UPS Backup Daisy chain fire hazard: Don't suffer fools!

[Gallymimus]

Guys,

Idiots kill me.  This guy on slickdeals went on to explain why plugging two UPS backups into each other in a daisy chain would "melt the wires in your home"

I can't take it.  It kills me.

https://slickdeals.net/f/10160548-cyberpower-10-outlet-850va-510w-ups-sinewave-pfc-mini-tower-90-free-shipping?p=100084240#post100084240

[MagicSmoker]

What is meant by daisy-chaining here? Surely not plugging the AC input of one UPS into the AC output of another - that's what is normally understood by daisy-chaining, but it doesn't make a whole lot of sense in this case...

[Gallymimus]

hahaha yes, that's exactly what it means.

It's a silly thing to do, but you do get, for the most part, the run time of both UPS units if you do that, but it certainly won't blow up the world.

[hamster_nz]

I think the premise is more along the "don't daisy chain power strips" line of thought. Plugging an X amp power strip into an X amp power strip does not let you draw 2*X amps from the wall socket, and may damage your building's wiring. Just for completeness....


However, interconnecting these devices is a violation of Occupational Safety and Health Administration (OSHA) regulations and the National Electrical Code because doing so can cause them to become overloaded, leading to their failure and a possible fire


http://www.roe26.net/testing/documents/fastfacts_daisychains.pdf

(from www.compliance.gov, who else!)

Edit: That document is priceless... well worth a giggle. eg:

Extension cords are sometimes used to energize power strips in locations far from outlets. Because electrical resistance increases with increased power cord length, interconnecting cords increases the total
resistance and resultant heat generation. This creates an additional risk of equipment failure and fire, particularly when paper and other combustible materials are in contact with the wires. Additionally, OSHA’s
regulations allow extension cords to be used only as temporary wiring for up to 90 days.

(i.e. you can have a fire hazard for 89 days, if you want...)

[Kleinstein]

One danger with daisy chaining extension cords, USPS or similar is that this will increase the series resistance. With a lot of series resistance, in case of a short there will not be enough current to trigger the fast (magnetic) part of the "fuse" protecting the outlet. So there is a small chance of starting a fire this way. With USPS this might not be so bad, as they should have additional fuses, that are usually smaller (e g. 6 or 10 A) than the one normally used on outlets.

The 90 Days rule is somewhat funny, but it is a way to quantify temporary. It is more like they expect you to remember the potential risk for 90 days.

[Red Squirrel]

Even with extension cords you can daisy chain as many power bars and splitters as you want - as long as they are rated for what they are carrying.  you might have 30 outlets available with tons of power strips daisy chained but it does not mean you can plug 30 space heaters, but you can plug 30 wall warts that are like a few watts each.

As for UPSes I'd say the danger of daisy chaining is that when the first UPS goes on battery and produces a square wave, the next UPS may not like the square wave and it will cause rapid relay switching back and forth.  Some UPSes are smarter in that regard and will stay on battery for a few seconds after a fault cleared, but some will be more aggressive to go back on AC.

If they are pure sine wave then it would not really be an issue.

Now if you plug a UPS into itself and add a batterizer to the battery, you essentially have unlimited portable power.

[Gallymimus]

One danger with daisy chaining extension cords, USPS or similar is that this will increase the series resistance. With a lot of series resistance, in case of a short there will not be enough current to trigger the fast (magnetic) part of the "fuse" protecting the outlet. So there is a small chance of starting a fire this way. With USPS this might not be so bad, as they should have additional fuses, that are usually smaller (e g. 6 or 10 A) than the one normally used on outlets.

The 90 Days rule is somewhat funny, but it is a way to quantify temporary. It is more like they expect you to remember the potential risk for 90 days.

I am quite curious how you think this can occur.  Certainly if you have a 20A circuit and run 21A through it a thermal/magnetic breaker may take some time to trip if at all, but that is why the wiring and outlets are suitable for far more than they are rated.  Can you paint a scenario where you are drawing maximum current, have a huge chain of power strips, UPS or whatever, and somehow create a fire in the wall?