Motor Short Circuit Ground Fault Protection

mbrooke

Batteries Not Included
Location
United States
Occupation
*
Has anyone ever seen or had problems with Short Circuit and Ground Fault protection tripping when set at 115-125% of the motor's Full Load Amps?
 

bwat

EE
Location
Western PA
Occupation
EE
Every case is different, but I would say if the cb is 115-125% of FLA and it doesn’t trip on across the line starting, you got lucky. Taking a look at cb trip curves and knowing the general relationship between LRC and FLA and you’ll see why. It’ll be almost a coin flip in a lot of cases.

I’ve had to fix a couple designs where the cb was tripping and it was because it was sized in that range.
 

kwired

Electron manager
Location
NE Nebraska
I think you will get away with the 115-125% more often with fuses than with breakers.

But again every installation has it's conditions.

I've seen many times where a 120 volt power tool, particularly air compressors, chop saws, and some miter saws or tables saws won't hold on starting on a 20 amp breaker (particularly on Square D standard breaker with lower mag trip setting than many others). This often when the source is fairly stout and there is relatively short branch circuit length. Sometimes just adding as little as a 25 foot extension cord to the circuit allows enough resistance to keep the surge below the trip curve.

Also seen many times where a 10 HP 480 volt 3 phase motor will hold starting on a 15 amp breaker, which is in the 125% range of the motor rated current or even 10 HP single phase 240 volt motors starting on a 60 amp breaker which is also in the 125% range. But also seen times where neither of those won't hold, same motor, same breaker - different installation conditions though.
 

paulengr

Senior Member
Has anyone ever seen or had problems with Short Circuit and Ground Fault protection tripping when set at 115-125% of the motor's Full Load Amps?
I’m a little confused here. A standard breaker will not coordinate with a motor, period. The thermal curve is the wrong shape. You cannot make it match the NEMA SG-1 curve or the equivalent under IEC. This is why NEC 430 gives a maximum long term trip rating of 250%. The breaker only covers short circuits. Then an overload relay is set to FLA on a modern adjustable relay. On old eutectic and bimetallic relays there is a calculation procedure to determine the correct rating. According to NEMA SG-1 there is a TOLERANCE of 115-125% but that’s not something you set. Second typically you can’t “set” the long term trip setting except on larger electronic breakers that have typically a rating plug and the ability to adjust the setting a small amount. So “setting” doesn’t sound right.

An MCP gets set to the inrush, typically 11-17x FLA. An MMS usually gets set to the FLA although I have seen one set similar to an MCP that was magnetic trip only without an included overload function (Sprecher and Schuh branded).

Finally getting to the question of tripping, short circuit (instantaneous) tripping can occur for three reasons. First is because there is a short circuit. Second a motor has a magnetizing inrush in the first 1-2 cycles. For this reason NEC allows instantaneous trip settings as high as 17x FLA. Theoretically it can be as high as 24x FLA. I’ve actually seen Siemens and Toshiba motors hit 22x FLA. Electronic breakers will trip in 1 cycle so tend to nuisance trip on high efficiency motors. Older thermal magnetic breakers and motor-specific devices (MCPs, MMSs) do not. So the choice of type of device is the second issue. Third and along the same lines a standard breaker typically has instantaneous tripping in the range of 6-10x the long term trip setting. So based on your information that would make it 6.9-12.5x FLA. That might work on an older motor but not a newer one even if it’s a slower thermal magnetic breaker.

As to ground faults, this is typical of a failed motor. When motor wiring is damaged it typically burns through the ground wall insulation which is a ground fault. However the typical mistake is that it gets tested using a standard multimeter. Using a multimeter unless it’s a dead short, it takes hours to charge the insulation capacitance to the point where you can detect a failure to ground. Insulation resistance meters use a higher voltage to charge the capacitance in under 1 minute for measurement. However in operation typically it takes very little current leakage to cause intermittent trips. So most likely it’s a real trip for an actual failure.
 

mbrooke

Batteries Not Included
Location
United States
Occupation
*
Also seen many times where a 10 HP 480 volt 3 phase motor will hold starting on a 15 amp breaker, which is in the 125% range of the motor rated current or even 10 HP single phase 240 volt motors starting on a 60 amp breaker which is also in the 125% range. But also seen times where neither of those won't hold, same motor, same breaker - different installation conditions though.

Any chance the breakers in these cases are getting hit at the magnetic trip region?

I somehow have the feeling that the thermal part of a breaker will hold even for heavy intertia loads.
 

mbrooke

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Location
United States
Occupation
*
I’m a little confused here. A standard breaker will not coordinate with a motor, period.
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?


The thermal curve is the wrong shape. You cannot make it match the NEMA SG-1 curve or the equivalent under IEC. This is why NEC 430 gives a maximum long term trip rating of 250%. The breaker only covers short circuits. Then an overload relay is set to FLA on a modern adjustable relay. On old eutectic and bimetallic relays there is a calculation procedure to determine the correct rating. According to NEMA SG-1 there is a TOLERANCE of 115-125% but that’s not something you set. Second typically you can’t “set” the long term trip setting except on larger electronic breakers that have typically a rating plug and the ability to adjust the setting a small amount. So “setting” doesn’t sound right.

An MCP gets set to the inrush, typically 11-17x FLA. An MMS usually gets set to the FLA although I have seen one set similar to an MCP that was magnetic trip only without an included overload function (Sprecher and Schuh branded).

Finally getting to the question of tripping, short circuit (instantaneous) tripping can occur for three reasons. First is because there is a short circuit. Second a motor has a magnetizing inrush in the first 1-2 cycles. For this reason NEC allows instantaneous trip settings as high as 17x FLA. Theoretically it can be as high as 24x FLA. I’ve actually seen Siemens and Toshiba motors hit 22x FLA. Electronic breakers will trip in 1 cycle so tend to nuisance trip on high efficiency motors. Older thermal magnetic breakers and motor-specific devices (MCPs, MMSs) do not. So the choice of type of device is the second issue. Third and along the same lines a standard breaker typically has instantaneous tripping in the range of 6-10x the long term trip setting. So based on your information that would make it 6.9-12.5x FLA. That might work on an older motor but not a newer one even if it’s a slower thermal magnetic breaker.

As to ground faults, this is typical of a failed motor. When motor wiring is damaged it typically burns through the ground wall insulation which is a ground fault. However the typical mistake is that it gets tested using a standard multimeter. Using a multimeter unless it’s a dead short, it takes hours to charge the insulation capacitance to the point where you can detect a failure to ground. Insulation resistance meters use a higher voltage to charge the capacitance in under 1 minute for measurement. However in operation typically it takes very little current leakage to cause intermittent trips. So most likely it’s a real trip for an actual failure.
Is inrush also a function of source impedance?

What about 15-20X breakers?
 

kwired

Electron manager
Location
NE Nebraska
Any chance the breakers in these cases are getting hit at the magnetic trip region?

I somehow have the feeling that the thermal part of a breaker will hold even for heavy intertia loads.
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.
 

mbrooke

Batteries Not Included
Location
United States
Occupation
*
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.

Makes sense now that you say it.

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