When 1 of the 3 phases in an electric distribution system is broken, what happens?

Question

When 1 of the 3 phases in an electric distribution system is broken, what happens?

Answer 1

When 1 line of the 3-phase electric distribution system is cut off or broken , low voltage will be experienced in one phase of the connected load. Three phase electric motors will trip or burn if without adequate protection.

Sensitive loads should have an undervoltage protection relay.

Answer 2

I've seen electric motors run sluggish with high current. This can sometimes trip the current overloads or trip the circuit breaker.

What I've done in the past to avoid this on larger motors is to attach a 480 VAC relay and put the contacts in series with run contact. I have that connected to my PLC input and I will shut the motor down if the PLC doesn't get a run signal in a certain period of time.

Answer 3

An open phase results in an unbalance condition.

For single-phase loads connected line-line or line-neutral in the two healthy phases, there is no disturbance.

For single-phase loads connected to the open phase, they will have essentially no voltage at their terminals and will stop functioning.

Some three-phase loads may continue to operate properly, but motors will experience a significant reduction in output torque and a marked increase in heating due to the negative sequence currents flowing in their stators.  If they are heavily loaded, they may stall.

Basically, if the particular system is designed to allow single-phase or two-phase operation, there will be no disturbance.

But, three-phase systems that are adversely affected by a loss of phase use protective relays to detect this condition and trip, thereby protecting the equipment from damage.

Answer 4

A lot of things can happen.

A 3 phase network with 120 degrees phase angles cancells out any neutal line voltage requirements; the breaking of one line can cause a voltage through the neutral wire; which is not normally present in transmission lines; this would cause all kind of problems to the area such as flicker, equipment damage, frequency change... However this is normally protected by the use of switch gear and cuircuit breakers that will shut down the entire line.

Answer 5

it is very simple the system will give a loud hum or the unit will chatter...listen

Answer 6

First of all, about 1/3 of your single phase circuits stop working.

Then 3 Phase Transformers and motors overheat, and some will trip their overload protection if the loads are heavy.