Backup Power - An Electrocution Risk?

[Ironic_Water]

Tanks are, obviously, a potentially wet environment. For wet environments, it's important to have GFCI outlets/breakers to protect against shocks on the circuit your tank is on.

However, I was thinking - a battery backup or UPS blocks the ability for the GFCI breaker to protect you.

Say you have a spill that causes a short. The GFCI detects this and shuts down power to the circuit. If you have a UPS on that circuit for your tank, that UPS would think there's been a power outage, and supply power with its battery... meaning you could potentially still be dead despite the GFCI having done its job.

So, are UPS something we should even have? I have one, and it's been great during power outages... but is it worth the risk?

 

[Jamie814]

So, are UPS something we should even have? I have one, and it's been great during power outages... but is it worth the risk?

Yes, still a valuable tool for power outages. A GFCI cord after the UPS will solve this issue. Power cords with them built in are readily available.

Something like this,
GFCI Extension Cord

 

[fryman]

Since almost all our equipment is DC nowadays, battery backup is best handled DC-DC. If you use a traditional UPS, you are converting DC from the battery to AC, then using an AC-DC convertor to power the equipment. Very inefficient, and also more expensive. I recommend using a LiFePO4 battery and DC-DC converters.

A well-designed battery system should have fuses installed.

I use a fuse at the battery positive terminal like this: BOJACK MRBF 50A, 58V DC Terminal

and for each positive wire, sized appropriately something like this:
Anyongora 16AWG Inline Fuse

 

[MaPePo]

So, are UPS something we should even have? I have one, and it's been great during power outages... but is it worth the risk?

Yes, still a valuable tool for power outages. A GFCI cord after the UPS will solve this issue. Power cords with them built in are readily available.

Something like this,
GFCI Extension Cord

After as in Wall GFCI -> UPS -> GFCI Extension cord -> tank controller?

 

[BeanAnimal]

Yes, still a valuable tool for power outages. A GFCI cord after the UPS will solve this issue. Power cords with them built in are readily available.

Something like this,
GFCI Extension Cord

Actually it will not. The UPS is a floating neutral. So in theory the hot wire has no reference to earth ground. You can still get shocked between hot and neutral coming from the UPS, but you are the load, and even if you are grounded. The GFCI will not read the fault.

 

[BeanAnimal]

After as in Wall GFCI -> UPS -> GFCI Extension cord -> tank controller?

As noted above, due to the floating neutral, the GFCI will NOT work as expected on the output side of a typical home or small business UPS that is running on battery. That said, if the fault occurs while running on mains, then the GFCI dow stream of the UPS will function and trip.

 

[Pistondog]

Actually it will not. The UPS is a floating neutral. So in theory the hot wire has no reference to earth ground. You can still get shocked between hot and neutral coming from the UPS, but you are the load, and even if you are grounded. The GFCI will not read the fault.

I think a gfci works by comparing current flow thru hot and neutral, if these dont match by some milliamps, it trips. Gfci should work on floating neutral and even ungrounded systems.

 

[BeanAnimal]

I think a gfci works by comparing current flow thru hot and neutral, if these dont match by some milliamps, it trips. Gfci should work on floating neutral and even ungrounded systems.

Yes, the GFCI works by comparing current on the hot and neutral conductors. The issue is that neutral is not referenced to ground in a floating neutral system.

In a bonded system (neutral to ground) the GFCI protects you any time you are grounded and receive a shock from a GFCI protected circuit, as the current "leaks" or "faults" to ground, even if that shock is from hot -> neutral contact (you are the load). Of course if you make contact with the hot and neutral and are NOT grounded, you get shocked because you are the load and there is no fault.

In the floating system a fault that puts you between hot and neutral of the UPS is not protected by the GFCI, even if YOU are grounded.

On the other hand (don't try this) the hot from the UPS will not shock you if you get between it and ground because it is not referenced to ground.

The bottom line, you can't count on the GFCI to work as intended on a floating neutral system. I hope that makes sense.