Switching the neutral for an AC unit

[sparkky302]

Hello everyone I have a quick question. I have a 120V Mitsubishi split unit (Mr. Slim). They show a 2 pole fused disconnect feeding this unit. Is it a violation to switch the neutral feeding the unit along with the phase conductor?
Thank You

 

[infinity]

It's fine along as it switches both simultaneously. I'm guessing that it's drawn that way to accomadate the possibility of a 240 volt unit.

 

[Dennis Alwon]

No, IMO this is not fine since it has fuses. It is fine to switch the neutral with the hot on a dp switch but I believe the fuses make it non compliant

 

[stickboy1375]

I dont understand why the manufactures just don't have a disconnect inside these units, either a switch or some type of plug connection for the motor, something as simple as a molex connector.

 

[Dennis Alwon]

Just fuse the hot conductors and tie the neutrals together.

 

[infinity]

No, IMO this is not fine since it has fuses. It is fine to switch the neutral with the hot on a dp switch but I believe the fuses make it non compliant

I agree, I missed the fuse part.

 

[templdl]

No, IMO this is not fine since it has fuses. It is fine to switch the neutral with the hot on a dp switch but I believe the fuses make it non compliant

I agree that the grounded conductor (neutral)should never be fused. If you think about the reasoning if that fuse happens to blow but the line side doesn't the circuit will remain energized through the load and back to the load side to be blown fuse which can't be a good thing.
I would not put is past that a manufacturer to knowingly do so since it is not unusual for a designer to know know the code requirements and the assembly people then follow the designs. If it is UL listed device which it probably is it most likely had been listed with a correct design. But, unless UL reviews a facility on a regular basis, things can unknowingly be changed and not caught. I worked for a transformer manufacturer as a sales and applications engineer and believe me things do slip through engineering ,assembly, and final inspection.

 

[don_resqcapt19]

UL says they review all manufacturer's installation instructions and that those instructions are 110.3(B) instructions:happysad:

 

[sparkky302]

Thank You eveyone for your input. I agree there is no need to have the neutral on a fuse.

 

[petersonra]

Thank You eveyone for your input. I agree there is no need to have the neutral on a fuse.

Sometimes you have to look very closely at the instructions and the diagrams to get what they are really saying.

 

[templdl]

UL says they review all manufacturer's installation instructions and that those instructions are 110.3(B) instructions:happysad:

That's the way it supposed to be but designer may change something that the designer doesn't see as relevant and it gets shipped. Then we install it asking ourselves why they did what they did.
When the manufacturers hire people with less and less experience and then add the turn over rates which also includes promotions of transferring from one position to another it often leads to the blind leading the blind sad to say.
And from my experience the UL inspector may be thorough and you dread him coming into the plant or you know that he would not be thorough and as long as there aren't any blatant violation there will be no issues. . It could be as simple as not using a UR component listed device in an assembly that is supposed to be UL listed because the plant manager and purchasing department changed vendors not realizing the implications. It could even be using insulating materials that were not part of the original design that was approved by UL. If the UL inspector isn?t sharp the deviation won?t get caught.

 

[don_resqcapt19]

That's the way it supposed to be but designer may change something that the designer doesn't see as relevant and it gets shipped. Then we install it asking ourselves why they did what they did.
When the manufacturers hire people with less and less experience and then add the turn over rates which also includes promotions of transferring from one position to another it often leads to the blind leading the blind sad to say.
And from my experience the UL inspector may be thorough and you dread him coming into the plant or you know that he would not be thorough and as long as there aren't any blatant violation there will be no issues. . It could be as simple as not using a UR component listed device in an assembly that is supposed to be UL listed because the plant manager and purchasing department changed vendors not realizing the implications. It could even be using insulating materials that were not part of the original design that was approved by UL. If the UL inspector isn?t sharp the deviation won?t get caught.

My point is that those instructions are often not reviewed and should not be 110.3(B) instructions. In my opinion, one that UL says is completely wrong, the only things that are "listing and labeling instructions, are those things that are in the actual product standard or the UL Guide Information (White Book), anything else is just a manufacturer's recommendation and should not be 110.3(B).

 

[templdl]

My point is that those instructions are often not reviewed and should not be 110.3(B) instructions. In my opinion, one that UL says is completely wrong, the only things that are "listing and labeling instructions, are those things that are in the actual product standard or the UL Guide Information (White Book), anything else is just a manufacturer's recommendation and should not be 110.3(B).

Point taken Don.
Thanks for the clarification.

 

[kwired]

I agree that the grounded conductor (neutral)should never be fused. If you think about the reasoning if that fuse happens to blow but the line side doesn't the circuit will remain energized through the load and back to the load side to be blown fuse which can't be a good thing.

Just how bad of a thing is it? (Yes I know you are not supposed to do it, and it can be a poor design choice, but in some situations it is not really all that dangerous, others it can be very dangerous.)

Placing a fuse in the grounded conductor at the service equipment is much different situation than doing so at this individual load, and even a circuit supplying lampholders where screw shell is connected to grounded conductor it becomes a little more of a hazard. Many appliances and other listed items do sometimes switch the grounded conductor (and not simultaneously switch the ungrounded conductor).

If disconnect still opens both lines - opening the disconnect before servicing or changing the blown fuse on the grounded conductor leaves it in a safe to work on condition. It is not any more or less dangerous than if the grounded conductor became open for any reason.

 

[Dennis Alwon]

I think they are generic instructions or diagrams for the 240V units that they used for the 120V units.

 

[templdl]

Just how bad of a thing is it? (Yes I know you are not supposed to do it, and it can be a poor design choice, but in some situations it is not really all that dangerous, others it can be very dangerous.)

Placing a fuse in the grounded conductor at the service equipment is much different situation than doing so at this individual load, and even a circuit supplying lampholders where screw shell is connected to grounded conductor it becomes a little more of a hazard. Many appliances and other listed items do sometimes switch the grounded conductor (and not simultaneously switch the ungrounded conductor).

If disconnect still opens both lines - opening the disconnect before servicing or changing the blown fuse on the grounded conductor leaves it in a safe to work on condition. It is not any more or less dangerous than if the grounded conductor became open for any reason.

If you fused the L and N at the equipment and the N fused blew and you ended up with a L-frame fault that wasn't of great enough magnetude to blow the L fuse can you be guaranteed what path the current would take? The examples that you have provided are of simple devices where the line conductor is not very likely to come into contact with a frame that is or is not grounded. On the subject of switching the grounded conductor the only instance that I can think of is with older appliances where they weren't using polarazed plugs yet. You made this statement:"Many appliances and other listed items do sometimes switch the grounded conductor (and not simultaneously switch the ungrounded conductor)."
Is this design been excepted by UL? You imply that you know for a fact that it is but I would be interested to know if this is something that would be listed by UL today. The key is the polarized or plugs and cords that have EGCs.

 

[kwired]

If you fused the L and N at the equipment and the N fused blew and you ended up with a L-frame fault that wasn't of great enough magnetude to blow the L fuse can you be guaranteed what path the current would take?

You sure you didn't mean to ask what happens if you have a N-frame fault? Are you likely to have both blown fuse and N-frame fault at same time - without exception of an incident where obvious physical damage has happened - then you could have potential shock and stray current hazards anyway if the neutral was opened in the event.

The examples that you have provided are of simple devices where the line conductor is not very likely to come into contact with a frame that is or is not grounded. On the subject of switching the grounded conductor the only instance that I can think of is with older appliances where they weren't using polarazed plugs yet. You made this statement:"Many appliances and other listed items do sometimes switch the grounded conductor (and not simultaneously switch the ungrounded conductor)."
Is this design been excepted by UL? You imply that you know for a fact that it is but I would be interested to know if this is something that would be listed by UL today. The key is the polarized or plugs and cords that have EGCs.

Only example that immediately comes to mind is the fact it is pretty common practice to switch the L2 (which can be the neutral) in control circuits. Usually as a limit or overload and not as the primary control contact. There are millions of motor starters with overload relay opening the grounded circuit conductor. A fault to ground on the coil side of that contact will result in undesired operation of the coil when the overload device is otherwise tripped. The likelihood of that happening is not normally that great but that is usually considered an acceptable installation.

I am not trying to say it is right or wrong, or that there is no hazards, I am just saying each particular situation has a different level of probable hazards, and sometimes it actually is done and is acceptable. Even a switch that simultaneously opens grounded with ungrounded can fail and open the grounded but not the ungrounded - mechanical things do fail.