Glossary

Safety extra-low voltage

Unit V · Also SELV, PELV

What is Safety extra-low voltage?

Extra-low voltage produced with safe isolation from the mains — for example via a transformer with separate windings. A fault on the mains side then cannot reach the extra-low-voltage side.

What matters about Safety extra-low voltage in practice?

The difference from plain extra-low voltage lies in the isolation, not in the level of the voltage. A power supply without safe isolation delivers extra-low voltage, but not safety extra-low voltage.

On board the term matters mainly where chargers and power supplies connect the 230-volt side with the DC side. How this isolation is implemented is stated in the device's data sheet.

What is the difference between SELV and PELV?

Both describe extra-low voltage with safe separation from the mains. With SELV the circuit is not earthed. With PELV it may be connected to the protective conductor or to earth. A DC system whose negative is connected to the bonding system therefore corresponds more closely to PELV.

Which voltage counts as extra-low voltage?

Up to 50 volts AC and up to 120 volts ripple-free DC count as extra-low voltage. Systems with 12, 24 and 48 volts are below this. That does not make them harmless: batteries deliver very high currents in a short circuit. The danger here is less electric shock than heat and fire, which is why every conductor needs a fuse close to the source.

Where does this matter on board?

Wherever 230 volts and the DC system meet: charger, inverter-charger and power supply. Only if the separation inside the device is safely designed does the DC side stay harmless when a fault occurs on the mains side. How the device is separated is stated in its data sheet.

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