How does a fused switch in an MCC protect a motor if only one phase is blown during a GROUND FAULT ?

[bdn2004]

Attached is a section of the Buff book on how to protect a small motor should it ground fault. It refers to a "bolted pressure contact switch". (I'm assuming this is a motor contactor?) It goes says the switch should be equipped with an anti-single phasing option and a shunt trip. That sounds reasonable but I'm not sure I've seen that done very often.

What really goes on during a ground fault that's protected by fuses ? Is it always going to single phase and you just have to happen on a strangely functioning motor/other load, and manually turn it off ? Or do they just all escalate into 3-phase faults and that's what trips the entire circuit ?

 

[CuriousBoy]

Hi

How is it possible for a ground fault situation to trip only a single phase fuse because the other two phases are then connected through the windings to ground?
then would two other fuses also trip?

 

[bdn2004]

CuriousBoy,

I'm still confused about that too. But look at this symmetrical components calculation for the Single Line to Ground Bolted Fault. The conditions are: Va = 0, Ib = 0 and Ic = 0. Which implies ALL the current is in Ia. How can that be? Maybe they are saying the magnitudes are so much greater in Ia, that Ib and Ic are insignificant. Even the Buff Book article I reference in the original post it makes this statement...

"In addition when a single fuse isolates a ground fault, the fault can still be fed from the other phases through the motor windings, even though the current magnitude has been greatly reduced."

I have greatly appreciated the discussion on single phasing motors, but as has been pointed out - that's not the same thing as a single line to ground fault at the terminals of motor causing one fuse to open and the others not opening.

 

[waross]

A single line to ground fault on a wye system is the same as a line to neutral load. Think of it as an overload. It doesn't matter if the cause was an overload or a fault, once the fuse blows.
A point of clarification. When we consider the effects of single phasing of a motor it is with the understanding that any fault that caused the loss of a phase no longer exists.
A flashover caused by a rodent, a bird or a dirty insulator will often blow a fuse but clear itself and leave the line unfaulted but with one phase open.
Single phasing:
You need three conductors to get two phases. Loads that incorporate a neutral connection will see two phases. With A phase lost neutral connected loads will still see B phase to neutral and C phase to neutral.
A motor has no neutral connection. With A phase lost it will see only single phase power supplied by B and C phases, line to line.
It is common to designate loads connected line to line with no neutral connection as single phase loads.

Motor contribution to a fault.
Back EMF.
A running motor generates a back EMF that is close to the applied voltage. As the motor comes up to speed the back EMF increases and reduces the line current.
When a three phase motor is spinning on single phase power, the magnetic field of the rotor is still cutting three separate and angularly displaced windings.
As a result the back EMF is three phase.
This is the voltage that shows up as a back feed into a fault.
The back EMF supplies the missing phase, but has issues with single phase loads.
The return path for line to neutral loads on the lost phase is through other single phase loads.
The line to neutral voltage may be stable or it may vary as if there was a floating neutral.
There may be damage to control transformers and/or control components.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[stevenal]

I attended an impressive demonstration some time ago. A running motor was single phased briefly by pulling a primary fuse. The motor got very loud and sounded nasty. The same motor was reconnected to a three phase VFD. When the fuse was pulled again, the motor continued to purr without a hickup. Prevention rather than protection.

 

[waross]

Can you retrofit existing motor starters with loss of phase detection relays or solid state relays or are you better off buying a whole new assembled starter bucket?

The biggest issue is finding someplace to mount the phase loss relay.
Some are fairly small.
There may be room in the MCC bucket, maybe not.
Wire the protection relay to open the motor starter coil circuit.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[bdn2004]

waross,

"A point of clarification. When we consider the effects of single phasing of a motor it is with the understanding that any fault that caused the loss of a phase no longer exists."

I think this is the part we're missing. And otherwise thanks for the thorough explanation.

 

[jraef]

As to retrofitting;

MOST (if not all) of the major motor control manufacturers now have SSOL options that will trip on a phase loss, and most (if not all) of them offer a direct replacement version. So for example if you have an Allen Bradley starter, NEMA or IEC, with an existing electro-mechanical OL relay, there is an SSOL made to directly replace it; same mounting and wiring. This is true for all of the majors now, I can't think of any mfr that does not offer that option. Even so, there are also stand-alone mounting options and feed-through power wiring so that you can just remove the old OL relay and mount the new SSOL wherever it will fit, then feed the motor leads through the CTs directly to the contactor load terminals. You may need the "wire stretcher" for some of those however.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[bdn2004]

This seems like a no brainer to order an MCC equipped with SSOL relays on every critical motor circuit. You would think the Vendors would be suggesting this.

But I’ve not seen a lot of this on new installations.

 

[dpc]

Unless fuses are being used in the motor starters instead of breakers, most single-phasing events will impact the entire MCC. So some type of loss-of-phase protection that shuts down the entire MCC might be more cost effective. I don't know the cost differential between solid-state OL relays and conventional.

 

[waross]

A customer in Central America was concerned about possible equipment damage due to somewhat unreliable power.
It was a sausage factory with a lot of refrigerated storage
I put in a simple relay that would detect loss of phase, phase reversal and over and under voltage.
That controlled a box full of interposing relays.
I then ran a pair of wires from each refrigeration contactor to the interposing relays.
Fortunately all of the refrigeration equipment was controlled by magnetic starters.
A few months later the utility upgraded their primary distribution system and increased the voltage considerably.
The crews replaced the transformers with new transformers.
They connected the new transformers for 240/416 Volts instead of 120/208 Volts.
It was electrical carnage.
All the fluorescent fixtures were lost.
Fax machines.
Copiers.
Computers.
You name it, it fried.
Except the refrigeration.
There was no damage at all to the protected equipment.
There was no loss of product.
As expensive and inconvenient as the damage was, the loss of product in just one storage room could have been much greater.
I was a hero for a day or so, but it never lasts. grin

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cuky2000]

Below are a couple of examples of one-line diagrams with large motor grounded.

It is that correct? The previous statement indicates that motor stator winding is not grounded.

 

[controlsdude]

Seems like your talking about two things
1 Loss of single phase from power supplied
2 Loss of single phase on a individual motor.

If you have loss of single phase from power supplied would not you just have the relay pull emergency stop on loss of phase.

If you have loss of single phase on a motor that is thru your overload device.