I am also not aware of any special hazards from this. A ground fault on one phase that blows a fuse would probably blow the other fuses. Single phasing will certainly increase the load on the other two phase conections and in my experience is usually due to loose conections or contact faults. 

As an aside this was a common way to get a small amount of three phase from a single phase supply. Find a suitable size three phase motor and arrange a single phase supply to two phases. Wrap some string around the motor shaft and give a good tug to get the motor spinning then turn on the single phase power. Three phase power was then available at the motor terminals. Don't try this at home unless you have a small industrial machine you want to use in your home workshop.

The protection for the other two phases will operate due to overload, but that's a lot slower than fault protection.

The problem is that the prospective fault current may be orders of magnitude lower than that from the grid, and fault protection may not therefore operate or if it does it will be delayed. In that time conductors overheat and insulation begins to fail. 

This used to be a very common way to get 3 phase from a single phase supply, these days of course we just use a drive. A pull string start is one way, but you can just use a motor start capacitor as you would with a single phase induction motor. If this condition occurs when the motor's already running then it will just continue on. 

it will be delayed. In that time conductors overheat and insulation begins to fail. 

If that can happen then surely your overload protection is flawed?

I do that the point that ADS might be somewhat compromised though if the voltage at the point of the fault can be fed from downstream of the protective device (maybe an issue even with some single phase motors spinning down, or grid-connected generators/inverters). I guess the simple answer is to use a protective device that opens all poles, including N (if any) so the remaining voltage is at least separated from Earth.

   - Andy.

unlike the grid,  the energy available from the spinning motor is limited - indeed as a 3 phase creator it is distinctly floppy -that's why we only do it if there is no easy way to get a proper 3 phase supply.

  There may be enough current to hurt someone on the created phase, at least from a big motor, but probably not to worry breakers with asymmetric arc traps or burn out wiring or anything like that.

Mike.

AJJewsbury said:

I guess the simple answer is to use a protective device that opens all poles, including N (if any) so the remaining voltage is at least separated from Earth.

Ideally yes. But imagine a fault upstream of the DB protecting the motor - that's more likely to be protected by e.g. BS88 fuses which by their nature don't provide all pole disconnection.

It would be very difficult or impossible with convention devices to meet ADS going backwards from the motor because Zs from the motor is likely to be very high.

I suspect the voltage won't hold up very well into a fault, so maybe not a shock risk. But it's still going to be feeding current which could overheat conductors.

And the basic principle of ADS doesn't work very well if a secondary source of supply isn't also disconnected.

Maybe I'm completely overthinking this. But there seems to be an absence of literature on this failure mode - either that or I'm looking in completely the wrong places.

30 years ago rotary converters were something you could buy. Now we are so dependent on drives that you get a weird fault and misbehaving motor running amok and it feels like I need more of a historian than an engineer.

mapj1 said:

indeed as a 3 phase creator it is distinctly floppy -that's why we only do it if there is no easy way to get a proper 3 phase supply.

Yeah I've pulled this trick myself quite a bit over the years. It's a handy way to get 3 phase at home because a small induction motor is a few £k cheaper than asking ENW to upgrade my supply.

And one thing I've found is that it works but it's really not very stable. Of course if you're doing it deliberately then you use very sensitive protection because I'd rather have erroneous tripping than let the magic smoke escape from my equipment.

In the factory where you could have a very large motor [or dozens of parallel motors] behaving badly, and no design consideration has been made for this type of fault condition because there's no literature to say we ought too. I wonder if damage does happen but people just fix the fault and don't do root cause analysis.

mapj1 said:

indeed as a 3 phase creator it is distinctly floppy -that's why we only do it if there is no easy way to get a proper 3 phase supply.

Yeah I've pulled this trick myself quite a bit over the years. It's a handy way to get 3 phase at home because a small induction motor is a few £k cheaper than asking ENW to upgrade my supply.

And one thing I've found is that it works but it's really not very stable. Of course if you're doing it deliberately then you use very sensitive protection because I'd rather have erroneous tripping than let the magic smoke escape from my equipment.

In the factory where you could have a very large motor [or dozens of parallel motors] behaving badly, and no design consideration has been made for this type of fault condition because there's no literature to say we ought too. I wonder if damage does happen but people just fix the fault and don't do root cause analysis.