ABB’s New Ultra-Fast Earthing Switch


Sectional View of ABB UFES | Source: ABB

In rare cases, failure inside a switchgear cubicle due to a defect, an exceptional service condition or maloperation may initiate an internal arc, which constitutes a hazard.

  • While the absolute protection of all personnel is by far the most important concern during such an event, users would also like to prevent system components from being destroyed. ABB’s new internal arc protection system ensures that this is the case. The system operates on the principle that the uncontrolled release of energy from an internal arc fault is prevented by rapid metallic 3-phase earthing. Characterized by a significantly low impedance, this type of connection causes the shortcircuit current of an arc fault to commutate immediately to the fast-acting earthing switch and extinguish the arc.
  • The new ultra-fast earthing switch (UFES) contains three complete primary switching elements  and an electronic unit type QRU (quick release unit). Each primary switching element is similar in dimensions (height 210 mm, diameter 137 mm), shape, and fastening points to a 24 kV pin-type insulator, and consists of a two-part vacuum chamber embedded in epoxy resin to protect it from the environment. 
  • Each element also features an integrated ultra-fast micro gas generator (SMGG), which is comparable in type and functionality to the gas generators in automobile airbags.

The SMGG drives a piston and is designed as a single-shot piston actuator. The electronic unit, based on durable and fast analog technology, is phase independent in structure, and ensures current and light detection and reliable tripping within the shortest possible time.

Arc fault duration in electrical systems and the associated consequences.

Using a conventional protection device 
  • Arc fault duration: 200–300 ms 
  • Detection by standard relay
  • Clearing of arc fault current by the upstream circuit breaker 
  • Possible dramatic consequences include – Fire/explosion, Serious injuries to personnel (depending on the switchgear design).


Fast-acting protection relay with supplementary equipment (eg, Ith-limiter)
  • Arc fault duration: 50–100 ms 
  • Fast detection by special protection relay 
  • Clearing of arc fault current by the upstream circuit breaker.

Ultra-fast earthing switch (UFES)
  • Arc fault duration: ≤4ms after detection
  • Ultra-fast detection by UFES electronic
  • unit (type QRU) 
  • Ultra-fast extinction of the internal arc by switching of the UFES primary-element
  • Final clearing of fault current by the upstream circuit-breaker.

Event Sequence Description | Source: ABB

ABB’s system is an active internal arc fault protection solution for medium-voltage switchgear systems,
which greatly increases both system availability and personnel safety. 

Various floating contacts are provided as interfaces to other units. These can be used: 
  • To transmit the status of the electronics to a remote control room
  • To send commands to a circuit breaker feeding into the arc fault 
  • As an interlock to block the reclosing of a circuit breaker directly after tripping.

  • Speed, safety and savings 
  • Authors: Dietmar Gentsch Volker Grafe Hans-Willi Ott Wolfgang Hakelberg Andreas BrandtMultilin HardFiber System
  • Publisher: ABB | Download

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