UPS and RCD protection question

[peter_4059]

Here's a question for the electricians out there - If I use a UPS system to run my scope I assume I'm exposing myself to a risk down stream of the UPS due to no earth leakage protection? Is it possible to use a plug in RCD switch on the outlet of a UPS device?

[acropolite]

Peter, I expect the output of the UPS is isolated from the source so any protection before the UPS probably won't work.
I cant see any reason why an RCD wouldn't work on the UPS output side, the RCD is only looking for a difference in current on either the active or neutral leg of the 240v supply. The only sure way to find out is to test it.

Edit. Just tested with my UPS and plug in RCD, Runs no problem, the test button trips the RCD with no ill effects to the UPS.
At least on my UPS, the second RCD neither stops the UPS nor trips the upstream switchboard RCD.

[peter_4059]

Thanks Phil. I'll get myself a plug in RCD and give it a try.

I think there are quite a few of us turning to UPS devices after experiencing problems with the power tripping. I assume a UPS could give you quite a kick and given the often dewy conditions I'd hate to not have earth leakage protection.

[acropolite]

Lethal, just as much as ordinary mains, it only takes a few milliamps to kill you, any ups is capable of delivering a fatal blow.

[DavidTrap]

Which is why I've set up my astro gear to run off 12V. I think it's only a matter of time before we hear of someone getting a shock from astrogear covered in dew. Most of our gear is rated for "indoor use only"!

DT

[midnight]

Peter,

It's a little technical but there are 3 main possibilities

1) Output is isolated with no earth. No output RCD will trip. (very cheap portable type inverters I have seen - like a double insulated device)
2) Output is isolated and an earth is provided which links back to the main input earth. No output RCD will trip (really no better than 1)
3) Output is isolated and an earth is provided which links back to the main input earth AND the output of the UPS neutral is bonded to the earth (ie an MEN link). Output RCD will trip

If your UPS has an output RCD (eg plug in type) and it trips, then this means you almost certainly have (3) above. But to be 100% sure, the earth on the output needs to be bonded to the input earth.

Why. Well the RCD is tested by diverting a current to the earth most likely in the cable. If this goes back to the inverter without being bonded to the input, then this is no good. You are most likely to get zapped through common earth or some other conductive path other than the earth in the cable. The return current can only come back via the input's 240V cable from your switchboard to complete the cct. Therefore it is important the earth on the output is bonded the input's earth. Unplug the UPS from the 240V input supply, ensure it is OFF and test this with a multimeter from the input earth terminal to the output earth and ensure it is continuous.

Hope this helps. If in doubt, engage an electrician always. I have an APC inverter and this is wired like (3) above with the input earth bonded to the battery (-) and output earth and a RCD works satisfactorily on the output without affecting the main RCD in the switchboard.

Darrin...

[alistairsam]

Hi,

Given the potential risks to running a UPS without an RCD on the output, is there any standard for UPS manufacturers to provide adequate protection?

I work in IT and use UPS's all the time, but wasn't really aware of this.
I'm surprised manufacturers like APC wouldn't have thought of this or included this in their products?

Is there a risk to running any equipment on UPS when not plugged into mains indoors, and does it get worse if run outdoors?

is an RCD similar to an ELCB?

At astrocamps, we often use powerboards in the field and that is exposed. is that considered risky?

[DavidTrap]

My concern is most of our astrogear, computers and power boards will be stamped with "Indoor Use Only". It's not uncommon for gear to be drenched in dew in our humid climate. Given my preferred dark sky site is off grid, I have little choice but to be running on 12V. Nonetheless, I don't mind running that way at home.

I've been zapped with 240V once through my own stupidity (not related to astro-gear) - doing my utmost to avoid that happening again in the future.

DT

[Poita]

I've only used APC brand UPS units and they are all tied to earth and RCD works with it.

[peter_4059]

It has got to be more risky outdoors - especially if dew is forming on equipment. UPS devices are becoming popular at astrocamps and I think a lot of people believe they are protected by the RCD's that the camp provides in the power distribution however this is probably not the case.

I believe RCD and ELCB are different names for the same device.

[acropolite]

Correct, ELCBR was the original name for the device, stood for earth leakage core balance relay, then ELCB earth leakage circuit breaker.

[123MB]

Hi Guys

Everyone please be careful with these installations.

Pressing the test button does not mean the RCD is going to trip on an earth fault. All the test button does is induce an imbalance in the trip coil. You need to test the RCD at the socket outlet(s) with an appropriate tester.

[123MB]

AS4763 - Safety of portable inverters
AS3001 - Electrical installations in portable structures
AS3000 - Wiring rules

[123MB]

There is nothing wrong with a fully isolated inverter and it is an acceptable source of isolated supply to AS3000 and AS3001. In many cases it can be safer than a bonded inverter or someone installing a neutral-earth link (as often happens).

These issues need to be considered on a case by case basis and electricians shouldn't be scared to put their hand up if they are not confident with application of AS3001

[ZeroID]

Indoors an RCD is a liabilty on a UPS output. The UPS is to provide power in the event of failure of supply so the system (server\pc etc) keep going long enough for a graceful shutdown with minimal damage or corruption. If the RCD trips which can happen as the UPS takes load then all is lost. Not so important in astro setup obviously especially if you are using laptops anyway.

In a standalone situation where the UPS is not connected to the mains but is providing power to mount\pc whatever then there is no earth return involved. The only chance of shock is back to the UPS itself even if it is still plugged to mains but the supply switch is turned off. Its a closed circuit in that set up. RCD performance may be unreliable as it has no earth reference point to detect imbalance.

My wife was the APC person in NZ for 5-6 years, now working for another UPS company. We run 5 at home including a 3kva double online conversion to the sound\entertainment system

I will run my Ob on a modified UPS soon, derive 12 volts direct from the batteries and everything else from the 240vac output.