3ph 4wire Y connected system losing phase

[Designer69]

gents,

good afternoon. We have a 3ph 4wire Y connected system that repeatedly loses a phase and the other 2 phases continue to operate without the OCPD tripping out the circuit.

is this crazy? this should not be happening should it?

thanks

 

[raider1]

Is the phase being lost at the utility transformer?

The only way to guarantee that the system will be shut down due to a lost of a phase is to use a phase failure relay or other device that will monitor the electrical system and shut down the system when a lost of phase is detected.

A standard OCPD such as a MCCB will only trip if there is an overcurrent due to a 3 phase load being single phased. If this does not happen then the OCPD will not trip.

Chris

 

[LarryFine]

We have a 3ph 4wire Y connected system that repeatedly loses a phase and the other 2 phases continue to operate without the OCPD tripping out the circuit.

That's because, for all 1-ph loads, the Wye system is effectively three 1-ph supplies.

is this crazy? this should not be happening should it?

Well, it's not good, especially if it's often, but the OCP opening is preferable to equipment damage. Is this a fuse or breaker?

If you're talking about a 3-ph breaker that doesn't open all lines when one trips, that's a different story. The internal common trip is defective. Make sure it's not three 1p's with a handle tie.

 

[Designer69]

If you're talking about a 3-ph breaker that doesn't open all lines when one trips, that's a different story. The internal common trip is defective. Make sure it's not three 1p's with a handle tie.

yes I thought the OCP would trip on ground fault.

but now that I think about it, the OCP is a fused disc.

I will get exact details but I always thought a Y system was never supposed to continue functioning with a phase out.

 

[LarryFine]

I will get exact details but I always thought a Y system was never supposed to continue functioning with a phase out.

No, but many 3-ph loads don't function well with a dropped phase.

 

[Designer69]

No, but many 3-ph loads don't function well with a dropped phase.

by no you mean that they can continue to operate with a phase out.

right?

 

[LarryFine]

No, I meant that a 3-ph Wye source is three 1-ph transformers, electrically speaking, and one phase going out doesn't stop the other two, unless they're de-energized by a breaker with common tripping.

Now, I'm talking about a source, not a load.

Whether a load shuts down when one phase is lost depends on whether it's either designed to do so, or is a load that cannot run (or continue to, which come motors can) without all three phases present.

 

[Open Neutral]

.....or is a load that cannot run (or continue to, which come motors can) without all three phases present.

Or is a load that smokes itself trying to run with one phase gone.......

I believe in lost/reversed phase detectors; you can guess why.

 

[ptonsparky]

Putting in the phase monitors will help prevent damage to those loads that are subject, but you really need to find out why. Poor connections is usually #1 on this list, that or an additional load, intended or otherwise.

 

[Hv&Lv]

Still not sure what type of load is being served. Thermal overload protection should protect the motors(if installed). Where is the loss of phase from? Utility? Dry pack? Only way to stop single phasing from utility side is to be controlled by three phase recloser. Secondary side has a couple of options, most arent installed until something burns up or out.

 

[ptonsparky]

Still not sure what type of load is being served. Thermal overload protection should protect the motors(if installed). Where is the loss of phase from? Utility? Dry pack? Only way to stop single phasing from utility side is to be controlled by three phase recloser. Secondary side has a couple of options, most arent installed until something burns up or out.

Newer starters are much better at detecting current imbalance, but I do not trust them for large or important motors. (hard to replace). Older NEMA starters were a good source of income for motor replacement.

 

[G._S._Ohm]

If I understand you correctly, with a Y connection and a neutral return wire, one phase being open circuited should not affect the others, due to some theorem for simplifying circuits the name of which I cannot recall.

The circuit can be reduced to three circuits, each with a high side and a neutral. The other two continue to operate.

 

[stevebea]

yes I thought the OCP would trip on ground fault.

but now that I think about it, the OCP is a fused disc.

I will get exact details but I always thought a Y system was never supposed to continue functioning with a phase out.

A 3 phase load such as a 3 phase compressor if running while the phase is lost will continue to run but will slow down, amperage will go up on the other two phase and the overloads will trip.

 

[kwired]

A 3 phase load such as a 3 phase compressor if running while the phase is lost will continue to run but will slow down, amperage will go up on the other two phase and the overloads will trip.

My experiences are that a higher torque requirement such as a compressor will stall. The current on remaining phases will increase however causing overload to trip.

I have seen applications with multiple fans where the fan continues to run because there is some help from other fans - sometimes the help is in the wrong direction and the one that is single phasing is actually drawing current on remaining phases but turning wrong direction. Careful overload selection can still catch this condition, but will have to be sized to normal operating load and not maximum motor current allowed.

 

[stevebea]

My experiences are that a higher torque requirement such as a compressor will stall. The current on remaining phases will increase however causing overload to trip.

I have seen applications with multiple fans where the fan continues to run because there is some help from other fans - sometimes the help is in the wrong direction and the one that is single phasing is actually drawing current on remaining phases but turning wrong direction. Careful overload selection can still catch this condition, but will have to be sized to normal operating load and not maximum motor current allowed.

Yes, I agree. Parallel 3 phase motorrs that are supplied from the same source can exchange current under some circumstances when one line is open. Motor #1 will supply unbalanced 3 phase current to Motor #2. Motor #2 may even be able to start but one phase will carry overload while the other two phases carry about normal current.

 

[kwired]

Yes, I agree. Parallel 3 phase motorrs that are supplied from the same source can exchange current under some circumstances when one line is open. Motor #1 will supply unbalanced 3 phase current to Motor #2. Motor #2 may even be able to start but one phase will carry overload while the other two phases carry about normal current.

I was not talking about that type of situation, and am not sure if I totally agree with it. Motors connected in parallel with a lost phase will both experience the loss of the same phase and will both operate in the same way if powering the same type of load. Light torque loads may continue to run with reduced performance but likely will not start on their own.

I was talking about multiple fans supplying air to same space, one fan blows a fuse for whatever reason on one phase, now the fan is operating in a single phase condition. It has little or no torque so the positive pressure supplied by the other fans overcomes the little or no torque and causes this fan to run backward. It may be able to run that way for long time without overloading the remaining energized windings. I have seen it before on grain storage buildings where air is pushed through the grain to keep temperature and moisture under control.