Nigel Charles wrote:
>
> The reason for an electrical pump to be connected to the regulator output is
> not for regulator failure but to cater for a major failure in the rest of
> the system. With the main fuse blown the regulator will cope quite
> adequately with just the load of the pump. In the case of a regulator
> failing to high voltage the other pump, isolated from the effects of the
> regulator by the OV unit, will be working just fine from battery power.
> If the load is too great for the pump connected to the regulator it will pop
> its c/b or blow its fuse. No big deal as the other pump is providing fuel
> pressure.
That's generally correct, but we may disagree on potential consequences.
We can't rely on the pump's CB to trip (a fast-acting fuse is
inappropriate for an inductive device). If a failed regulator were to
supply 7 amps to the pump after the OV trips, it can take up to one hour
to trip the recommended 5A breaker. If we're lucky the pump will fail,
or it may run very hot with fuel inside plus excess pressure to flood
the carbs. A bad scenario is right after takeoff with both pumps on.
The only indication of a very serious problem may be a panel indication
that the OV has tripped, but not whether one pump is under stress.
Alternatively without an OV device doing us such a favor, the battery
will clamp the volts to that pump, with similar panel indication of a
problem, but not potential engine stoppage at the worst possible time.
It's easy to add a circuit to prevent excess volts to the pump while
keeping it safely running off a runaway regulator, but I side with Jos
Okhuijsen's post questioning the net safety benefit of add-ons like this
- one just to prevent bad results of the other.
Another observation is a Kitplanes article by Jim Weir proposing OV
protection applied only to a bus with avionics and selected instruments.
This leaves the 914 wiring as specified by Rotax, so we need not
debate who knows more than they do.
Regards,
Fred F.
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