Motor Short Circuit Ground Fault 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?

One other issue with using fuses for overload protection is first fuse blows, then the motor single phases until another fuse blows.

 

[mbrooke]

One other issue with using fuses for overload protection is first fuse blows, then the motor single phases until another fuse blows.

Right- but does this damage the motor?

 

[Jraef]

Right- but does this damage the motor?

Potentially, yes. If the motor is fully loaded when that happens, the severe imbalance in currents creates Negative Sequence Current in the rotor, which in turn produce negative torque and "fights" the positive torque. So the ROTOR BAR current increases above what is taking place in the stator and the motor can be damaged without the stator current ever exceeding the specified values and clearing another fuse.

If the motor is only loaded at roughly half of its rating, it might be able to run forever on single phase power. But if that were the case, why would one fuse have blown? Bottom line, it would be FAR more common for the motor to burn up than not in that situation.

 

[Jraef]

I never recommend trying to use fuses as the running OL protection for a motor, it's too difficult to get the exactly correct fuse value to do it right, so it generally comes down to over sizing them to keep them from blowing on inrush. Letting an overloaded motor run longer than the OL damage curve dictates greatly reduces the motor life. OL relays/heaters are purposely designed to protect based on the damage curves of standard motor design parameters, fuses are not.

 

[mbrooke]

Potentially, yes. If the motor is fully loaded when that happens, the severe imbalance in currents creates Negative Sequence Current in the rotor, which in turn produce negative torque and "fights" the positive torque. So the ROTOR BAR current increases above what is taking place in the stator and the motor can be damaged without the stator current ever exceeding the specified values and clearing another fuse.

If the motor is only loaded at roughly half of its rating, it might be able to run forever on single phase power. But if that were the case, why would one fuse have blown? Bottom line, it would be FAR more common for the motor to burn up than not in that situation.

Same with heaters?

I'm not doubting you, but its seems like Bussman and now Eaton is very optimistic about what fuses can do.

 

[mbrooke]

I never recommend trying to use fuses as the running OL protection for a motor, it's too difficult to get the exactly correct fuse value to do it right, so it generally comes down to over sizing them to keep them from blowing on inrush. Letting an overloaded motor run longer than the OL damage curve dictates greatly reduces the motor life. OL relays/heaters are purposely designed to protect based on the damage curves of standard motor design parameters, fuses are not.

Makes sense.

In your experience, do 115% and 125% fuses blow on motor inrush or motor run to speed?

 

[mbrooke]

If a motor's FLA is right at or even below a fuse's rating, damage can still take place for a motor under overload, correct?

 

[paulengr]

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?

The protection issue is not during starting and the first few seconds. It’s when you are mildly overloaded with say a bearing going out or something jammed and the motor burns up repeatedly because fuses don’t react to anything longer than a minute or so. This is the realm of overload relays. Conversely overload relays have thermal mass too so they are utterly useless against electrical faults.

Overload relays at the size you are talking about are also about equal to the price of a single fuse without the fuse holder. And if one fuse trips the motor single phases until the second fuse trips. So you are automatically out the price of more than the overload relay would have cost you. And you needed a disconnect anyways by Code and to change fuses so just buy an MMS. One device does it all unless your transformer is so large that the contactor keeps welding or SCCR is the issue. Or use thermally protected motors (very cheap option) and any fuse or breaker you want that meets Code. A Klixxon relay is so cheap some of the motors we sell come with it as a standard option.

Believe me I’ve tried this. It does work but you are not saving money. As I said earlier I think fuses are very underutilized but the cost of replacement cannot be denied and although they exist current limiting breakers are still way overpriced and too limited to displace fuses.

Look I have a customer that thinks that it’s OK to run a motor off a fused safety disconnect along with the rest of the control panel. He might be right as it is out in a scrap yard processing waste concrete. What is the fire risk? None. Does it make it right? Absolutely not. He is that cheap. He sure spends a lot in repairs though because every mechanical issue is also automatically an electrical issue too. Good customer. Spend a dollar to pinch a penny.

 

[mbrooke]

Thanks, I appreciate your experience.

When dealing with 5-400HP motor, is there any case where fuses result in a lower capitol cost? Or it still applies as will smaller motors?

It sounds like fuses will respond only to moderate to severe overloads without motor damage, while low level overloads will not open a fuse whose time current curve starts at 125-165%? In other words all fuses have a "blind spot"?


I don't mean to put you on the spot, but why is Eaton so optomystic?

 

[bwat]

Another wrinkle to add to this. Sometimes people will use fuses in a UL508 panel that feeds motors because it can sometimes be easier to get a higher SCCR rating for the panel when using fuses.

 

[paulengr]

Thanks, I appreciate your experience.

When dealing with 5-400HP motor, is there any case where fuses result in a lower capitol cost? Or it still applies as will smaller motors?

It sounds like fuses will respond only to moderate to severe overloads without motor damage, while low level overloads will not open a fuse whose time current curve starts at 125-165%? In other words all fuses have a "blind spot"?


I don't mean to put you on the spot, but why is Eaton so optomystic?

The wisdom (25 years ago) is fuses are cheaper than breakers. MMSs weren’t really around. So you trade off reusability and initial cost for better SCCR and better trip times on electrical faults. Today breaker costs have come down and the MMS has decidedly tilted things against fuses compared to back then. At least on small motors under 50 HP where they are available. So breaker popularity has definitely increased. The MCP is almost automatic in an MCC. It is not a “breaker” by Code but it’s almost the same thing. In 4160 starters for instance breakers are generally not even an option. And at several hundred dollars for 3 fuses nobody wants to change one. Around 500+ HP is where the “wisdom” leans heavily in favor of higher voltages. That is what IEEE Red book recommends. So that covers over 400 HP.

Under that range if there is no MMS I tend towards breakers. Fuses are cheaper at around 60+ HP but most of the time we are selling soft starts or VFDs so overload protection is “free” (built in by Code) and we just need to consider SCCR to decide breakers vs fuses.

 

[mbrooke]

MCPs? MMS as in those combo enclosures with everything inside? I'm bad with acronyms lol.

Where would an MCC with just a disco, fuses and contactor cost? I want to say this is the cheapest, but perhaps not so.

 

[paulengr]

MCPs? MMS as in those combo enclosures with everything inside? I'm bad with acronyms lol.

Where would an MCC with just a disco, fuses and contactor cost? I want to say this is the cheapest, but perhaps not so.

MCP=Motor Circuit Protector. It comes in a molded case housing. You might call it a circuit breaker but since it only has instantaneous (magnetic) tripping but not an inverse time function it is technically not a circuit breaker. NEC calls it supplemental protection. They are strictly components, part of a Listed assembly.

MMS=manual motor starter. Generally contains everything necessary to be a starter for on/off control. To make it automatic you add a contactor on the load side.

Cheapest by far for installation is to use an industrial control panel with IEC contactors fed by MMSs. In larger motors use fuses with disconnect capability instead. It is far more compact and merges the controls and motor control usually with PLC controls in one spot. That is why these are so popular.

Hard to estimate MCCs because it’s a highly competitive market as long as you always order it with a main lug only (MLO) or main breaker. This triggers the manufacturer to give bid prices.

 

[Jraef]

Makes sense.

In your experience, do 115% and 125% fuses blow on motor inrush or motor run to speed?

With heaters, the heaters act on an overload relay to drop out the contactor coil, which disconnects all 3 poles and the motor will not run single phased.

Same with heaters?

I'm not doubting you, but its seems like Bussman and now Eaton is very optimistic about what fuses can do.

With heaters, the heaters act on an overload relay to drop out the contactor coil, which disconnects all 3 poles and the motor will not run single phased. When attempting to use fuses AS the running OL protection (instead of an OL relay), you set up the possibility of only one fuse clearing and allowing the motor to continue to run single phased.

 

[Jraef]

I'm not sure what you are reading that is promoting fuses as running OL protection for motors, but it's just bad juju. In a motor circuit, you must have 4 main elements: a means of disconnect, a means of control, a means of protecting from short circuits, and a means of protecting from long term running overload of the motor. Those do not necessarily have to be separate devices, you can combine the elements, but they all need to be there and the devices you use must be qualified (listed) for the task(s). So for example a Circuit Breaker provides Short Circuit Protection Device (SCPD) and is ALSO a disconnect device, so it is 2 of the 4 elements right there. The same would be true for a fused disconnect switch; SCPD and Disconnect as one unit. If a Circuit breaker is ALSO listed as a Manual Motor Starter, it can ALSO be the controller (manual on-off control) and have an adjustable thermal OL protection too (I have done this), so you have all 4 elements in one device (so long as manual control is all you need). You can ALSO use a HP rated Fused Disconnect as the manual controller and SCPD, but if the fuses are sized to that narrow window to be within 125% of the motor nameplate FLC, it too CAN be all 4 elements in one.

As said, the SCPD can be fuses or a circuit breaker. Fuses in some cases are better than Circuit Breakers (i.e. when the Available Fault Current is extremely high), but always run the risk of single phasing a motor. If fuses are used only as the SCPD in a classic motor starter with a contactor and OL relay, it's a good idea to have Single Phasing protection with it to prevent that damage to the motor, and most of the new Solid State Overload Relays provide that now. But using fuses without something else to protect from single phasing, as would be the case when trying to use them AS the OL protection too, exposes you to that risk.

But just because you CAN use fuses in lieu of an OL relay, doesn't mean you should and there is no cogent argument to doing so, other than a "penny wise - pound foolish" approach to trying to save money on component costs. You can shave a few dollars from the cost of a motor controller, but you risk a much more EXTREME cost of lengthy down time and rewinding an expensive motor. Not worth doing, no matter what the fuse manufacturer wants you to think.

 

[ptonsparky]

Makes sense now that you say it.

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

Depends on theFan and hwIt is set up. Total distance to fan fro m xfmr. We ask for a pump panel sized xxxHP. We get fuses. Why would you spec Js?

 

[paulengr]

Depends on theFan and hwIt is set up. Total distance to fan fro m xfmr. We ask for a pump panel sized xxxHP. We get fuses. Why would you spec Js?

Is are cheaper and fit in your pocket easier than RKs.

 

[mbrooke]

Is are cheaper and fit in your pocket easier than RKs.

Any difference between the two performance wise?

 

[paulengr]

Any difference between the two performance wise?

Between RK1 and J, both give you type 2 (no damage) protection. The Js have a maximum of 309KA and RK only 100KA but how many times do you need 100kA?

RK5 is not as fast so not type 2, more damage before it trips, etc. RK1s are less common though.

On some manufacturers you pay extra for Js because they come with RK fuse holders and you have to buy the J clip kit.

 

[hornetd]

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.

This reads as if Paul and some of the people responding to him are talking about 2 different things. Isn't the Ground Fault Protection of the circuit overcurrent protection named in relation to the possibility of a hard fault rather than a much lesser level of current LEAKAGE?

--
Tom Horne