- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Being that your experience is outside of the US, you may have not come across this. Older style NEMA bi-metal or eutectic melting alloy overload relays that have the replaceable heater elements do not have differential tripping, the feature in IEC OL relays that will bias the trip point in order to help prevent motor damage from single phasing. But over here, standard off-the-shelf NEMA designed motors in sizes where people tend to get cheap about things like OL relays are designed with a 1.15 Service Factor, which makes them more tolerant to the added heating effects of negative sequence currents that take place in a 3 phase motor that loses a phase while running, a concept that doesn't exist with IEC motors. So basically, our basic OL relays don't have it so our motor designs are made to be more tolerant, or vice versa. IEC motors have no Service Factor built-in so even basic OL relays have to cover that potential problem.
This also raises a semantics issue for all: standard IEC bi-metal OL relays that do provide this differential protection technically do NOT protect against the loss of a phase, they only protect against that additional heating in a 3 phase motor that results from losing one phase while running, and only IF the motor is fully loaded. What the differential bar does is move the trip curve pickup point lower than normal so that the load current only needs to be somewhere around 75-80% of the setting instead of 100% before it begins it's trip process. But if your motor is only loaded to 50% when the phase is lost, it may run forever and not cause that OL relay to trip. So it is not truly protecting against a loss of a phase, it is protecting against the DAMAGE that may happen on a fully loaded motor if a phase is lost while running. Another important issue is that it will not PREVENT a motor starter from turning on in a lost phase situation, which can damage a motor and cause other operational issues as well because the starter will say the motor is running but it will not spin. So if there are other more important issues to guard against resulting from a phase loss, one should NOT rely upon the often misquoted "single phasing protection" of a standard IEC bi-metal OL relay, use a solid state OL relay that has single phase protection (most do) or use a CURRENT based phase loss relay (voltage based phase loss relays can be fooled too easily).
