Utility voltage sags and swells

[kdubya]

I was asked by a facility manager as to why only one of four compressors in a roof-top unit "cooked" while the others still function properly. The insulation on the load wires melted within the flexible conduit to this unit.
The compressor manufacturer states the cause is due to dips and swells from the utility. (I should add that phase protection is not installed at the unit).
My question is: Generally speakingshould the overcurrent and overload protection be sufficient to prevent a utility "event" or only when in combination with phase protection.

 

[480sparky]

..........My question is: Generally speakingshould the overcurrent and overload protection be sufficient to prevent a utility "event" or only when in combination with phase protection.

I don't think either one is capable of detecting sags & swells, let alone being able to so anything about it. I'd look into over- and under-voltage protection.

 

[drbond24]

I don't think either one is capable of detecting sags & swells, let alone being able to so anything about it. I'd look into over- and under-voltage protection.

I agree.

I'm not sure I agree with the diagnosis from the compressor manufacturer, however. Do they just mean the small variation in voltage you might get from nominal? That little bit of change won't cook anything. Even if it did, an overvoltage from the POCO would cook EVERYTHING. It wouldn't just pick one compressor and ignore everything else. It would be more of a system-wide event.

 

[gar]

090528-1637 EST

If you have a wire with a circuit protector at the input end of the wire, then that wire should never melt its insulation even if the load is in excess of its normal value.

This assumes that there is no excessive voltage to cause insulation breakdown, and that the circuit protector was designed to protect the wire under all load and temperature operating conditions.

I have included a lot in these statements without going into detail. Everything gets back to thermal considerations.

.

 

[ATSman]

I was asked by a facility manager as to why only one of four compressors in a roof-top unit "cooked" while the others still function properly. The insulation on the load wires melted within the flexible conduit to this unit.
The compressor manufacturer states the cause is due to dips and swells from the utility. (I should add that phase protection is not installed at the unit).
My question is: Generally speakingshould the overcurrent and overload protection be sufficient to prevent a utility "event" or only when in combination with phase protection.

The answer is no. That is why motors usually > 50HP have UV, phase unbalance, phase loss and phase reversal protection. Since it is a rooftop unit the motors are probably < 50HP and hense no protection of this kind. I have seen motors burn up due to single-phasing or UV and no breakers tripped. This is because the current levels that cause the winding damage (in the remaining winding) are under the O.C. device settings.
If it continues to be a problem I would recommend adding a voltage monitor that has all the features mentioned above. Several manufacturers make them, Eaton, Square D, GE, Basler, and are relatively inexpensive.
Cutler-Hammer (Eaton) has the Cat# SVM3AC unit for 480V systems. Since these are voltage relays, no CT's are required, just the 3-Phase control wires to the input and the wires to the breaker shunt trip.

 

[benaround]

It sounds like bad connections to the roof top disconnect to me.

 

[Besoeker]

090528-1637 EST

If you have a wire with a circuit protector at the input end of the wire, then that wire should never melt its insulation even if the load is in excess of its normal value.

This assumes that there is no excessive voltage to cause insulation breakdown, and that the circuit protector was designed to protect the wire under all load and temperature operating conditions.

I have included a lot in these statements without going into detail. Everything gets back to thermal considerations.

.

I agree. If the current was high enough to melt the insulation, then it should have been enough to trip the motor protection device.