PoE for Improved Smoke Alarm Project

[waoj]

How does PoE isolate grounding?  Can that isolation be trusted, and to what limit?

Backstory:
We had to take down all of our smoke alarms, because one of our dogs absolutely freaks out and urinates and defecates everywhere any time there is any beeping.  including the battery-getting-low beeps.

We could alternately replace all batteries indiscriminately every 3 months to prevent it.  But I'm not able to waste that much money or time on so many batteries.  Some are high enough there is a significant risk of injury to access them.  We also have a mildly disabled senior living with us who would be severely injured if she fell off a chair so there must not ever be any reason for her to get up to change a battery without us.

I need alarms that use AC AND battery, so that if the power goes out as part of a fire the alarm still works, and also if the battery fails or runs out, the alarm also still works.

Idealy in my mind, there would be a mechanism for the alarm to charge its battery back up to 100% when it reaches 50%, and only beep if that charge fails and it gets down to 30% or something.  But yet I've seen no such product!  Why?

I'd also love something that omited the battery beep entirely in lieu of an email or a z-wave trigger.  Or perhaps a pre-emptive silent alarm, before the battery is low enough to cause beeping.

I have considered a commercial fire detection system, but this is a home.  I can't install something that doesn't look home-like.

So, in order to have fire protection again this is what I've come up with:
9v usbc rechargable batteries
PoE to USB converter
PoE switch scripted to keeep the ports down most of the time, but energise once a month for 4 hours, to top up the batteries.

Before putting it in the ceiling I was testing and found there is continuity between the neutral AC wire, and the USB ground.  And that got me thinking....

I don't want to tie grounds and neutrals together  So I tested from usb ground to ethernet pins (with ethernet not connected) and there was NO continuity. 

That implies that the PoE adapter is transmitting energy from rj45 to USB without a shared ground/neutral path.  (Or its only shared when live)

I need to break out the ethernet to test it while its powered I guess....  But How does this isolation (generally) work.  and does the switch (typically) do the same sort of isolation? 

Would it be able to isolate 120vac if for instance someone miswired a smoke alarm, bringing the USB shield to 120vac?  I'm not worried about any higher voltage than 120/240 since I'm in North America.

If PoE is ground-isolated on BOTH sides, then I guess I'm comfortable with it.  (How would I verify that through non destructive testing?)

I'm still slightly more inclined to do away with the AC input entirely and just use battery alarms but code may require hardwired alarms here.  And that still doesn't help me with the scenario of a failed battery...  Batteries are chemical so perhaps the heat of a fire could fail the battery before the smoke set off the sensor just thinking....

Hard wired is also necessary to interconnect them in a reliable way.

Admittedly, fire code is probably out the window anyway the second I think of doing/modifying anything about the smoke detector...  But I'm an adult, and free, for now, to take my own risks.

I'm also not interested in putting the dog down or doping it up to ignore the beeping.

How can I better understand PoE isolation?
and
What else could I do to improve this plan?
How would PoE isolation prevent a fault if for example you power a device in another building and then plug in an external powered hard drive to that device and the buildings had different ground potential for whatever reason?

[ejeffrey]

Just get a smoke alarm with a 10 year sealed battery.  At least in the US these are pretty much standard now. 

[waoj]

I considered that but:
No interconnect.
No redundant power.
I would need 8 of them.  I don't particularly like throwing working things away even every ten years.
They're still going to beep when they get low though the dog will be dead by then probably.

I do already have an enterprise PoE switch.

[ejeffrey]

If you already have AC available where the smoke detectors are and really want to use it, you can use a hard wired smoke alarm, and then replace the backup batteries with lithium primary cells which should also last 10 years in this application.  But I wouldn't mess with any "hacked together" power system using PoE adapters.  The pure battery powered ones are fine.  If heat from a fire causes the battery to fail, the smoke detector is already dead and well past it's chance to warn anybody.

[ejeffrey]

It's recommended to replace smoke detectors every 10 years anyway.  There are plenty of battery powered smoke detectors with interconnect.  There is no need for redundant power with a 10 year battery.

Yes it will beep after 10 years.  Mark on your calendar to replace it in 9 years.

[ejeffrey]

If you are worried enough about safety to be worried about the reliability of the sealed lifetime battery smoke detectors definitely don't hack something together with PoE!

[waoj]

Do you have more failed batteries or failed Poe devices?

[ejeffrey]

A smoke detector with a sealed battery will have far fewer failures within 10 years than anything you cobble together as you have suggested.  Probably 1000x lower failure rate if not more.  It's not just "PoE devices":  it's the PoE switch (those have a relatively high failure rate) the rechargable battery (few have 10 year lifetime) the PoE tap, the recharging circuit and your ability to wire it all together I  a way that won't fail.

Commercial smoke detectors are _incredibly_ reliable. 

Also while you are at it, make sure to get photoelectric smoke detectors rather than ionization detectors.

[NiHaoMike]

You could use NiMH batteries and wire a resistor across the blocking diode in the detector in order to counteract the self discharge. Not strictly compliant with not modifying the smoke detector at all, but it's a simple mod with far less to go wrong.

There are smoke detectors designed to connect into home alarm systems, maybe look into those? That brings the problem of having to add an alarm panel, which often aren't DIY friendly to install.

[waoj]

I had NiMH batteries before I bought the lithium usbc ones actually.

I considered running small conductors out to each smoke detector to remotely monitor the battery level, and recharge it in situ.  but it occurred to me those wires during a fire could melt and short together disabling the detector.

I wish I could view battery status remotely.  but its more important that the unit can function autonomously when everything around it has failed.

[ajb]

You can get

I considered that but:
No interconnect.
No redundant power.
I would need 8 of them.  I don't particularly like throwing working things away even every ten years.

You can absolutely get 10yr smoke detectors with AC+interconnect.  Although there's not much benefit to AC power with 10yr sealed batteries aside from the interconnect -- as already mentioned, you should be replacing your smoke detectors every ten years anyway, as there are other components besides the battery that will age out.  They may still "work" after 10yrs, but the risk of failure will increase in ways that are hard to predict. 

Also while you are at it, make sure to get photoelectric smoke detectors rather than ionization detectors.

You actually want both -- ionization detectors are better at detecting fast, flaming fires, while photoelectric detectors are better at slow, smouldering fires.  You can get dual detectors, or use a combination of both types -- but ionization detectors are best kept away from kitchens to avoid false alarms. 

Admittedly, fire code is probably out the window anyway the second I think of doing/modifying anything about the smoke detector...  But I'm an adult, and free, for now, to take my own risks.

Yes, modifying a fire safety device is no way to endear yourself to the local code authorities.  Your insurance company might like the idea though, since it'll save them from having to pay your claim if there's a fire. 

[ejeffrey]

For instance this: https://www.amazon.com/Kidde-Hardwired-Detector-Interconnectable-Indicators/dp/B0DDW2K42G

Two packs of four and problem solved.

A mix of photoelectric and ionization is indeed the most comprehensive but if you are going to pick one the photoelectric is much more important.  It will detect most house fires faster and be less prone to false alarms. In the cases where the ionization detector is faster it seems like the difference is usually much smaller   But yeah if you want the best possible use both.

[themadhippy]

but ionization detectors are best kept away from kitchens to avoid false alarms.

Best to avoid any type of smoke detector in the kitchen,unless you want everyone to know you've burnt the toast,a heat  detector is a much better option.

[floobydust]

I think better (no false smoke detection) for a kitchen is UV flame detection (electric stove not natural gas I think?).  The Hamamatsu UVTRON sensors can detect a candle flame many metres away. I haven't seen these make it to residential use though.

I did design my own smoke alarm interconnect to alarm panel tie-in circuit, because what is commercially available is only a mains-powered relay module thus does not work if there is a power outage. The relay-based module is low-tech and uses a capacitive dropper that runs hot and of course the deal-breaker is it needs mains.
The circuit must not in any way endanger the smoke alarm's operation (interconnect), it should of course be UL tested. But it is possible to safely monitor the interconnect line.

Operating costs pissing around with 9V batteries in smoke alarms is such a hassle.
Smoke alarms are shit quality, always check the many product recalls especially for the "10-year" battery ones that croak and die.
https://www.cpsc.gov/search?search_api_fulltext=smoke+alarm

I see 12/24VDC smoke alarms that could be powered from an alarm panel and these might be cheaper to use in the long run, say 5-7 years compared to 9V backup systems.
If you have POE you have a UPS I would think.