Single phasing protection of three phase motors

[keith kropf]

It would seem that it is good practice to always protect a motor from single phasing, and way back in the 70's I worked for a summer designing single phase detection for one of the big electrical equipment manufacturers. But I haven't found any spec that requires it. Is there a requirement by NEC, OSHA, or ? that three phase motors be protected against single phase power input?

 

[beanland]

Single-phasing

Single-phasing

NEC does not require it. Remember the NEC is developed by NFPA to protect buildings from burning down and costing lives and insurance companies money. Almost every utility has a clause in their service rules that says that they do not promise to have 3-phase power available at all times and that end users are required to install protective equipment to protect motors from single-phasing. Most users never bother and end up burning out the motors. That says something for the generally excellent power service we enjoy in USA.

 

[mdshunk]

I think there's some stuff in the elevators, escalators, and moving walks codes, but other than that, I haven't seen squat. Naturally, it's on print specs here and there, and in many machine control cabinets.

 

[danickstr]

what is a single phasing protection device?

edit: i found this link

http://www.wisconsinpublicservice.com/business/phase.asp

 

[brian john]

For the cost I think common sense dictates the use of them.

 

[davidr43229]

Quote:

The NEC does require it within sections 430.40, 430.52, 430.53 & 430.55

In a single phase condition the other 2 legs go up 173% to 200% in current, which is an overload. These sections address a Single-Phase condition & overloads or OCPD sizing to prevent damage in just this sort of event.
http://www.bussmann.com/library/docs/Psp.pdf

 

[Cody K]

We use a voltage monitor that cost's about 70$ which when voltage on any three legs drop below set value it trips a set of dry contacts and gives us a PLC input. Then we shut down all motors.

Or you could hard wire the start circuit thru such contacts. Money well spent.

 

[Jraef]

Voltage monitors can be fooled by phantom regeneration of a missing phase by other rotating equipment (think of a rotary phase converter). The best Phase Loss Monitors are those that are based on current.

 

[danickstr]

seems like 3 current transformer rings might serve as a low cost version if someone rigged up a monitor, or you could just buy one. SSAC is one palce I found that sells them.