According to some chap called Paul Cook:
In TN systems it is preferable for RCBOs to operate in overcurrent mode when providing indirect shock protection. When they are working in overcurrent mode they are voltage independent, whereas in RCD mode they are not; they need a large enough voltage being applied to drive the circuitry inside. The IEC stipulate that voltage dependent RCDs should operate at voltages as low as 50V, so that the effects of a collapsing voltage in a fault condition are nullified. But if you get an open-circuit neutral, it is possible that an RCBO will not operate in RCD mode with a fault to earth, so the design for a TN system is supposed to ensure that RCBOs operate in overcurrent mode for indirect shock protection, and for this reason the earth fault loop impedence restrictions are the same, and a Type C or D RCBO may not provide a 0.4s disconnection time.