Help! Not sure if this is allowed by codes, UL 508a Experts?

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I have a panel I am looking at that controls a motor. The panel is 480V 3 phase and so is the motor. They have 3 independent circuit breakers, on each phase, before the motor contactor and motor. In my experience, this is not typical or recommended since the possibility exists that one phase could trip and possibly damage the motor by single phasing it.

So I decided to try and look it up in UL 508A, NEC 70, and NFPA 79, the only thing I can find are these references:

7.2.10 Power Circuits in NFPA79 where it says "all ungrounded and grounded conductors open with no pole operating independently"

240.15 Ungrounded Conductors in NEC 70 says: "(B) Circuit breaker or overcurrent device shall open all ungrounded conductors of the circuit both manually and automatically...."

My dilemma is the customer wants the panel to meet UL508A requirements, but I can not find any reference in UL about this issue. Am I overlooking it? My understanding is that since NEC is law binding, all other codes should follow. So I want to tell them that this is not UL, but it would be nice to have proof.

If any one has any ideas please let me know, or if you know any white papers, case studies, etc... that talk about this in more detail, please let me know. The only other code is UL498 for circuit breakers, but I do not have this code...

Thanks,
Jay
 

petersonra

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is this being used as a disconnecting means? if so, take a look at 2.15 in UL508a.

I would not be worried all that much about single phasing. The same thing could potentially happen with a set of fuses and no one worries about that.
 

Jraef

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At a basic level, single pole circuit breakers are not UL listed for protecting 3 phase circuits, so that alone prohibits their use on a UL508a listed panel in that manner. I'd be shocked if the panel bears such a label, at least legally.
 

petersonra

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At a basic level, single pole circuit breakers are not UL listed for protecting 3 phase circuits, so that alone prohibits their use on a UL508a listed panel in that manner. I'd be shocked if the panel bears such a label, at least legally.

CBs are generally listed by voltage and current ratings, not by phases.

A single pole CB with appropriate voltage rating can be hooked up to a single phase of a 480V 3 phase system. I don't see anything in UL508a that would prohibit one from using 3 appropriately rated single pole CBs to protect a 3 phase load. They would be prohibited from serving as a disconnecting means though.

I am not suggesting it is a good idea, just that I am not convinced it is prohibited.

How is this any different than 3 separate fuses?
 

Frank DuVal

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I have not seen an overload that does detect single phase condition. It does trip on overcurrent, which is a byproduct of a three phase motor running on one phase. If the overload is not set right, of course there is no protection. If the motor can supply its load while running on a single phase, without drawing excess current, all is well. But, the next time it starts....
 

Jraef

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I have not seen an overload that does detect single phase condition. It does trip on overcurrent, which is a byproduct of a three phase motor running on one phase. If the overload is not set right, of course there is no protection. If the motor can supply its load while running on a single phase, without drawing excess current, all is well. But, the next time it starts....
Well, not 100% true. If the motor current under single phasing conditions is near, but BELOW the OL trip setting, the OL will NOT trip, but the negative sequence current created inside of the motor because of the missing phase will heat up the motor disproportionately to the current flow and can damage it. That's why you want to have single phasing protection. Almost all Solid State OL relays will now provide single phasing protection. IEC bi-metal OLs do in a fashion as well, but what they really do is just BIAS the OL trip pickup point to be lower than it would be on 3 phase so if the motor is FULLY LOADED it will trip on a loss of phase. But if still BELOW the setting, it would not trip however. That's a common misconception, fostered by misleading advertising on the part of IEC OL manufacturers.

After a little more reading, I am agreeing with Bob Peterson on the technicality of the 3 single pole breakers. IF NOT USED AS THE DISCONNECT, there is nothing in UL508a prohibiting this fiasco. UL508a does say that all breakers must be UL489 listed, and UL489 does require certain MARKING of the breaker's capabilities (i.e. a single pole 480V breaker can ONLY be used on a 480/277V system, not 480V Delta), but it too does not expressly forbid using 3 separate single pole breakers on a 3 phase circuit, as long as they are not to be considered the disconnecting device.

Still, it make NO sense to do that, because it means you must use 3 separate 1 pole breakers (which will cost more than a 3 pole breaker), THEN you ALSO must provide ANOTHER means of disconnecting power to that panel, then mount and wire all of those extra parts. Foolishness exemplified, but not technically illegal if all of those conditions are satisfied.

XxaudioholicxX,
Do you have another disconnect in that panel ahead of these single pole breakers then? If so, what is it? Because there are rules about that as well, for example you would need a disconnect listed under UL98, not the less expensive UL508 versions, because a UL508 listed disconnect can only be used AFTER the Branch Circuit Protective Device.
 
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Ingenieur

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is this being used as a disconnecting means? if so, take a look at 2.15 in UL508a.

I would not be worried all that much about single phasing. The same thing could potentially happen with a set of fuses and no one worries about that.

Fuses may be sized so that if single ph occurs the other lines rise > fuse rating
that may not be the case with a cb
ol's should catch it too

460/10 hp/14 A
fuse 17.5-25 (30 for heavy load start)

single ph i 24 a
fuse may catch it
ol will
 
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Jraef

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Fuses may be sized so that if single ph occurs the other lines rise > fuse rating
that may not be the case with a cb
ol's should catch it too

460/10 hp/14 A
fuse 17.5-25 (30 for heavy load start)

single ph i 24 a
fuse may catch it
ol will
Again, ONLY if the motor is at FULL LOAD.

If the motor is only at 1/2 load when the phase is lost, then the increase due to single phasing will not meet the fuse or the OL trip point.
 

Ingenieur

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Again, ONLY if the motor is at FULL LOAD.

If the motor is only at 1/2 load when the phase is lost, then the increase due to single phasing will not meet the fuse or the OL trip point.

then the equipment or protection is not sized appropriately
 

GoldDigger

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then the equipment or protection is not sized appropriately
Did you not fully read Jraef's comment about negative sequence current internal to the motor? If some of the winding current does not travel through the supply wires, how can any OL that only senses line current detect it?
Single phasing is an *anomalous* condition, which is hard for a passive thermal OL to detect while still allowing starting and full load operation.
 

Ingenieur

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Did you not fully read Jraef's comment about negative sequence current internal to the motor? If some of the winding current does not travel through the supply wires, how can any OL that only senses line current detect it?
Single phasing is an *anomalous* condition, which is hard for a passive thermal OL to detect while still allowing starting and full load operation.

No it isn't
negative sequence current?
why?
they will be in ADDITION to line current
will heat the motor and damage it if left unchecked
proper fusing and ol setting will mitigate to large extent

it's not the complicated
imho we tend to make complex engineering problems where they don't exist
http://www.cooperindustries.com/con...BUS_Ele_Protection_Against_Single-Phasing.pdf

all I'm saying is basic engineering due diligence which requires 20% of the effort will solve 80% of the scenarios

A simple thing: adjust the ol's for nml running load not motor fla
 

Jraef

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then the equipment or protection is not sized appropriately
Huh?

If I have a 10HP motor 14A FLA, I size the OL to protect a 10HP motor based on an FLA of 14A. Assuming a 1.15SF motor, the OL pick-up point will be 125% of that, so the OL will not even start to count down until the current exceeds 17.5A.

If, at the time I lose a phase, the motor was only drawing 9A because it was lightly loaded, the current will increase to 15.58A on the other phases. That is STILL below the pick-up point of 17.5A. Nothing would happen there. The motor will run for a long time like that, yet there is nothing inappropriate about the selection of the OL relay heaters or setting. The same would be true for fuses, assuming a 18A fuse and no OL relay.

At the same time the effect of motor heating from unbalanced voltage in percent of rise is going to be (2 x Vunbal)2. With no voltage on the lost phase, the voltage unbalance is 100%, making the heat rise 400% of normal. So even if you have a motor rated for 40C ambient with a 115C rise (Class F Insulation) the maximum insulation temperature is 155C. Now let's say it was operating at only 90C before the phase loss, so a 50C rise. After the phase loss the winding temperature will rise 400% of that 50C, so now a 200C rise, which means you will absolutely* exceed the maximum winding temperature of that motor, WITHOUT ever tripping the OL relay.

*Unless the ambient is -45C (-49F)
 

Jraef

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Ah, but he is telling you that any motor that is *ever* lightly loaded is the wrong motor (or should have a VFD.)
:angel:
Ok, valid point, IF that unloaded condition is permanent (smaller motor) or sustained with occasional higher loading (VFD). But this was discussing the control panel design and the effects of one breaker tripping, not the motor / drive selection.
:roll:
 

Ingenieur

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Huh?

If I have a 10HP motor 14A FLA, I size the OL to protect a 10HP motor based on an FLA of 14A. Assuming a 1.15SF motor, the OL pick-up point will be 125% of that, so the OL will not even start to count down until the current exceeds 17.5A.

If, at the time I lose a phase, the motor was only drawing 9A because it was lightly loaded, the current will increase to 15.58A on the other phases. That is STILL below the pick-up point of 17.5A. Nothing would happen there. The motor will run for a long time like that, yet there is nothing inappropriate about the selection of the OL relay heaters or setting. The same would be true for fuses, assuming a 18A fuse and no OL relay.

At the same time the effect of motor heating from unbalanced voltage in percent of rise is going to be (2 x Vunbal)2. With no voltage on the lost phase, the voltage unbalance is 100%, making the heat rise 400% of normal. So even if you have a motor rated for 40C ambient with a 115C rise (Class F Insulation) the maximum insulation temperature is 155C. Now let's say it was operating at only 90C before the phase loss, so a 50C rise. After the phase loss the winding temperature will rise 400% of that 50C, so now a 200C rise, which means you will absolutely* exceed the maximum winding temperature of that motor, WITHOUT ever tripping the OL relay.

*Unless the ambient is -45C (-49F)

what what what??? lol
it should be sized for the operating point

if a 10 A motor is running under normal operating conditions at only 5 amps
the motor is not sized properly
and the overloads should be set at 1.15 x 5 = 5.75 not 11.5
the overloads are to protect against a mechanical overload also
if the machine runs normally at 5 A and steps to 11 there is a problem

we make this more complicated than it is to show off our big brains
my neck is sore from carrying mine around :happysad:
 

SceneryDriver

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Code violations aside, isn't this just a really bad idea anyway? If two individual poles trip, the motor still potentially has voltage to ground. It wouldn't be running, but it would be live, and could hurt the person troubleshooting it. How much can a proper 3-pole breaker possibly cost, in comparison to all the time you've spent researching this? Replace the (3) single-pole breakers with (1) 3-pole breaker and be done with it.



SceneryDriver
 

petersonra

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Code violations aside, isn't this just a really bad idea anyway? If two individual poles trip, the motor still potentially has voltage to ground. It wouldn't be running, but it would be live, and could hurt the person troubleshooting it. How much can a proper 3-pole breaker possibly cost, in comparison to all the time you've spent researching this? Replace the (3) single-pole breakers with (1) 3-pole breaker and be done with it.



SceneryDriver

neither the code nor UL508a would allow its use as a disconnecting means.
 
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