Selectivity in the distribution board: what is it and what is it for?

It is a well-known term for electricians: selectivity. Installation circuit breakers (or: fuses) in the distribution board must be selective with respect to each other downstream. But what exactly does that mean?

Searching in the dark

A system is selective when it is set up in such a way that, for example, a short circuit somewhere will result in only the circuit where the short occurs being switched off, while the other circuits remain unaffected. The aim is to ensure that the entire house does not immediately go without power in the event of a short circuit. For example, if the toaster in the kitchen has a short circuit in the evening after sunset, you don't want the entire house to be in the dark while you desperately check all the appliances to see where the fault is coming from.

A non-selective distribution board: the miniature circuit breakers have the same value as the main circuit breaker. Result: a short circuit in the toaster leads to a power outage in the entire house

The same house but with a selective distribution board. Result: a short circuit in the toaster only affects the kitchen; the lights stay on in the rest of the house

From 40 to 25 to 16

The power grid in a house is therefore set up with the largest circuit breaker at the beginning: the main circuit breaker. In new builds, this one usually has a value of 3 x 25 amps (i.e.: three phases come in, and when the current on one or more phases exceeds 25 amps, the main breaker trips). In the distribution board, the current is distributed over all circuits in the house, such as the kitchen, the living room, the shed, etc. These circuits are usually single-phase and are protected by 16 amp circuit breakers. The step from 25 to 16 amps is considered to be large enough to prevent the main circuit breaker from accidentally tripping. (For example, if all circuits on the distribution board were fused at 22 Ampere behind a main fuse of 25 Ampere, then in some cases the main fuse could still trip before the circuit fuse, because there is always a certain inaccuracy and inertia in the fuses themselves.) That is why electricians use the rule of thumb that for each step downstream the circuit breaker should be approximately 1.6 x smaller than the previous one. In practice this often goes like this: from 40 A to 25 A to 16 A.

3.6 kW inverters

For solar panel installations, this means that the main circuit breaker determines the maximum power of the inverter or inverters that can be installed. If your house has a main circuit breaker of 25 Ampere, the inverter will probably be installed behind a 16 Amp breaker. The inverter should therefore never produce more than 16 A of current itself, otherwise the fuse will blow. The fuse does not know whether the current is flowing in one direction or the other: The fuse blows as soon as it measures that more than 16 Ampere of current is flowing through it, regardless of direction. With the mains voltage being 230 Volt in the Netherlands, this amounts to 16A x 230V = 3680 Watt. That is why there are so many types of inverter that are exactly that size: the SolarEdge 3680HD, the SMA Sunny Boy 3.6, and the Zeverlution 3680.

The rules of the NEN1010

Some installers claim that the selectivity rule does not apply to solar panels. After all, the inverters of the solar panels do not consume power like the other devices in the house, they actually feed power in. That is why they think it's OK to install an inverter that can feed in more than 16 A in a house with a 25 A main connection, like for instance a single-phase Sunny Boy 5.0 which supplies up to 22 Ampere.

However, they're missing an important point: The inverter is an electrical appliance just like a washing machine, a refrigerator and all the others, and can therefore in principle also get a short circuit itself. Also, the cable that runs to the inverter from the distribution board can get a problem. That is why the NEN1010 rulebook (Dutch regulation for low voltage installations) specifically states that, for the purposes of protection against electric shocks, the inverter must be considered to be a consumer of power, even though it actually isn't. And that's why the fuse of the inverter must also be selective with respect to the main fuse.

An example

This does have certain consequences for the inverter you purchase. An example: You have a small single-phase network with a main connection of 1 x 25 Ampere. According to the rules of selectivity, the inverter will be placed behind a 16 Ampere breaker. The inverter may therefore never produce more than 16 Ampere, otherwise the breaker will trip unintentionally. This means that one inverter can never produce more than 3680 Watts.

This won't work: a 5 kW single phase inverter in a home with a 25 A main circuit breaker. The rules of selectivity would require the inverter to be placed behind a smaller circuit breaker which would trip when the inverter produces full power

As far as the main circuit breaker is concerned, you could easily install more power than that. The main fuse is 25 Ampere, so a calculation shows that you could feed in a maximum of 25A x 230V = 5750 Watts. Let's say you would like to have a system that can produce a maximum of 5 kW: How would you connect that?

A single 5 kW inverter is unfortunately not possible: If you were to place it behind a 16 Ampere breaker, the breaker would trip on the first sunny day. But what is possible is to place two inverters of 2.5 kW each. These two are each potected by their own 16 Ampere breaker, which means they comply with the rules of selectivity.

This is the solution: installing two 2.5 kW inverters allows you to feed in the same amount of power without breaking the rules of selectivity

The consequences of applying the selectivity rules in this case are that you need to get two inverters instead of one, with two AC cables and two circuit breakers. It's a bit more expensive, but in this way you at least comply with the rules for a safe distribution board.

There are many other conceivable combinations for specific cases. Zonnefabriek will be happy to advise you on the possibilities for your own home.