In AW189, B787 and few other aircraft/helicopter, the physical breaker has been replaced by virtual breaker, which can be controlled by the MFD (Multi-Function Display).

I tried to google the answer, but I could find nothing about it: how does a virtual breaker physically work? I know how you control it, but what really happens when you press the breaker switch button on the MFD?

  • $\begingroup$ For details about electronic involved, look for thyristor in DC or triac in AC, as the power switching component is likely one of these latch devices. Those semiconductors are used for quite a long time for power switching (e.g. in once-pervasive strobe). The gate of those devices would be triggered by a simple current measurement (the voltage at the pins of a calibrated resistor) this measurement would replace the thermal effect in a thermal circuit breaker. $\endgroup$
    – mins
    Apr 17, 2017 at 13:40
  • $\begingroup$ Could you please edit question and give some context (a link) or explanation of what "AW189" is. "MFD" or "B787" is pretty obvious, but AW189 is not. What is it ? $\endgroup$
    – kebs
    Apr 18, 2017 at 13:51
  • $\begingroup$ @kebs I update the question. $\endgroup$ Apr 19, 2017 at 8:20
  • $\begingroup$ The interesting part of the question would be: What happens if the virtual circuit-breaker for the MFD is triggered? ;-) $\endgroup$
    – U. Windl
    Sep 27 at 8:25

2 Answers 2


Virtual breakers use solid-state relays:

A solid-state relay (SSR) is an electronic switching device that switches on or off when a small external voltage is applied across its control terminals.

The relay may be designed to switch either AC or DC to the load. It serves the same function as an electromechanical relay, but has no moving parts.

According to Boeing for its 787:

The remote power distribution units (RPDUs) are largely based on solid-state power controllers (SSPC) instead of the traditional thermal circuit breakers and relays.

The standards for the solid state power controllers in aircraft is set by BS ISO 8816.

Related: Where are the circuit breakers on the 787?


A virtual circuit-breaker is a relay (solid-state or otherwise) where the current going through it is monitored by a computer of some sort. When the computer sees too much current going through it, the computer trips the relay open. (Think of it as 'active CB' versus 'passive CB'.)

The engineer/pilot can access the computer to find out which CBs have been tripped by the system, and can try to get the system to close them again, just like resetting a real CB.

Advantage 1 - you can guarantee that the CB will trip at the target amperage, a real CB will often have a large error in its trip amperage.

Advantage 2 - real CBs have to be mounted where they're accessible to the crew, virtual CBs can be mounted anywhere that's convenient for the systems design engineer.


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