Inverters and Residual Current Devices (RCD's): what's the deal?
Residual Current Devices (RCD's) serve to shut off electricity when there is leakage current: current that leaks out because, for example, someone stuck their finger in a wall socket. The RCD continuously measures how many amps of current enter the cable (via the phase wire), and how much current comes out again (via the neutral conductor). There is always a slight difference between the two (caused by the leakage current), but that difference is normally very small (in the range of a few milli-Amps). If the difference becomes too large (for example, when current leaks out through the finger in the socket), then the RCD trips and the power to that cable is cut off immediatley, protecting the person from being electrocuted.
On the distribution board, all circuits are protected by both circuit breakers (fuses) and also by RCD's. According to the rules of the NEN1010 (the Dutch Standards for low voltage installations), a maximum of 4 circuits may be installed behind one RCD.
The rule of having a maximum of 4 circuits behind one RCD implies that if there were still room for a new circuit behind an existing RCD, you could connect the inverter for the solar panels on that same RCD. Yet we often choose to put a separate RCD for the inverter. Why is that?
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What is an RCD?
Residual Current Devices (RCD's) serve to shut off electricity when there is leakage current: current that leaks out because, for example, someone stuck their finger in a wall socket. The RCD continuously measures how many amps of current enter the cable (via the phase wire), and how much current comes out again (via the neutral conductor). There is always a slight difference between the two (caused by the leakage current), but that difference is normally very small (in the range of a few milli-Amps). If the difference becomes too large (for example, when current leaks out through the finger in the socket), then the RCD trips and the power to that cable is cut off immediatley, protecting the person from being electrocuted.
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Circuit beakers and RCDs
On the distribution board, all circuits are protected by both circuit breakers (fuses) and also by RCD's. According to the rules of the NEN1010 (the Dutch Standards for low voltage installations), a maximum of 4 circuits may be installed behind one RCD.
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A separate RCD for a solar inverter?
The rule of having a maximum of 4 circuits behind one RCD implies that if there were still room for a new circuit behind an existing RCD, you could connect the inverter for the solar panels on that same RCD. Yet we often choose to put a separate RCD for the inverter. Why is that?
What do circuit breakers and residual current devices actually do?
Whereas an RCD reacts to leakage currents, the circuit breaker protects against overcurrents. For example, a 16 amp circuit breaker trips as soon as a current of more than 16 amps passes through. This protects cables from overcurrents (danger of overheating). Also, if, for example, the phase wire and the neutral wire were to touch each other (creating a short circuit), then a very large current would immediately flow through them. This would not trip the RCD (because there is no leakage current), but instead it would trip the circuit breaker in this case. That's why you need both devices to ensure safety.
An RCBO ('Residual Current circuit Breaker with Overcurrent protection', also called a 'GFCI circuit breaker' in the US) is a device that combines both functions in one. In Dutch they are sometimes called 'alamat'. They have two limit values that both lead to activation (tripping): the overcurrent I (in the range of several Amps), and the leakage current ΔI (in the range of 30 or more milli-Amps).
single phase RCBO: inside the red circle you can see both the overcurrent value of 16 A as well as the leakage current value of 0,03 A, i.e. 30 mA
Inverters and earth leakage protection
Almost all inverters today are so-called TL models, where TL stands for “TransformerLess". These are generally the quietest, lightest and most efficient models available. However, what you need to be aware of is that a TransformerLess inverter can lead to a phenomenon known as “capacitive leakage current”: an induced current from the panels that resembles leakage current and can trigger an RCD when there is actually no safety reason to do so.
For many TransformerLess inverters, it's better not to put them behind a 30 mA (milli-Ampere) RCD because it will trip too quickly, especially in humid weather, as moisture on panels increases capacitance. Zonnefabriek often chooses to put the inverter behind a 300 mA RCD. This still protects the cable against leakage currents, without unnecessarily activating the RCD for no reason.
Sometimes there is some confusion about the rules of the NEN1010. Many people assume that only RCDs of no more than 30 mA may be used in the home. This is not entirely true: the 30 mA RCD serves as additional protection, and this additional protection is mandatory for all circuits which lead to: wall sockets, lighting points, and movable electrical equipment.
Since the inverter is connected to a separate circuit (without lights or sockets) and mounted in a fixed location, the obligation for additional protection does not apply here. In addition, the inverter itself is also equipped with internal residual current measurement that causes the device to shut down when a sudden change in leakage current is detected.
Some installation companies choose not to put the inverter behind a residual current device at all. However, that ignores the fact that the cable itself must also be protected against faults. An circuit breaker might be sufficient for this, but only if the so-called loop impedance of the fault circuit is sufficiently low. If this is not certain (and it often is not in the case of so-called TT systems), an RCD of at most 300 mA must still be used. That's why Zonnefabriek installs these as standard.
To install or not to install an RCD for the inverter?
This choice depends on the type of system in your power grid: a TN system or a TT system.
- Is your power grid a so-called TN system? Then you do not need to install ground fault protection for the inverter. In a TN network, the grounding of your home system is provided by your grid operator, and you are not dependent on an earth pin for grounding.
- You can recognize a TN system by a sticker on the meter box with text such as “grid operator offers earth.” New construction homes are more likely to have TN systems than older homes.
- Not sure which system you have? Then it's best to assume it's a TT system just to be safe, and therefore install an RCD
On this distribution board the red frame indicates the sticker with the text: 'distribution grid operator provides earthing' (netbeheerder biedt aarde aan). This indicates that we are dealing with a TN grid.
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