The Storm Season Is Here. Is Your Smart Home Protected From the Bit Nobody Sees?

Storm season on the Highveld is starting, and the usual story is "we had a bang during the storm and now half the house doesn't work."

Nobody ever phones to say the surge protection did its job. That is the point of it. It is the most boring line on a quote, and it is the line most often cut first.

So this is a post about the boring line.

How much lightning are we talking about?

Property24 ran a piece on lightning density that used the South African Weather Service ground flash density map from 2017. It put Johannesburg at around 15 flashes per square kilometre per year, against roughly 3 for Europe. The same article says that is an exceptionally high figure for a country's main economic centre.

It also quoted insurer numbers: Santam paid out R47 million on 3,055 lightning damage claims through April 2021. That works out to roughly R15,400 a claim. Those are old figures and one insurer, so treat them as a sense of scale rather than a current statistic. But the direction is hard to argue with.

Add up what is now plugged into a modern home: inverter, batteries, routers, access points, cameras, a NAS, a gate motor, a stack of KNX actuators. The surge has more places to go than it did ten years ago, and more of them cost real money.

KNX is low voltage. Doesn't that make it safe?

Partly. The bus runs on safety extra-low voltage, 29 V maximum, and the devices are built to tolerate a fair amount of abuse. That is a real advantage over mains-powered gadgets.

But a lightning strike does not need to hit your house to hurt it. A nearby strike induces a surge in any long conductor, and a bus cable is a long conductor. DEHN, the German surge protection manufacturer, publishes a white paper on exactly this. Its point is blunt: KNX systems can be destroyed by coupling, particularly when a bus line segment runs past 1,000 m. Most homes are nowhere near that, but a big property with outbuildings, a gate house and a pool plant room can end up with surprisingly long runs.

The damage also rarely arrives through the bus alone. It comes in on the mains supply, through the KNX power supply, and out through the actuators and whatever they switch.

Think of it as a manager at every door

Here is how I explain it on site. Surge protection is the manager who stands at each door of the building and says "you can come in, but not at that speed."

The mains entry is door one. The cable between buildings is door two. The bus line that leaves the house to reach a gate or a pool controller is door three. If you only post a manager at the first door, the surge walks in through the others.

What the guidance says to do

I am working from the DEHN white paper and from South African coverage of the wiring code. I am not a lightning protection designer, and a proper design for a large site should come from one. With that said, this is what the sources agree on.

Protect at the distribution boards. DEHN recommends surge protective devices (SPDs) at the main distribution board, at the KNX distribution board, at actuators and heating controls, and wherever you cross from one lightning protection zone into another.

Pick the right type. A Crown Publications article on SANS 10142-1 explains that where a building has external lightning protection, a Type 1+2 device goes at the main board. Where it does not, at least a Type 2 device is recommended. The same article gives minimum conductor sizes of 6 mm² copper for Type 2 and 16 mm² for Type 1+2. Read the current edition of the code or ask your electrician to confirm what applies to your installation, because the article does not make clear what is mandatory and what is advised.

Protect the bus too. KNX has dedicated bus arresters. DEHN's paper lists a KNX-specific one with a 5 kA nominal discharge rating. Where a bus line leaves a building, my own practice is one at each end. That is my reading of their zone-boundary advice, not a quote from it.

Route cables to avoid loops. This one costs nothing and gets missed constantly. DEHN says bus cables and the low-voltage cables leading to the same devices should be installed close together, because a wide loop acts as an antenna for induced surges. Metal structures connected to earth can create the same problem.

Between buildings, use screened duct. For cables routed in an earthed metal duct or pipe, DEHN says surge arresters are enough and full lightning equipotential bonding is not required. Plain cable clipped to a wall is a different story.

Think about the plugs, too. Lightning King, a South African supplier, describes Type 3 devices as point-of-use protection for individual electronics. They sit behind the board-level protection, not instead of it. The same supplier suggests replacing surge protectors every three to five years or after a significant surge. That is a supplier's view rather than a standard, so check your device's datasheet. Many have a status window that tells you when they have done their job.

What I do on a KNX job

My view, from the integration side. It is not a tested standard.

The surge protection goes on the quote as its own line, with a name on it, not buried in "sundries". It is the cheapest insurance in the whole project, and it should not be the first thing to disappear when the budget gets tight.

I want the distribution board layout drawn before the first fix, so the SPDs have a place to go and the bus and mains routes are planned together. And when there is a bus run to an outbuilding, I ask the question early: how is this protected at both ends?

After a storm, I want the status indicators on the SPDs checked. A protector that has taken a hit and is now sitting there dead gives you a false sense of safety, which is worse than none.

Where this leaves us

A smart home is only as resilient as the weakest door into it. KNX is a very robust system, and the bus is more forgiving than most. It is still electronics on long cables in one of the more lightning-dense cities on earth.

If you are building or already living in a home with KNX, solar or a lot of networked kit, ask one simple question: where are the surge protection devices, and what are they rated for? If nobody can answer, that is worth a conversation before the storms get going, not after.

If you want to talk it through for your project, I'm happy to. Just a conversation about what the project needs.

wayne@knxlogic.co.za | 082 564 3982 | www.knxlogic.co.za

Sources: Property24, "Mapped: SA's most lightning-dense areas" (30 Sept 2021, citing SAWS 2017 ground flash density map and Santam claims to April 2021); DEHN International, "Surge protection for KNX systems" white paper (knx-wpx019-en); Crown Publications, "Using surge protection and how it is described in the wiring code (SANS 10142-1)" (1 Mar 2023); Lightning King, "How to choose the right surge protection for your property in South Africa" (7 Oct 2024).