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 ?

 

[dpc]

You can Google bolted pressure switch or Pringle switch. It's not a contactor. Generally, this would be done to protect an entire MCC or service, not an individual motor, against single-phasing. Circuits protected by fuses will lose a phase on a ground fault when the fuse in the faulted phase melts. In theory, the motor overload relay should detect the increased current in the remaining phases and operated before the motor is damaged. In practice, it doesn't always work. Loss-of-phase protection or unbalance relays are often provided for additional protection.

Motors will overheat rapidly if single-phased and it can be devastating to a facility if adequate protection is not provided.

 

[thermionic1]

The last bolted pressure switch or Pringle switch I saw was in Jan of this year. It was the 2000A main for a high school. There was a tag hanging on it stating that it would not operate (dated 2012). The ground fault relay associated with it had both the pick up and time dial jacked to MAX. I made a point of telling the facilities manager about the issue and danger. I even gave them the number of a former employer that was qualified to repair the situation.

I was back at the facility in June and the tag was still on the switch. I hope you don't see these things too often.

 

[cuky2000]

Also check the NEC Table 430.37, requires three over-load protective devices, one in each phase, for the protection of all three-phase motors

 

[dpc]

the current in the remaining two phases increases to 173% of normal current

But "normal" current is often 50% of FLA. The main threat to the motor is the negative sequence current created by the unbalanced voltages, not the absolute magnitude of the current. These negative sequence currents can damage motors at current levels below motor FLA. Standard overload relays or fuses are not reliable protection against single-phasing of motors. We have seen failures of numerous motors in facilities when the utility drops a phase.

 

[cuky2000]

Hi dpc,

Can a 50% loaded motor survive for the long term in a single phasing event?

 

[cuky2000]

Per the excerpt below, the zero-sequence current is null during single phasing in an induction motor. See also page 6 of the link below that mention the current increase of 173% that is in agreement with the enclosed excerpt.

http://www.cooperindustries.com/con...BUS_Ele_Protection_Against_Single-Phasing.pdf

 

[waross]

Attached is a section of the Buff book on how to protect a small motor should it ground fault.

I wonder about the author of that article.
If he considers a single motor fed by a bolted pressure switch small, how many thousands of HP is a big motor.
The bolted pressure switches that I am aware of start at around 1000 Amps.
A basic bolted pressure switch is just that. A switch that has a bolt providing pressure on the knife blade contacts instead of spring tension.

When closing, the first movement of the mechanism moves the blades to the closed position on either side of the stationary contacts.
Further travel operates the small captive wrenches that tighten bolts to provide high contact pressure.
When opening, the first movement of the operating mechanism moves the wrenches to loosen te bolts.
Further travel pulls the blades away from the stationary contacts.
The basic switch is manually operated. This switch is palletized for shipment. You can see the operating handle stowed across the bottom of the switch.
Motor operation, stored energy operation, shunt trips etc. are not part of the bolted pressure principle.
These features are added or optional extras.
I had to put a bolted pressure switch back into service after it sat unused for years. It took over a week to strip it down, clean and lubricate it and set it up to operate.
It was very similar to the switch in the photo.

The springs in the second photo are not stored energy springs.
They are over center springs to give fast closing and opening.
A note on Cuky's graphic;
That applies to single phasing of a single motor.
If the lost phase is on a feeder feeding the motor and other loads, it gets more complicated. Not better, but more complicated.
The motor acts as an induction generator and tries to power the non motor loads on the same feeder.




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

 

[bdn2004]

How can zero sequence current be null on a phase to ground fault on a grounded system ?

 

[bdn2004]

And the Article in the Buff book actually says

"when applying bolted pressure contact switches,
there is a possibility of blowing the fuse in only one phase during a ground fault. This problem is called single phasing and it can be injurious to motors because many older motor overload protection devices do not react in time to protect the motor for this condition. 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. Thus, the switch should be purchased with the anti-single phasing option and an electrical shunt trip. At the same time, a ground fault unit can be purchased to trip the switch. Coordination of the ground fault unit follows the established procedures so that the faulted circuit is isolated from the system by the nearest protective device, and the other parts of the systems are not affected (Shown on Fig 245)"

Attached is Fig 245 - he's gotta be talking about items D & E which are 600A and 2000A main fused switches.

 

[waross]

Single phasing issues are not unique to bolted pressure switches.

when applying bolted pressure contact switches

Okay. That puts it in context.
I apologize if I missed that the first time around.
Single phasing issues apply to any fused feeder.
A fused disconnect switch.
A primary feeder protected by fuse cutouts.
A power circuit breaker with current limiting fuses.
Not so much power circuit breakers, these often have special fuses that release a striker that will trip the breaker when a fuse blows, but not every one.
A broken connection or conductor.
In any system where there is a possibility of losing one phase, you can either use some type of phase loss detection and action, or you can live with the consequences.
It's done both ways.

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

 

[cuky2000]

Hi bdn2004,

This is not a phase to ground fault. If the motor is delta connected or wye ungrounded (as the most connection in the industry) there is not zero-sequence current. If the motor neutral is solidly or impedance grounded, then zero sequence current is expected.
See below the equivalent zero-sequence connection for the application described.

 

[dpc]

Can a 50% loaded motor survive for the long term in a single phasing event?

Not in my experience. I'm sure there are multiple factors, but motors and generators are quite sensitive to negative sequence current due to the "double frequency" effect. The skin effect increases the rotor resistance to the negative sequence currents and the effective negative sequence impedance is basically the locked rotor impedance, so the resulting low impedance coupled with high resistance causes a disproportionate amount of heating compared with positive sequence current. Loss of phase protection should be viewed as essential for any large motor, IMO.

 

[bdn2004]

Say if the A-phase lead comes off at the terminals onto the grounded motor housing, with a grounded source - there is ground current. The fuse clears the A phase. But the feeder welds to with the grounded surface. The fault is still fed through the other two leads that didn't trip. Is that not right and is there not a ground component?

Seems like the situation you describe could happen on a phase to phase fault - and only one of the fuses blow.

 

[waross]

In the worst case, the motor shares a feeder with non motor loads and the feeder loses a phase.
The motor may overheat and burn up fairly quickly with no load on the motor.

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

 

[cuky2000]

Bdn2004,
For single phasing event, a distinction should be noticed between the grounded source and the ungrounded load. This is what I believe will happen in this case for ungrounded motor load:

a) Local fuses protecting the motor do not detect ground component.
I[sub]a[/sub]=0 (open phase), I[sub]c[/sub] = -I[sub]b[/sub] & I[sub]n[/sub]=0 (neutral to ground).
[sub]Therefore, no zero-sequence current component will be circulated at the neutral to ground load side for an ungrounded motor.[/sub]

b) Upstream protective devices of grounded sources might detect a ground current component and a good design should consider coordination with other protective devices in the system.

[highlight #FCE94F]How is the motor connected?[/highlight]

 

[waross]

Three phase motors do not have a neutral connection.
The significance of a ground fault is that it takes out one phase.
There are other things that may take out one fuse or one phase.
An induction motor is also an induction generator.
That's what back EMF is.
The back EMF is normally less than the applied voltage, (unless the motor is being over-driven)
When one phase is lost on a feeder, the back EMF on that phase of the motor feed the other non-motor loads on the feeder.
This can badly overload a motor electrically even though it has no mechanical load.
These effects are completely unrelated to any ground fault or zero sequence current that caused the phase loss.
Rather than zero sequence we have negative sequence and an induction generator that may be heavily loaded on one phase.
It does not matter whether the motor is wye connected or delta connected.
If there is a case where a motor has a connected neutral, it is a rare and unusual case.

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

 

[jraef]

LOL, BPS used in a "small" motor starter.... had me going too.

What I always suggest now is for users to implement Solid State Over Load relays that have true phase loss protection in them. IEC type bi-metal OL relays with a "differential trip" mechanism will eventually trip if the motor is almost fully LOADED, but not if it isn't. NEMA eutectic melting alloy OL relays and NEMA bi-metal ones may not. Most SSOLs now will trip on any phase current being lower than some threshold value, 20-30% is typical. This way they can't be "fooled" by voltage regeneration on the missing phase from an already spinning motor as Voltage Monitor relays can.

I have seen many times when an OL relay does NOT in fact trip on a single phase condition because the motor is too lightly loaded and the increased current is still below the trip threshold, but the motor still burns up because of the added heat caused by negative sequence currents. This is a well documented phenomenon, I think I first learned it in the GE protective relaying books decades ago, and those were old books back then.


" 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]

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?

 

[aussiejohn2]

All,
I have seen the consequences of "single" (actually double) phasing in an industrial facility (paper) that lost a lot of motors. Event was due to false opening of one phase on the utility side. Their incoming transformer was Yy so the single phasing was reflected through to them. However I have also seen that utility overhead distribution practice makes very widespread use of single phase drop out fuses, without causing any apparent problems to customers. I have never been able to reconcile these two observations. Many utility customers are single phase residential, but there are typically still plenty of commercial, irrigation, and other three phase loads. The voltage unbalance at the customer is not as severe, as there is usually a Dy transformer in between. Has anyone done the maths to see if this keeps the negative sequence low enough?
John.