Mod note: Posters Location is Switzerland.

Hello everyone,

First of all, thank you for your time ! Please forgive me if my explanations are too bulky, I'm doing my best to make things simple.

I recently bought pieces here and there to make my own (online) UPS. As any noise would make my neighbors go crazy, the only way to go was with a battery/inverter combo that is used as backup power (meaning when utility power goes down, the UPS takes over). I plan to have a very small load, roughly 250W (two computers and some networking equipment) but safety is not a feature, and since there is switching going on I fear my knowledge isn't enough to properly set up the whole thing.

My main questions are:
- Is there any need for a neutral-ground bond ? When I asked the inverter's manufacturer, they told me there is no need. To be honest, I don't really understand its purpose even though I googled quite some pages. Also, thanks to the Schuko plug design, I can't seem to find which hole is neutral or hot.
- Considering the low amperage that will be going through the relay (slightly above 1A AC), is there any risk of arcing ? If yes, since both AC inputs share the same relay, is there any risk of "bridging" between the two when arcing occurs ?
- Is it safe to have the load always directly grounded to a wall outlet AND the inverter itself also directly grounded to a wall outlet, but therefore with no ground link between load and the inverter ? See the second picture below, you will better understand.

Here you will find an overview of the setup without the switching system: setup w/o ATS

It's pretty straight forward, seems safe to me. However, if the battery or the inverter were to fail, then power would be lost. That's why I opted for a DPDT relay to address this issue: setup w/ ATS

In this configuration, power is drawn from utility and ground is standard. The UPS is in "standby mode". Once power goes down, the relay switches to the UPS thus keeping computers on but the grounding is kept standard, meaning in case of surge the inverter is never "used" for grounding. It still has short-circuit and overcurrent protection thanks to the inline breaker.

To make it easier for you to help me out, here is a datasheet list of some components I use:
UPS: RIPEnergy Swing PRO 350
Relay: REL-IR/L-230AC/2X21
Battery charger: Generic Lead-Acid battery charger
Battery: Panasonic 12V 38Ah AGM battery

So the goal here is to see whether setup #2 is safe to operate, and if not how I could fix it. Also, if my main questions are relevant regarding setup #2.

Thanks in advance !

Regards,

SwissGuy.

 

I don't understand the need to build a UPS for networking equipment. Why would a store bought unit make noise that could be heard by your neighbors?

 

I run a few servers that need to be always on, so there you go.

Regarding a retail product, it would certainly cause much less hassle but for the same price (~1K CHF) I get over 1hr of backup power whereas any online UPS would've lasted only around 15min. Also, I like building my own solutions when possible. I learn a lot.

 

I would be very much surprised if the mechanical relay could switch fast enough to avoid shutting down the computer. That alone would keep the second option from being viable.

The 200 ampere fuse you show on the positive battery wire is useless. The inverter only draws a maximum of 32 amperes at 12 volts (nominal output) or 15.5 amperes at 24 volts. I would go no larger than a 40 ampere slow-blow fuse and even then it is the internal protection circuitry that will protect the inverter.

Earth grounding is of no consequence because this would be a "separately derived system" with no reference to the earth UNLESS you were to earth ground one of the two output conductors...something I would not advise.

I would prefer to use the two batteries in series with an inverter that accepts a 24 volt input. Higher voltage means less voltage drop.

 

That's all well & good but my question was about the noise that you mentioned. What noise?
Do you offer free shell accounts on any of your servers?

 

@Pulpo: forgot to add: when I said noise, I was referring to a generator-based UPS (ex. Diesel).
What do you mean by shell account ? A virtual server ?

@Furd: as per ATX specifications, the minimum hold-up time for a compliant power supply is 16ms. If I add up the response and release time of the relay I chose, I get 22ms which in return leaves an 8ms margin. Most PSU's fall in the 20-40ms range so no worries about that. There's also another relay with 8+3=11ms total transient time which could be safer to use: Finder 56.32.8.230.4040.

Thought the same about the fuse, seemed oversized to me. As it was included with the battery cord, I didn't mind changing it but it would indeed be a good idea. Unfortunately, the inverter only accepts 12V input so I can't really add up another battery in series.

When you say "to earth ground one of the two output conductors" do you mean making a neutral-ground bond ?

 

Yes. Commercial (utility) power systems have one side of the line grounded to earth for the purpose of limiting the high voltage under fault conditions AND to facilitate the opening of overcurrent protective devices (fuses or circuit breakers) when a ground fault exists. By using the first incarnation of your proposed project (full time battery charger>battery>inverter>computer) you have effectively isolated the power source (the inverter output) from the utility line. This is called a separately derived system in the US. Since the separately derived system is all on/in a single premises there is no advantage or reason to ground one side of the line. Some inverters will not allow grounding of the output, you would have to check on your specific inverter to know for certain.

 

Swiss Guy, A shell account is a Unix shell which is command line only. I really didn't expect you to be offering that but I took a chance anyway.

When you finish building the UPS are you going to test it by shutting the breaker to see if it works?

 

I see, thanks for making that clear.

So as you say, the first version of the setup is indeed fully isolated from utility line, thus rendering the earth pointless. But what if the inverter failed and sent HV spikes instead of 220V ? Wouldn't a ground connection we useful in that case ?

When I refer to the user manual (here), on page 31 section 48.3, if I understand correctly the case must be grounded for earth "functionality" to be present on the AC output side.

 

@Pulpo: I know what a shell account is, but it requires a computer/session to "run on". Dispatching accounts for a physical machine wouldn't be wise (in my configuration), as there would be no isolation from the internal machine and network. That is, you would have access to the actual computer and could, for example, scan ports of a given machine. However, if I fire up a VS connected to a VLAN and give you an access to it, you are effectively isolated from sensitive parts of the network (like the router or other computers that are present).

I would use rssh anyway, but if I am to host services for people other than myself, I would prefer having a dedicated VLAN and VS's for security purposes.

Regarding the UPS, I will indeed test all possible configurations to make sure there's no way power goes down. It would be imprudent not doing it.

 

Would I do that?
_____________

 

@Pulpo: You're certainly a nice and honest person, but as any other IT service provider would do I'd rather have a VLAN for the customers or nothing at all.

 

SwissGuy, you are 100% correct. A VLAN would be the way to go. I was just playing. Don't give anyone remote access.

 

@Pulpo: I knew you were hiding something, heh.

Anyway, I've ordered the parts I need and after having tested the whole thing, I'll post an update.