (top of my head), some datacenters go off the grid regularly at peak (provider) load times and get more favorable rates for doing it. I guess it ensures your fuel doesn't go stale and all your transfer systems work.

bluGill 6y ago
Good point. Many power companies will give you a nice discount year round if you will disconnect from mains on demand. If you are on the fence about a good backup generator this will more than make up the difference.

gerdesj 6y ago

If done right it is routine. In our small data centre we have a board with five lights and a key. You turn the key and the mains is cut off. The lights give the status. The UPSs beep for a few seconds "on battery", the genny fires up in the boiler room and when it stabilises, it takes the load. We leave it running for 5-15 minutes, go out and check the fuel levels and top up accordingly. It's diesel so safe to hot refuel. When done, turn the key back and then the UPSs take the load again and then hand back to the mains. The genny runs for another 15 minutes and then shuts down. That should avoid flip flopping. You then fill in the test form on our wiki and job done. We do it weekly to fortnightly. An ESP8266 based thingie reports status back to the central monitoring (GPIOs and ESPHome -> Home Assistant)