I recently got an old APC BK300 UPS (Input:120VAC & Output:120VAC 300W) along with many other vintage electronics stuffs. So I tried opening the the UPS completely.I noticed that the battery a 12V 7Ah BB battery SLA was completely dead. Well I kinda added som edistilled water and revived it and it seems to be working fine now after a day of charging with a high power automotive charger and put it back on the UPS after resealing the SLA. I removed the UPS buzzer since it was kinda annoying. Connected the SLA and powered up the UPS and connected a 130V 100W bulb and it was glowing bright.I usually do some power electronics stuffs from mains and having been looking to replace the mains as source with an inverter or UPS(always feared getting electrocuted). So does a UPS has enough juice to kill me like power from mains 110V outlet? Also there are two transformers inside the UPS a really big one and a small one on the circuit board.Which one is the inverter transformer? I've posted some pics below. IMG 1




  • \$\begingroup\$ I guess, enough to kill not just one person. \$\endgroup\$ Apr 6, 2017 at 18:08
  • 1
    \$\begingroup\$ The UPS puts out the same thing the outlet does. You can be electrocuted by the outlet, so you can be electrocutedby the UPS. \$\endgroup\$
    – JRE
    Apr 6, 2017 at 18:11
  • \$\begingroup\$ A few mA can kill you. This will definitely kill you if you're not careful and take basic precautions. \$\endgroup\$
    – Envidia
    Apr 6, 2017 at 18:12
  • \$\begingroup\$ estimates are ~5ma through the heart to kill you, that's a mere 0.6W.. so...... \$\endgroup\$
    – Trevor_G
    Apr 6, 2017 at 18:21
  • \$\begingroup\$ Which makes me wonder... do standard UPS units include ELCBs in their outputs? I do see a site wiring fault indicator there but that may not be the same thing. \$\endgroup\$
    – Trevor_G
    Apr 6, 2017 at 18:22

2 Answers 2


SHORT ANSWER: YES! Also be careful handling the circuit board even when its not plugged in. The capacitors can shock you!

I would bet that the small transformer is there for the small electronics that control the annoying speaker you removed (step down transformer) and the big one is simply for safety and is for the inverter. (Isolation transformer)

LONGER ANSWER: 120 VAC from any wall socket alone will shock you and can kill you. Although there is the debate of "volt vs amps" being more lethal, its known that amperages between .1 and .2 amps (100 or 200 mili-amps) will kill you. That UPS is can put out a whole 2.5 amps which is roughly 100 times more than the lethal amount

Formula: power = (voltage * amperage)...

so solving for amperage is...

Amperage =( 300 watts / 120volts ) which is 2.5 amps

The big tansformer is probably an isolation transformer. Its there to keep you from drawing more than 2.5 amps in case of a short. the little one is there to get the voltage down to something suitable for a voltage regulator to manage for the logic circuits that control the UPS. (5 volts maybe)

  • \$\begingroup\$ I agree with you mate. If it can light a 100W bulb then its got the juice but the only reason I asked this noob question was because this was from a battery source. The 110V mains usually gives in 10-15Amps easily. \$\endgroup\$
    – RSB
    Apr 6, 2017 at 19:02

Killing you is not that hard. You don't have to be burned to charcoal state for that. Enough stopping your heart by 60Hz voltage.


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