My DB board

When I had my inverter installed I decided to put in a whole new DB board. In this post I'll talk about why and how I wired it up.

The main reason for redoing the DB board is that I wanted to be able to switch individual circuits on and off of the inverter.

An easier solution would be to get an automatic changeover switch box and switch the entire supply to the DB board, but I wanted more fine-grained control:

I have a pumped solar geyser which needs a bit of power to pump the water through the collector, so it makes sense to run this off the inverter in summer. Obviously I don't want the geyser element running off the inverter though so in winter it will stay off.

We also have a gas stove with an electric ignitor, so it's nice to be able to turn it on easily.

It gets pretty hot where I live so the aircons can also be run off the inverter. We have a Samsung inverter aircon in the bedroom which only draws a few hundred watts in economy mode, so it's no problem running off battery.

So this is my db board circuit, it's reasonably straight forward. The idea is there are two completely isolated supplies with changeover switches that select where each subcircuit is powered from. It's very important that the inverter and utility supplies can never be connected to eachother.

I have indicator lights on the supplies because I read somewhere that that's a legal requirement on dual supply systems, and it's nice to know when each supply is active.

I then ordered all the stuff, basically copying the breaker ratings from the old board. My old db board used minirail circuit breakers, but they seem to be a South African thing, so you don't get changeover switches in that size, so I got DIN breakers instead.

Can you follow what's going on?

I mounted the new DB board on hinges over the recess from the old board so the wires behind it can be accessed and worked on if necessary.

I then printed out labels, which turned out to be wrong (or at least I need to swap the wires on the lights and plugs changeover). I should get round to printing another label...

As you can see I'm in loadshedding now- only the red light is on... Time to make some coffee :)

Installation

Here's a rundown of the parts involved:

Inverter (Inverter, AC charger, MPPT solar charger): R13000

Batteries (4x100Ah): R6000

Battery cabinet: R1000
- http://www.bidorbuy.co.za/item/178019194/battery_cabinet_105ahx4.html

Battery cabling and lugs: R500
Make sure to get the lugs crimped properly. Don't forget you'll need cables between each battery. Make sure you get the right type of lug for your battery, I'm using 6mm ring lugs, but some batteries use post terminals (eg a car battery)

DB Board, MCBs, Changeover switches, wiring: R3000
I'll try write another blog post about the DB board, but if in doubt get an electrician to do this.

Electrician to wire in DB: R2500
I got wildly varying quotes. This is the cheapest for installation and certificate of compliance.

Total R26K

I bought the stuff and wired up the DB board myself to save a few bucks. It's quite a tedious job.
I just got an electrician in to pull out the old DB and wire in the new one.

*This is an old photo, the breakers on the bottom left are place-holders for a changeover switch

Online vs Offline UPS

There are two different types of UPSes: online mode and offline mode.

Offline mode is the simplest and cheapest: it has a relay switch that supplies your appliances directly from Eskom utility power while it's available, then switches over to battery+inverter power when utility power fails.

The process of switching over takes a bit of time, only a fraction of a second, but in this time your stuff will have a brief power cut. Usually it's not a big deal but it could cause your computer to reset, your microwave timer to reset etc.

A fast changeover could also cause a surge: appliances with motors in them typically store a bit of power as a magnetic field, and release that power later. 

AC power is delivered in waves, where power cycles back and forth, but normally a cheap inverter and the utility won't be synchronised. If your washing machine is spinning, the current it generates once power is cut could be added on top of the power the inverter generates as it kicks in and cause a surge. Newer appliances are protected against this sort of thing though. I'd only worry if I had an ancient old fridge or washing machine.

A safer, more expensive option is an online UPS where the appliances are always run off the inverter, and the inverter takes its power from the utility when available, or from battery when its not. This means power is never interrupted, and everything stays nice and smooth.

Offline UPSes are cheaper because they only need a small charger circuit. They usually take quite a long time to recharge after a power failure.

Higher end inverter units will usually let you run in either mode. The tradeoff with running in online mode is that it uses more standby power because all the power you use needs to be converted from AC to DC back to AC again. My cheapo Axpert aka MPPSolar unit uses about 50W in online mode, which is about R40 a month's worth. Decent inverters might only draw 10 to 20W.

I tend to leave my inverter completely disconnected because I know when the load shedding is coming anyway. It's important to keep the batteries topped up regularly though.