Permanent Overvoltages

Permanent overvoltages are increases in potential of over 10% of the nominal value of the power supply, which remain for several cycles or even permanently. They are a result of phase decompensation, which is usually caused by the neutral conductor breaking, its faulty connection or faults in transformer centres.

Permanent or power frequency overvoltages are those that last a relatively long time and may cause damage to the installation and the electrical equipment.

  • Equipment destruction
  • Fire
  • Explosion in classified areas
  • Equipment overheating
  • Decrease in service life
  • Power supply interruption.

In order to protect equipment against the effects that these overvoltages may cause, Power Frequency Overvoltage Protectors must be installed, which disconnect the installation from the power supply network.

Network disconnection can happen in two ways:

  • By means of a main protection device. This device could be a main circuit breaker or a residual current device and must include a shunt release.
  • Self-reclosing device. This device could be a contactor and is very useful in second residences, public lighting and, generally speaking, in non-assisted areas.


Protection against overvoltages is mandatory according to the article 16.3 of the Spanish Low Voltage Electrotechnical Regulations (REBT). It has usually been interpreted as applying to transient overvoltages given the part of the regulation where they appear (ITC-BT-23). However, the articles of the Regulations refer to both transient and permanent overvoltages.

Internationally, the application standard is EN 50550 “Power frequency overvoltage protective device for household and similar applications (POP)”.

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