One of the key selling points for the SolarEdge Inverter / Power Optimizer system is its fire safety capability.
The SafeDC feature is a fail-safe mechanism ensuring voltage automatically drops to a safe 1 volt level as soon as the inverter and/or grid power is shut down.
The SolarEdge system architecture depends upon the inverter constantly telling the power optimizer behind each module to let the power through and send it down to the inverter. In the event that the SolarEdge inverter is turned off, the optimizers stop receiving these signals and cut off the bulk of the power on the roof, bringing every module down to a safe 1 volt.
In the event of a fire this is an excellent feature because when the fire service cut off AC power to the property, the inverter shuts down, stops communicating making the power optimizers and modules shut down on the roof. This means that there will not be any high voltage DC cables inside the building – one less concern for the fire-fighter.
This same mechanism means that a SolarEdge inverter is essentially a DC isolator. If the SolarEdge inverter is turned off, the DC optimizers stop sending power and the system is isolated.
The technical details of the SafeDC system are described in the SolarEdge SafeDC Explained and in the SolarEdge Inverter Installation Guide (see appendix D) documents which you can download from the links shown below.
SafeDC Operation
Three simple steps are needed to operate the SafeDC mechanism:
1. Flip the switch at the bottom of the inverter. As a result the inverter stops feeding in power and disconnects from the main, the power optimizers immediately stop producing power and the capacitors begin to discharge.
2. Wait for the voltage to reach a safe level of less than 120Vdc. The DC voltage can be read on the LCD display. In case of a grid shutdown the display won’t work. Wait three minutes before performing the next step.
3. Disconnect the PV input ports. This results in galvanic separation between the PV array and the inverter. Each power optimizer outputs a safe voltage of 1V, thus the output voltage of the array is below hazardous values. The danger of arcs has now been eliminated, ensuring the installation is safe.
Technical Compliance
This mechanism has been certified by Bureau Veritas as being a DC isolator in accordance with IEC 60947-3:199 when using modules of up to 95 Voc on a single phase inverter and 70 Voc on a three phase inverter: Essentially most PV modules currently available in the UK.
The MCS Guide to PV Installations states that a separate DC isolator is not needed when an inverter possesses an integrated DC isolator, provided the DC cable is also connected to the inverter via plugs that can only be opened with a tool and which are labelled with a visible warning sign or text saying “Do not disconnect under load”.
SolarEdge inverters are connected via MC4 plugs and the certified SafeDC mechanism is in essence an integrated DC isolator. So installing separate DC isolators with a SolarEdge Power Optimizer Inverter is purely optional.
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