Motor Short Circuit Ground Fault Protection

[kwired]

Makes sense now that you say it.

Any idea if a 125% class J fuse would hold on a crop dryer fan?

Don't know and RK5 is much cheaper than finding out, presuming you are talking about those that are 10 HP and up.

Some the small axial type fans probably even hold on RK5 as they are not usually not that difficult to accelerate, are usually lighter weight aluminum instead of steel, some newer ones are even aluminum hub but some sort of non metallic blade, not sure exactly what kind of material it is. Some those don't even have start capacitors in the motors (when single phase) they just use a PSC motor, and they are up to ~10 HP, they can get away with it because it is easier to accelerate.

 

[paulengr]

What about a low peak fuse?

You say breaker will not coordinate with a motor, but I know of cases where the short circuit and ground fault protection was at 125% and motors held on that circuit. Are you referring to overload protection of the motor?




Is inrush also a function of source impedance?

What about 15-20X breakers?

A fuse will coordinate just fine for short circuit protection. OP stated ground fault and short circuit, and fuses don’t do ground fault. Using a low peak fuse helps a lot. As far as short circuit protection a low peak fuse is almost the perfect device for motor protection when used in conjunction with overload relays, especially “smart” ones.

Inrush is a function of phase angle. The values I stated assume that there is no transformer/feeder impedance limitation. It is solely based on motor magnetization current. It is almost purely reactive. So it is 90 degrees offset from voltage. So if the contactor closes at peak voltage you get almost no magnetization current. If it closes at the zero crossing, current is at a maximum. That’s one phase and you have to consider all 3 to arrive at worst case. Then based on your trip point gives us what phase angles are a problem...lower trips mean a greater percentage of starts will nuisance trip. Many more plants just reset without a second thought which is clearly an OSHA violation.

You can use electronic trip breakers but you need to use one with an adjustable instantaneous delay set for 1 cycle or be careful on motor selection to avoid anything high efficiency or “world market”. They exist but since there is no standard for this it’s specific to some manufacturers and models. At this point in time you are better off avoiding the motor selection method unless you are using a motor that is faster to rebuild than to purchase (made to order). Or switch to fuses where there are no issues. At worst the fuse temporarily current limits. Or bypass the issue and install a soft start. I work for a motor shop. I would be happy to sell you a motor that won’t trip your breaker if I could but at this point it will have to be sourced from the rebuild market.

15-20x breakers fix the instantaneous problem but then we run into LRC. If starting time is fast enough (small motors with low Wk2 can start and avoid tripping. The maximum allowed starting time by NEMA standards is 10 seconds without using specially modified motors or wound rotors. Typical using NEMA’s preferred torque criteria is 2-4 seconds assuming across the line starting but if you compare that to a breaker inverse time curve it will hit the “knee” in the curve.

If you look in this book they have some great illustrations showing the problems with standard breakers and motor protection. I don’t subscribe to all they claim (very fuse forward reference) but most of it is accurate, if not sometimes half truths.

Selecting Protective Devices (SPD) Handbook

Based upon the 2017 NEC, this 300 plus page handbook shows how to properly select and apply overcurrent protective devices.

www.cooperindustries.com

 

[paulengr]

Magnetic trip yes, thermal trip only on heavy inertia loads.

Most the crop dryer fans (at least 10 HP and larger) I run into you can't even use class 10 overload relays, takes too long to accelerate the load before it trips, you must have class 20 overload relays. Those have pretty heavy impeller and it takes a lot just to get them up to speed.

The 10 HP 480 three phase motors I see that hold on a 15 amp breaker or the 10 HP single phase on a 60 amp breaker are always on a pump or other low inertia and low starting torque load.

You said it all already. Class 20 is the default when the motor is not marked otherwise. Class 10 is for hermetically sealed motors and some others that are very thermally sensitive. Similarly motors that are modified for heavy inertia starting (crusher motors) will be specifically marked class 30.

NEMA SG-1 states that all overloads trip in 12 minutes or less at 200% FLA. The class number means it trips in that many seconds at 600% FLA. So a class 10 overload trips in 10 seconds at 600% FLA. These points “set the curve”. Note though that IEC Class 10 is not NEMA class 10. It’s actually more similar to NEMA class 20. Make sure you know which standard you are dealing with.

 

[paulengr]

Makes sense now that you say it.

Any idea if a 125% class J fuse would hold on a crop dryer fan?

Yes.

Download NEMA SG-1. It’s free. That’s the standard for starters. It gives specific fuse sizes for every standard motor size for type 2 starter ratings.

One problem with contactors is that they tend to weld themselves at around 30x FLA (dead shorts) before the breaker/fuse ever trips. Certain fuses (J, RK-1, some others) are rated to limit current and trip before the contactor is destroyed. This is called type 2 (no damage) starting. If the SCCR is low enough obviously this works, too. But contactors don’t list an SCCR rating. The type 2 rating is similar.

 

[mbrooke]

Yes.

Download NEMA SG-1. It’s free. That’s the standard for starters. It gives specific fuse sizes for every standard motor size for type 2 starter ratings.

One problem with contactors is that they tend to weld themselves at around 30x FLA (dead shorts) before the breaker/fuse ever trips. Certain fuses (J, RK-1, some others) are rated to limit current and trip before the contactor is destroyed. This is called type 2 (no damage) starting. If the SCCR is low enough obviously this works, too. But contactors don’t list an SCCR rating. The type 2 rating is similar.

Do you have a link?

 

[mbrooke]

A fuse will coordinate just fine for short circuit protection. OP stated ground fault and short circuit, and fuses don’t do ground fault. Using a low peak fuse helps a lot.

Fuses do indeed provide ground fault protection- at least as recognized by the code in article 430. Why do you say they don't?

As far as short circuit protection a low peak fuse is almost the perfect device for motor protection when used in conjunction with overload relays, especially “smart” ones.

What about cases where a low peak fuse also does motor overload protection? Is the concept practical? I know ti strays away from my initial question but its something I think about.

Inrush is a function of phase angle. The values I stated assume that there is no transformer/feeder impedance limitation. It is solely based on motor magnetization current. It is almost purely reactive. So it is 90 degrees offset from voltage. So if the contactor closes at peak voltage you get almost no magnetization current. If it closes at the zero crossing, current is at a maximum. That’s one phase and you have to consider all 3 to arrive at worst case. Then based on your trip point gives us what phase angles are a problem...lower trips mean a greater percentage of starts will nuisance trip. Many more plants just reset without a second thought which is clearly an OSHA violation.

What about where the motor is denergized at its peak with residual magnetism then power applied at the opposite point in the sine wave? I've heard inrush will be two times zero crossing.

 

[topgone]

Makes sense now that you say it.

Any idea if a 125% class J fuse would hold on a crop dryer fan?

The best answer would be to refer to fuse manufacturers' published time-current curves on minimum fusing!

 

[mbrooke]

The best answer would be to refer to fuse manufacturers' published time-current curves on minimum fusing!

Link for bussman by any chance? 125% appears to hold on normal inrush from the data I'm going by.

 

[paulengr]

Do you have a link?

Good idea since I got the category wrong. It’s ICS 2.

Controllers, Contactors and Overload Relays Rated 600 V

Controllers, Contactors and Overload Relays Rated 600 V

www.nema.org

 

[mbrooke]

Good idea since I got the category wrong. It’s ICS 2.

Controllers, Contactors and Overload Relays Rated 600 V

Controllers, Contactors and Overload Relays Rated 600 V

www.nema.org

Thanks and its alright

 

[paulengr]

Fuses do indeed provide ground fault protection- at least as recognized by the code in article 430. Why do you say they don't?




What about cases where a low peak fuse also does motor overload protection? Is the concept practical? I know ti strays away from my initial question but its something I think about.



What about where the motor is denergized at its peak with residual magnetism then power applied at the opposite point in the sine wave? I've heard inrush will be two times zero crossing.

Whether or not fuses provide ground fault protection is inadvertent. It provides overcurrent protection period. One can say that since during single phasing currents will be at least 173% of normal load and torque drops so current will be even higher resulting in a second or third fuse trip, is that single phase protection?

Typically ground fault tripping is set to 10-15% of total current. This is far below fuse tripping. OCPD works well under 10 HP but if we have say 1 ohm 0.5 ohms in both the ground and phase conductors which is often highly optimistic at 480 V a ground fault would be 277 A. At an LRC of277 A tripping falls to the overload at typically 20 seconds with an FLA of 46 A, roughly a 40 HP motor. Should you really wait that long?

Low peak fuses cannot do overload protection. They are worse than breakers. The curve is way too steep: Most examples where this appears to be the case are typically thermally protected motors or special applications like fire pumps where overload protection is not used. The fuse/breaker argument would be won if this was the case.

As to pumping...inrush is inrush not LRC. This is in the flux current path, not torque. Cranes often have anti-pump timers for this. See NEMA ICS 2. They also have an entire guide on crane control in the ICE series.

A more common issue than pumping a starter is plugging....suddenly reversing. The motor is operating at negative slip and becomes a generator with torque/speed reversed which can turn into excessive “start” times but torque remains the same curve. Current is reversed and goes through LRC as the motor decelerates then reversed.

 

[mbrooke]

I'm confused though... why is it called ground fault protection even at 300%?

 

[mbrooke]

Whether or not fuses provide ground fault protection is inadvertent. It provides overcurrent protection period. One can say that since during single phasing currents will be at least 173% of normal load and torque drops so current will be even higher resulting in a second or third fuse trip, is that single phase protection?

Typically ground fault tripping is set to 10-15% of total current. This is far below fuse tripping. OCPD works well under 10 HP but if we have say 1 ohm 0.5 ohms in both the ground and phase conductors which is often highly optimistic at 480 V a ground fault would be 277 A. At an LRC of277 A tripping falls to the overload at typically 20 seconds with an FLA of 46 A, roughly a 40 HP motor. Should you really wait that long?

Low peak fuses cannot do overload protection. They are worse than breakers. The curve is way too steep: Most examples where this appears to be the case are typically thermally protected motors or special applications like fire pumps where overload protection is not used. The fuse/breaker argument would be won if this was the case.

As to pumping...inrush is inrush not LRC. This is in the flux current path, not torque. Cranes often have anti-pump timers for this. See NEMA ICS 2. They also have an entire guide on crane control in the ICE series.

A more common issue than pumping a starter is plugging....suddenly reversing. The motor is operating at negative slip and becomes a generator with torque/speed reversed which can turn into excessive “start” times but torque remains the same curve. Current is reversed and goes through LRC as the motor decelerates then reversed.

I'm confused though, I've heard of low peaks being used to protect motors from overloads, there is even a catalog. I'll double check.

What fuse is best for motor overload protection?

 

[kwired]

I'm confused though, I've heard of low peaks being used to protect motors from overloads, there is even a catalog. I'll double check.

What fuse is best for motor overload protection?

Unless you know it will serviced by qualified persons IMO none of them are. Those that don't really know what they are doing will likely only replace very few times before they figure out what else fits the same fuseholder that will "not trip as easily". Same guys will also figure out they can turn the dial up on motor overloads that have such adjustable setting, and I have even run into some that will up size thermal overload elements if they are readily available to them.

 

[mbrooke]

There is that, but for the sake of the discussion lets say its qualified folks.

 

[paulengr]

I'm confused though... why is it called ground fault protection even at 300%?

If we step back a minute induction motors are wired delta, whether they are or not. So we have I1+I2+I3=0 normally. If there is a ground fault they will not sum to zero. A ground fault adds to this. Hopefully the motor current plus ground fault current exceeds the motor FLA to trip the overload. If the ground fault is large enough it will trip a fuse. If it is a dead short to ground it will be equal to the phase-phase short circuit bolted fault current divided by 1.732 in a solidly grounded system. That does happen but the most common motor fault is where the stator has a ground fault which is often not solidly shorted and the ground return path isn’t all that great either. It happens but it’s just not a reliable way to detect ground faults in motors. I’m not sure why you would believe that it is.

 

[Jraef]

I'm confused though... why is it called ground fault protection even at 300%?

The term "motor branch-circuit short-circuit and ground-fault protective device" in the context of 430.52 just means a short to ground, as opposed to a "bolted fault" that is phase-to-phase short. They want the device to be able to protect against both, equally, i.e. fuse or mag trips on a circuit breaker. Don't confuse it with "Ground Fault" as used in GFCI or GFEP, they are not the same thing. This is a common common misconception by the way.

 

[paulengr]

I'm confused though, I've heard of low peaks being used to protect motors from overloads, there is even a catalog. I'll double check.

What fuse is best for motor overload protection?

None.

http://www.cooperindustries.com/content/dam/public/bussmann/Electrical/Resources/technical-literature/bus-ele-br-3002-spd-2017.pdf

Nice charts showing you why. See page 7-117, 7-118 (low peak charts showing type 2), 7-87 to 7-88 that show why fuses and breakers won’t do overload protection by themselves.

There are a bunch of fractional HP rules that I would have to check that might allow this but nothing 1 HP and larger unless it’s something like a fire pump where overload protection is done by oversizing the wiring and let it burn up.

If you don’t mind the disadvantages you really can’t beat a good fuse to handle short circuits and a good advanced motor relay to handle everything else for the best motor protection money can buy. But there is definitely a place for MMSs and breakers too. Just don’t fool yourself into thinking that you can get away without having two devices. One can be an overload relay or thermal switches or something else but the fuse can’t do it all. There are some MMSs that can because they are combination Short circuit and overload but it’s really two devices in one box.

 

[mbrooke]

None.

http://www.cooperindustries.com/content/dam/public/bussmann/Electrical/Resources/technical-literature/bus-ele-br-3002-spd-2017.pdf

Nice charts showing you why. See page 7-117, 7-118 (low peak charts showing type 2), 7-87 to 7-88 that show why fuses and breakers won’t do overload protection by themselves.

There are a bunch of fractional HP rules that I would have to check that might allow this but nothing 1 HP and larger unless it’s something like a fire pump where overload protection is done by oversizing the wiring and let it burn up.

If you don’t mind the disadvantages you really can’t beat a good fuse to handle short circuits and a good advanced motor relay to handle everything else for the best motor protection money can buy. But there is definitely a place for MMSs and breakers too. Just don’t fool yourself into thinking that you can get away without having two devices. One can be an overload relay or thermal switches or something else but the fuse can’t do it all. There are some MMSs that can because they are combination Short circuit and overload but it’s really two devices in one box.

Hhhhmmm...interesting... Choice 5 (125% Class J or RK-5) graph on page 7-118 shows green (motor start and run) to the left with the dual element fuse to he right. Red motor damage curve follows the fuse. I'm thinking there must be combinations where the fuse fits in between the green and red curves considering the space.


However- if you look at page 7-88 it mentions that the fuses will open before the motor damage curve is reached. 3-36 says RK-5 fuses provide excellent overload protection for motors. Page 3-7 says that a 40 amp time delay fuse for a motor with 32.2 amps of FLA "...plus overload protection (eliminating the code requirement for separate motor overload protection" Below that Bassman re-iterates that time delay fuses provide overload protection plus backup overload protection preventing motor burn ups. Its seems like they go out of their way to make a distinction between overload protection and last resort overload protection that won't emit smoke. Meaning a time delay fuse could do what the heaters do.

It seems that there are combinations that will indeed work?


I mean look at it like this. Why provide expensive motor protectors when a time delay fuse can save two birds with one seed feeder?

 

[mbrooke]

The term "motor branch-circuit short-circuit and ground-fault protective device" in the context of 430.52 just means a short to ground, as opposed to a "bolted fault" that is phase-to-phase short. They want the device to be able to protect against both, equally, i.e. fuse or mag trips on a circuit breaker. Don't confuse it with "Ground Fault" as used in GFCI or GFEP, they are not the same thing. This is a common common misconception by the way.

Right, GFCIs remove a phase to chassis ground fault with current differential while breakers and fuses remove a phase to chassis ground fault through thermal or magnetic means activated via a low impedance path back to the source.