# The formula for the residual circuit breaker

### A fault current’s wave­form is crucial to the selec­tion of the right residual current circuit breaker – also for EC motors.

Helmut Lipp, R&D Manager Devel­op­ment Elec­tronics at ebm-papst in Mulfingen

Fault currents can be dangerous for people, machinery and entire systems. For personal safety, Type A residual current circuit breakers with a maximum trip­ping current of 10 mA or 30 mA are required. Type B with 300 mA is required for fire safety, but such high currents can lead to irre­versible harm or even death in humans, so the choice in indus­trial settings is not always easy.

This is also an issue when elec­tron­i­cally commu­tated (EC) DC motors are used. For EC devices with passive power factor correc­tion (PFC), Type A serves its purpose reli­ably. However, if for example EC fans are oper­ated in parallel, active PFC is used to avoid harmonic currents in the power grid. To decide which circuit breaker is the right one here, the topology of the commu­ta­tion elec­tronics needs a closer look since it deter­mines the current wave­form in the event of a fault. Depending on the DC link voltage CZWK across the capac­itor, a voltage builds up between the posi­tive terminal of the capac­itor and the PE poten­tial; this voltage has a wave­form that differs with the topology.

With active PFC, the DC link voltage CZWK across the capac­itor can be set to values of UZWK > √ 2×Uac,rms. Here the voltage poten­tial is always greater than 0 V. The minimum value is Umin = UZWK -√ 2×Uac,rms. If there is an insu­la­tion fault, this is the same condi­tion as if a resistor were inserted between the poten­tial UZWK+ and PE. The fault current behaves anal­o­gously to the voltage.

For an inverter with active PFC, the current drops to a minimum value Ifault = √ 2× UZWKmin/Rfault. We have measured the behavior of the circuit topology, taking an insu­la­tion fault into account; a fault current Ifault > 30 mA was set. With this fault current, the Type A residual current circuit breaker did not trip.

In the same exper­i­ment using a Type B circuit breaker, it discon­nected the circuit from the power supply at a time T2 after the fault occurred. In order to provide the neces­sary safety for people, machinery and systems with EC drives connected to circuits using elec­tronics with active PFC as boost converter, these residual current circuit breakers Type B are the right choice.

EC motor with active PFC. The Type A residual current circuit breaker fails to trigger.

The Type B residual current circuit breaker discon­nects the circuit from the power supply at a time T2 after the fault occurred.