Crossed Neutrals: The Hidden Cost of a Battery Upgrade

At least 11 circuits share 8 available positions in the brass neutral bar

Solar and in particular battery installation often dictates you must have a switchboard update.

Can this expose otherwise hidden issues in your house? Not always, but as night follows day, one thing can lead to another.

Safety switches save lives. As much as we might curse at them for flicking off the lights from time to time, better that than stopping your heart.

However the other heart stopper you might encounter is the bill you get, after a day of electrical fault finding which follows a switchboard rebuild.

The problems we’re explaining in this article arise when electrical upgrades expose flaws in your house wiring that weren’t evident in simpler times.

If you remember when talkback AM radio was king (before home filter taps were a thing), the adverts instead featured electrical companies spruiking capital-S Safety. The equipment was expensive and there was a market for retrofits on houses that didn’t have any. It was a pretty simple job to add a single safety switch which covered the entire* house.

Except The Lights*

For years electricians would leave the lights out of the safety switch circuit, and it was legal to do so. The reasoning was simple enough, lights were usually out of reach¹ but because they’re fitted to the ceiling or eaves, they’re prone to getting wet.

It doesn’t take much to trip an RCD, so if your outdoor light gets damp in a rainstorm, you don’t want the entire house to go dark. Worse still you don’t want a pool of possum piss making your downlights damp… and the freezer thawing out because the safety switch won’t reset.

Batteries Dictate Updates

The last update of the AS/NZS3000 wiring rules changed the type of safety switches we have to use. Any modification to the supply means downstream circuits must be made compliant, so whatever wiring is connected to your new battery supply needs to have the latest type A protective devices, which supersede the type AC your switchboard is likely full of already.

If your installer requests pictures of the switchboard, the images must be good quality. If they can’t read these symbols, the quote could be $2000 off the mark.

Why The Change?

As batteries have proliferated through modern life, in power tools, bikes and an ever-increasing number of cordless appliances, the need to charge them means we have a lot more DC power supplies plugged in everywhere. They’re not inherently dangerous, but they do bring some risk.

If there’s a fault in the equipment, there could be DC electricity injected into the AC supply. The problem is that older safety switches are blind to DC, so they won’t trip if DC current is leaking out of the circuit and into your body.

In classic electrical industry fashion, these things are confusingly named. Though the words type A or type AC aren’t used on the hardware, if you look in your switchboard there are symbols that denote the difference.

Type B are the ones used for EV charging, often built into the EVSE box itself.

The Core Problem

A safety switch uses a transformer core to measure what’s going out on the (red/brown) active wire, which is in balance with what returns on the (black/blue) neutral wire. In your switchboard, all the loads are divided up with circuit breakers on the active supply to protect the wiring, but every circuit comes back to a common neutral busbar, so when the whole house is on one safety switch, there’s no problem.

However if you have more than one, two or three safety switches, there is a neutral connection for each one. In modern switchboards, RCBOs give you a safety switch for every single circuit.

In decades past, electricians would wire up houses or modify existing circuits, knowing everything was going back to one common neutral bar.

When the next trade divides all the outgoing circuits from the switchboard to have their own neutral, they might find there are circuits which share a neutral wire, and this causes the safety switch to trip every single time a particular light or power point is used.

Current flowing through the line and back along the neutral is normally balanced. When electricity “leaks” out of the functional circuit and disappears down the earth wire, the imbalance energises the search coil & trips the RCD.

Crossed Neutral Curse

In this example, our company’s youngest electrician rebuilt a whole switchboard in a dental surgery, which at some point in its 150-year (?) life had been two separate tenancies. Late in the afternoon he found the two outgoing light circuits would trip, but the day was waning and the surgery had to open tomorrow.

People have different skill sets, not all electricians are great fault finders, however training as an auto electrician is a masterclass methodical problem solving. So a week later I got the short straw to scrabble around in the roof looking for the problems I didn’t make. I’ve done this a few times but it’s often hot, dirty work which requires you to root through the roof like the rats have before you.

Crossed neutrals certainly aren’t restricted to lighting, they’re equally a problem in power circuits. In some instances it can be simpler, faster and cheaper to rewire a problem circuit instead of painstakingly searching every cable, switch, junction box and light fitting.

North Versus South

Switching breakers on/off I found the building was divided up the middle, with lights grouped on each side. Thankfully, there was no connection to the general power circuits, compressor, vacuum pump, X-ray machine, etc., etc.

It was a long trip, scrabble through the roof, down the ladder, along the hall and out through the front door to get to the switchboard, check a breaker and return.

However the central hallway had switches on both sides and light fittings right under a box gutter in the middle of the roof. Knowing that there would be a neutral jumped from one side to the other, somewhere, I began taking lights off the ceiling and switches off the door frames.

Climb over that duct, don’t wreck it, but find some wiring under the filth if you can.

Sometimes you have to partially disconnect wiring, or try to pull enough slack to measure a single conductor with an inductive clamp ammeter, and if you don’t have an apprentice with a walky-talkie, it means a lot of walky walky, going to and from the switchboard after every single change.

So part of the problem is working out where you are in the roof in relation to the walls and the existing wiring, especially when the hallway lights fittings are right under the box gutter.

Solutions Sound Simple

You just have to find somewhere, sometime in the past, somebody connected a neutral wire. For example, the 3 hallway lights and 3-way switching might share a wire with the back door light, which is now incorporated into the rumpus room and supplied from another portion of the house.

The fine idea of separating outside lights and dividing the house for greater redundancy can suddenly be a maddening adventure I assure you, especially when you peel off a roof sheet to find non-compliant wiring strewn around, which needs more attention.

The upshot is probably one of three options.

1. Leave all the lights on one circuit protected by one RCBO
2. Rewire various light circuits, eliminating the spurious connection in the process
3. Disconnect some wiring and do your rumpus activities in the dark

The fundamental point is this: every circuit in your house needs to have its own return circuit. These days neutral wires can’t be shared, and as an electrician, I can’t legally leave a circuit without protecting it with a safety switch.

While your legacy wiring might be legal because it was installed under old standards, battery systems trigger requirements that a less-than-ethical solar retailer might gloss over.

When getting quotes for a battery-backed solar system, make sure the upgrades you’ll need are documented and check the fine print on the contract to understand who’s liable if the upgrades go sideways. There might be 5 hours labour quoted for a switchboard rebuild and that can double when you need to search out a crossed neutral.

Thinking of adding a battery to your solar setup? Read more in our detailed guide to solar batteries.